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Crucial factors for the feasibility of commercial hybrid breeding in food crops
Abstract
There is an ongoing societal debate about plant breeding systems and their impact on stakeholders in food systems. Hybrid breeding and hybrid seed have become controversial topics as they are believed to mostly serve high-tech agricultural systems. This article focuses on the perspective of commercial plant breeders when developing new cultivars of food crops. Arguably, hybrid breeding is the most effective breeding system for genetic improvement of crops, enhancing yields, improving product quality and increasing resistance against (a)biotic stresses. Nonetheless, hybrid breeding is not commercially applied in all crops. We analyse how biological and economic factors determine whether a commercial plant breeder opts for the hybrid system or not. We show that the commercial feasibility of hybrid breeding depends on the crop and business case. In conclusion, the commercial application of hybrid breeding in crops seems to be hampered mostly by high costs of seed production. Case studies regarding the hybrid transitions in maize, wheat and potato are included to illustrate these findings. This Perspective analyses how biological and economic factors determine whether a commercial plant breeder will opt for a hybrid breeding system, and finds that the cost of seed production is a key factor.
... In the future, there may be divergent breeding priorities for specific agroecological zones. However, breeding, specifically for poorly developed niche markets, such as hybrid potatoes adapted to tropical lowlands, is considered unrealistic from a business perspective due to low returns and high risks [7]. Therefore, Solynta is aiming at public-private partnerships that may offer ways to organize the resources needed to address food security and climate change as major global challenges. ...
... The potential of hybrid potato has now been generally recognized, and fundamental hybrid breeding programs have been launched in the largest potato-producing countries, China, India and the USA [53,54,60]. However, building new hybrid breeding programs from scratch requires long-term efforts and high commercial investments [7]. Before we can expect improved hybrid potato varieties to reach the agro-industrial market, these varieties will first have to meet current, high-level and tight-fitting retail and potato processing standards. ...
... In this respect, the company follows a strategy similar to that of other globally operating potato breeding firms, which in their breeding programs hardly focus on the particular needs of African farmers but introduce best-bet varieties from their portfolios in those countries where there is a market demand [19]. As we learned from our exploration of hybrid potato prospects in the international development context, in the absence of an immediate business case, there is a need for public-private collaboration to facilitate the development and dissemination of potato varieties that are suited to the specific conditions and needs of smallholder farmers (see also: [7]). To further strengthen Solynta's pilot activities and partnerships in Sub-Saharan Africa, a joint program has been proposed, seeking to bring together local public and private stakeholders in a new collaborative platform initiative called Sepia [73]. ...
Citation: Stemerding, D.; Beumer, K.; Edelenbosch, R.; Swart, J.A.A.; de Vries, M.E.; ter Steeg, E.; Almekinders, C.J.M.; Lindhout, P.; van Dijk, L.C.M.; Struik, P.C. Responsible Innovation in Plant Breeding: The Case of Hybrid Potato Breeding. Plants 2023, 12, 1751.
Abstract: As an emerging innovation, hybrid potato breeding raises high expectations about faster variety development and clean true potato seed as a new source of planting material. Hybrid breeding could, therefore, substantially contribute to global food security and other major sustainable development goals. However, its success will not only depend on the performance of hybrid potato in the field, but also on a range of complex and dynamic system conditions. This article is based on a multidisciplinary project in which we have studied the innovation dynamics of hybrid potato breeding and explored how these dynamics may shape the future of hybrid potato. Inspired by the approach of responsible innovation, we closely involved key players in the Dutch and international potato sector and other relevant actors in thinking about these potato futures. An important and recurrent theme in our work is the tension between the predominant commercial innovation dynamics in plant breeding and promises to respond to the global challenges of food security, agrobiodiversity and climate change. In this article, we, therefore, discuss responsible innovation strategies in (hybrid) potato breeding, which may help to bridge this tension and finally reflect on the implications for the field of plant breeding in general.
... Hybrid seed is a type of seed which have potential of enhancing global food and nutritional security. Hybrid seed are produced by crossing of two homozygous parental lines of two different gene pools (Steeg et al. 2022). These are also known as F 1 seeds and have more yield, better uniformity and resistance to insect, pests as compared to their parents due to heterosis. ...
... Emasculation involves the manual removal of the male flower or reproductive part from the plant. Steeg et al. 2022 Emasculation is done in the crops where the female is not sterile. However, in case of the crops with a sterile female line, emasculation is performed. ...
Welcome to the world of plants and their fascinating transformations! This book is your guide to understanding how humans have played matchmaker with plants for generations, creating new and improved varieties through the art and science of plant breeding. Plants, just like people, have traits that make them unique. Some have tasty fruits, while others resist pests or thrive in specific environments. Plant breeding is like a gardening adventure where scientists and farmers team up to create the perfect plant companions. In these pages, we'll explore different methods of plant breeding, from the ancient art of selecting the best seeds to cutting-edge technologies like genetic engineering. You'll learn how scientists cross plants, tinker with their genes, and use other clever tricks to develop crops that are more productive, disease-resistant, and better suited to our ever-changing world. So, whether you're a budding botanist or just curious about the magic behind your favorite fruits and veggies, join us on this journey through the world of plant breeding. Get ready to discover the secrets of how humans have shaped the plants around us, creating a symphony of colors, flavors, and resilience that fills our gardens and plates. Happy reading and happy planting!
... These are related on one hand to the need to introgress traits, such as disease resistances and quality traits, from wild relatives, on the other hand to the desire to develop hybrid wheat varieties. When compared to traditionally inbred varieties, hybrid wheat is estimated to have a potential yield increase of 10% [19]. Furthermore, hybrid wheat, as with other hybrid crops, offers enhanced resilience and provides greater yield stability, even in the presence of environmental stresses and the extreme impacts of climate change. ...
... However, the breeding and widespread adoption of hybrid wheat is lagging behind. One reason is a costly production of hybrid seeds due to wheat's inherent tendency for self-pollination and the production of only one seed per flower [19]. Overcoming these barriers and establishing effective breeding techniques for large-scale hybrid wheat breeding requires substantial research and development efforts. ...
By using gene editing technologies such as CRISPR/Cas, precise modifications can be made in the genome. CRISPR/Cas is especially valuable for targeted mutagenesis in polyploids, as it can induce mutations of multiple alleles simultaneously, to obtain regenerants that are homozygous for the desired mutation. A range of gene-edited traits have been developed in hexaploid bread wheat, including various nutrition and health-related traits, plant architecture, pest and disease resistance, tolerance to abiotic stress, and traits that enable more efficient breeding. Wheat is also known as a cause of some human diseases, particularly coeliac disease (CD), with a prevalence of 1–2% of the population. In the EU alone, at least 4.5 million people suffer from it. CD is a chronic inflammation of the small intestine, induced and maintained in genetically predisposed individuals by the consumption of gluten proteins from wheat, barley and rye. As there is no cure, patients must follow a life-long gluten-free diet. The dominant epitopes in gluten proteins that trigger the disease, have been characterized, but they cannot be removed by classical breeding without affecting baking quality, as it concerns over 100 gluten genes that occur partly as blocks of genes in the genome of wheat. Using gene editing, two studies have shown that it is possible to modify the epitopes in several alpha- and gamma-gliadins simultaneously, while deleting some of the genes completely. In some lines more than 80% of the alpha-gliadin genes were modified. These proof-of-principle studies show that it is feasible to use gene editing, along with other breeding approaches, to completely remove the CD epitopes from bread wheat. Gene-edited coeliac-safe wheat will have economic, social and environmental impact on food security, nutrition and public health, but the realisation will (partially) depend on new European legislation for plants produced by gene editing.
... Since Shull first coined the term "heterosis" in 1914 (Shull 1914;1948), this phenomenon has been applied in plant breeding to develop hybrids that outperform their inbred parents. However, despite the success of hybrid breeding in many major crops with selfing/outcrossing mating systems, for example maize, sunflower, tomato, sugarbeet and oilseed rape (Steeg et al. 2022), the mechanisms driving heterosis have yet to be fully elucidated. Several factors contributing to heterosis Communicated by Yiqun Weng. ...
... Coding and noncoding features which were differentially expressed or methylated in this study provide new insights into early expression of heterosis in oilseed rape seeds and seedlings from a molecular viewpoint and constitute an extensive multiomics atlas for oilseed rape breeding. The extent of these features in an allopolyploid model crop like B. napus also have potential implications in other polyploid crops where heterosis still remains to be exploited, such as wheat and potatoes (Steeg et al. 2022). Patterns of expression and methylation dominance levels could also contribute a new level of understand regarding allele-specific gene expression (Fan et al. 2020;Sands et al. 2021), isoform expression (Vitting-Seerup and Sandelin 2019; Yao et al. 2020;Golicz et al. 2021), gene fusion and dosage (Mahmoud et al. 2019;Serin Harmanci et al. 2020;Bird et al. 2021b) as well as non-germline omics variations among F 1 plants and populations (Higgins et al. 2018;Cortijo et al. 2019;Orantes-Bonilla et al. 2022;Quezada-Martinez et al. 2022). ...
Key message
Transcriptomic and epigenomic profiling of gene expression and small RNAs during seed and seedling development reveals expression and methylation dominance levels with implications on early stage heterosis in oilseed rape.
Abstract
The enhanced performance of hybrids through heterosis remains a key aspect in plant breeding; however, the underlying mechanisms are still not fully elucidated. To investigate the potential role of transcriptomic and epigenomic patterns in early expression of hybrid vigor, we investigated gene expression, small RNA abundance and genome-wide methylation in hybrids from two distant Brassica napus ecotypes during seed and seedling developmental stages using next-generation sequencing. A total of 31117, 344, 36229 and 7399 differentially expressed genes, microRNAs, small interfering RNAs and differentially methylated regions were identified, respectively. Approximately 70% of the differentially expressed or methylated features displayed parental dominance levels where the hybrid followed the same patterns as the parents. Via gene ontology enrichment and microRNA-target association analyses during seed development, we found copies of reproductive, developmental and meiotic genes with transgressive and paternal dominance patterns. Interestingly, maternal dominance was more prominent in hypermethylated and downregulated features during seed formation, contrasting to the general maternal gamete demethylation reported during gametogenesis in angiosperms. Associations between methylation and gene expression allowed identification of putative epialleles with diverse pivotal biological functions during seed formation. Furthermore, most differentially methylated regions, differentially expressed siRNAs and transposable elements were in regions that flanked genes without differential expression. This suggests that differential expression and methylation of epigenomic features may help maintain expression of pivotal genes in a hybrid context. Differential expression and methylation patterns during seed formation in an F1 hybrid provide novel insights into genes and mechanisms with potential roles in early heterosis.
... Hybrid breeding is the technology of choice for plant improvement due to joint benefits to farmer and commercial stakeholders. Hybrid breeding provides farmers a uniform crop with superior performance across multiple traits while attracting commercial breeders with the incentive of intellectual property protection and a highly effective system for achieving long-term genetic gains [1]. Recently, Bradshaw described the theoretical background, with emphasis on quantitative genetics issues that drive decisions in a hybrid potato breeding program [2]. ...
... Male sterility is common in potato germplasm. In hybrid breeding, male sterility is used in many crops to facilitate open field seed production, resulting in lower production costs [1]. Depending on the exact route to a ware crop, discussed in Section 4 ( Figure 3), true seed production cost may be an important factor. ...
Research on diploid hybrid potato has made fast advances in recent years. In this review we give an overview of the most recent and relevant research outcomes. We define different components needed for a complete hybrid program: inbred line development, hybrid evaluation, cropping systems and variety registration. For each of these components the important research results are discussed and the outcomes and issues that merit further study are identified. We connect fundamental and applied research to application in a breeding program, based on the experiences at the breeding company Solynta. In the concluding remarks, we set hybrid breeding in a societal perspective, and we identify bottlenecks that need to be overcome to allow successful adoption of hybrid potato.
... In rye, hybrid breeding is a success story in terms of the selection of favorable alleles for grain yield. In sharp contrast to the competitiveness of hybrid compared to line breeding in wheat [17], the key parameter that triggered the transition to hybrid rye was the high added value of hybrid varieties compared to open-pollinating cultivars [18], sufficient to cover the costs of seed production. An increase in productive tillers is a main factor for the strong genetic gain in grain yield of hybrid rye. ...
... Hybrid rye breeding supports agricultural biodiversity for a resilient food and farming system with continuously improved hybrid cultivars, to the benefit of farmers, consumers, and the environment. Notably, hybrid breeding represents a technology with the potential to strengthen global food and nutrition security [17]. This cutting-edge technology it is not restricted to multinational breeding companies, as demonstrated by the hybrid rye breeding program of a medium-sized enterprise described in this review. ...
Rye is the only cross-pollinating small-grain cereal. The unique reproduction biology results in an exceptional complexity concerning genetic improvement of rye by breeding. Rye is a close relative of wheat and has a strong adaptation potential that refers to its mating system, making this overlooked cereal readily adjustable to a changing environment. Rye breeding addresses the emerging challenges of food security associated with climate change. The systematic identification, management, and use of its valuable natural diversity became a feasible option in outbreeding rye only following the establishment of hybrid breeding late in the 20th century. In this article, we review
the most recent technological advances to improve yield and yield stability in winter rye. Based on recently released reference genome sequences, SMART breeding approaches are described to counterbalance undesired linkage drag effects of major restorer genes on grain yield. We present the development of gibberellin-sensitive semidwarf hybrids as a novel plant breeding innovation based on an approach that is different from current methods of increasing productivity in rye and wheat. Breeding of new rye cultivars with improved performance and resilience is indispensable for a renaissance of this healthy minor cereal as a homogeneous commodity with cultural relevance in Europe that allows for comparatively smooth but substantial complementation of wheat with rye-based
diets, supporting the necessary restoration of the balance between human action and nature.
... Hybrid seed is a type of seed which have potential of enhancing global food and nutritional security. Hybrid seed are produced by crossing of two homozygous parental lines of two different gene pools (Steeg et al. 2022). These are also known as F 1 seeds and have more yield, better uniformity and resistance to insect, pests as compared to their parents due to heterosis. ...
Welcome to the world of plants and their fascinating transformations! This book is your guide to understanding how humans have played matchmaker with plants for generations, creating new and improved varieties through the art and science of plant breeding.
Plants, just like people, have traits that make them unique. Some have tasty fruits, while others resist pests or thrive in specific environments. Plant breeding is like a gardening adventure where scientists and farmers team up to create the perfect plant companions.
In these pages, we'll explore different methods of plant breeding, from the ancient art of selecting the best seeds to cutting-edge technologies like genetic engineering. You'll learn how scientists cross plants, tinker with their genes, and use other clever tricks to develop crops that are more productive, disease-resistant, and better suited to our ever-changing world.
So, whether you're a budding botanist or just curious about the magic behind your favorite fruits and veggies, join us on this journey through the world of plant breeding. Get ready to discover the secrets of how humans have shaped the plants around us, creating a symphony of colors, flavors, and resilience that fills our gardens and plates. Happy reading and happy planting!
... The male sterile lines used to produce hybrid seeds have an important impact on the development of modern agriculture. Male sterility technology has been successfully applied to major crops such as wheat (Ter Steeg et al., 2022), maize (Wu et al., 2016), and rice (Song et al., 2020) and has achieved great success (Zhang et al., 2011). ...
The male sterile line of soybean is crucial for hybrid seed production, and has allowed significant advancements in soybean germplasm innovation and yield increase in China. In this study, we created two bulks of sterile plants and collected a natural population consisting of 100 elite soybean germplasms. There were significant phenotypic differences between the sterile and natural populations resulting from flowers and pods. The sterile plants exhibited fleshy spherical pods and large black-green leaves in the maturity stage, while the leaves of the fertile plants fell off. After I2-KI staining, the pollen of the sterile line turned light brown and yellow, while the pollen of the fertile line turned black. On the basis of the SNP sequencing results, the sterility genes were located on eight chromosomes. Additionally, they were fine-mapped to 13 regions on six chromosomes using 72 pairs of SSR markers. Five genes involved in auxin response and pollen development were predicted as candidate genes underlying soybean sterility. These candidate genes for soybean sterility will help with gene cloning and functional analysis and accelerate the widespread use of hybrid seed production and yield increase in soybean grown in cool regions.
... Despite several efforts in recent decades, hybrid breeding programmes in wheat showed only limited success, and the interest of growers in these varieties has increased only marginally through time (Gupta et al., 2019;Ter Steeg et al., 2022). This is mainly due to the fact that the market success of hybrid wheat is influenced by the level of seed production costs and the profit from an increased grain yield compared to inbred lines for the farmers . ...
Hybrid breeding of wheat ( Triticum aestivum L.) is limited by its self‐pollinating nature. Past cross‐pollination improvements mostly focused on optimizing male traits. We tested the hybrid seed yield of 100 diverse elite lines treated with a chemical hybridization agent (CHA) and pollinated by non‐sterilized male plants in multi‐environmental field trials. Plant height and phenological traits of female plants were also assessed. In parallel, control experiments without CHA sterilization were conducted to measure per se yield of the tested material. Hybrid seed yield variation is of quantitative genetic nature, and, despite the large environmental influence, this trait has a strong genotypic component and is highly heritable ( h ² = .77). The lack of correlation between hybrid seed yield and per se yield suggests a non‐shared genetic control. Phenological traits and their interactions are important factors explaining together ~1/3 of hybrid seed yield variation. In contrast to plant height and flowering traits, which are influenced by major genetic factors, no significant marker–trait associations were found for the hybrid seed yield, thus suggesting a highly polygenic genetic architecture and the need of larger populations to investigate female hybrid seed yield.
... In speed breeding, the xenia effect can be used to speed up the breeding process by maximizing the genetic diversity of plants in a shorter amount of time. By cross-pollinating different varieties of a plant, breeders can produce offspring with unique genetic combinations that have desirable traits such as higher yields, disease resistance, and improved quality [71,78,79]. This can significantly reduce the time and resources required for traditional plant growth methods and facilitate the development of new crop varieties that can meet the ever-increasing needs for food worldwide. ...
After crosses, the identification of true hybrids is not only the most important step in the initiation of a breeding program but also plays a crucial role in the improvement of hybrid varieties. However, current morphological or molecular-based hybrid identification methods are time-consuming and costly approaches that require knowledge and skill, as well as specific lab equipment. In the current study, xenia, direct or immediate effect of pollen on seeds was used to identify true hybrids in the genus Pisum L. for the first time without growing F1 plants. The current study was therefore aimed to (i) elucidate the xenia effect on seeds in intra- and interspecific crosses between P. sativum L. subsp. sativum var. sativum or var. arvense L. Poir. and its wild relatives, including P. sativum subsp. elatius (M. Bieb.) Aschers & Graebn. and P. fulvum Sibth. & Sm., and (ii) illuminate the beneficialness of the xenia effect in a practical improvement of the genus Pisum L. The pea cultivars, including P. sativum subsp. sativum var. sativum and P. sativum subsp. sativum var. arvense, were therefore crossed with P. sativum subsp. elatius and P. fulvum, and the occurrence of the xenia effect was studied on the seeds of fertilized female plants immediately after the crosses. It was concluded that using the xenia effect for the early detection of true hybrid immediately after crossing was not only the fastest, most reliable, and least expensive option as early selection criteria, but that xenia also provided information about dominant seed and pod traits after double fertilization.
... However, it can be assumed that larger breeding companies worked at diploid level at that time, and today in their pre-breeding programmes. Fairly recent is attention to the development of hybrid potato varieties at diploid level Bethke et al. 2022;ter Steeg et al. 2022). ...
The Netherlands has a world-leading position in potato breeding, but little is known about the factors that led to this success. This paper analyses the factors that have influenced the development of potato breeding in the Netherlands. This study is based on research of the grey and scientific literature and interviews with various representatives from the Dutch potato breeding sector. We distinguish four periods: (i) Before 1888, no potato breeding in the Netherlands existed whereas in other countries first crosses occurred. (ii) 1888–1940, more individuals started breeding out of interest and hobby to overcome the commonly observed degeneration of potato. (iii) 1940–1967 the emergence of a corporate set up of breeding by private companies collaborating with small breeders. (iv) 1967–present, towards full-fledged breeding industry supported by the new Seeds and Planting Materials Act (ZPW) in 1967 including the breeders’ rights. Many factors including cultural practices, diseases, and market that determine the strategy of breeding have been analysed. The development is most of all ‘crop driven’ to maintain the level of production. But it was also ‘export driven’ leading to the development of an export-oriented seed potato sector. The conclusion is that three elements were dominant in the development of a strong potato breeding sector: (1) the broad cooperation among all players in the potato chain, (2) the design of the institutional infrastructure, and (3) the remuneration of the breeding work through legislation regarding plant breeders’ rights. The study ends with an outlook on future trends, one of them leading from an open to a more closed business culture.
... This debate may also influence the governance of potato genetic resources, which have traditionally been treated as a commons [67]. Of course, as pointed out by ter Steeg et al. [68], launching a hybrid breeding program for re-engineering a crop will necessitate a significant initial investment. ...
The genetic improvement of crops faces the significant challenge of feeding an ever-increasing population amidst a changing climate, and when governments are adopting a ‘more with less' approach to reduce input use. Plant breeding has the potential to contribute to the United Nations Agenda 2030 by addressing various sustainable development goals (SDGs), with its most profound impact expected on SDG2 Zero Hunger. To expedite the time-consuming crossbreeding process, a genomic-led approach for predicting breeding values, targeted mutagenesis through gene editing, high-throughput phenomics for trait evaluation, enviromics for including characterization of the testing environments, machine learning for effective management of large datasets, and speed breeding techniques promoting early flowering and seed production are being incorporated into the plant breeding toolbox. These advancements are poised to enhance genetic gains through selection in the cultigen pools of various crops. Consequently, these knowledge-based breeding methods are pursued for trait introgression, population improvement, and cultivar development. This article uses the potato crop as an example to showcase the progress being made in both genomic-led approaches and gene editing for accelerating the delivery of genetic gains through the utilization of genetically enhanced elite germplasm. It also further underscores that access to technological advances in plant breeding may be influenced by regulations and intellectual property rights.
... Maize (Zea mays L.) is an important food, feed, and fuel crop worldwide. Improving grain yield is a top priority in modern breeding [1]. Grain yield is determined by grain size, composed of three secondary traits: grain length, gain width and grain thickness [2]. ...
Background
The mechanism of grain development in elite maize breeding lines has not been fully elucidated. Grain length, grain width and grain weight are key components of maize grain yield. Previously, using the Chinese elite maize breeding line Chang7-2 and its large grain mutant tc19, we characterized the grain size developmental difference between Chang7-2 and tc19 and performed transcriptomic analysis.
Results
In this paper, using Chang7-2 and tc19, we performed comparative transcriptomic, proteomic and metabolomic analyses at different grain development stages. Through proteomics analyses, we found 2884, 505 and 126 differentially expressed proteins (DEPs) at 14, 21 and 28 days after pollination, respectively. Through metabolomics analysis, we identified 51, 32 and 36 differentially accumulated metabolites (DAMs) at 14, 21 and 28 days after pollination, respectively. Through multiomics comparative analysis, we showed that the phenylpropanoid pathways are influenced at transcriptomic, proteomic and metabolomic levels in all the three grain developmental stages.
Conclusion
We identified several genes in phenylpropanoid biosynthesis, which may be related to the large grain phenotype of tc19. In summary, our results provided new insights into maize grain development.
... To overcome this problem, the current study aimed to induce polyploidy in guava seedlings using colchicine. Polyploidy, the state of an individual plant having more than two sets of chromosomes, is often related to improved genotypes in terms of fruit size, pollen viability, self-compatibility, vegetative growth and stress tolerance (Vishwakarma et al. 2021;Ter Steeg et al. 2022). Polyploidy in P. guajava was reported to be uncommon but the genus Psidium itself is represented by di-, tetra-, hexa-and octoploid species (Itayguara and Forni-Martins 2006). ...
Polyploid plants are noticeably superior in horticultural traits than diploid plants and could be utilized in future breeding programs. Hence, this study aimed to develop polyploidy guava progenies with superior horticultural traits in terms of yield and quality of fruit. In the present study, ploidy manipulation was attempted in P. guajava cv. ‘Arka Kiran’ using colchicine with three different methods (method 1—seed treatment, method 2—seedling meristem treatment, and method 3—seed treatment followed by seedling meristem treatment). A higher concentration of colchicine treatment decreased the percent seed germination and seedling survival. This investigation resulted in the generation of nine mixaploids (nuclear DNA range between 300–450) and three tetraploids (450–600) with method 1, 11 mixaploids and one tetraploid with method 2, and 14 mixaploids and four tetraploids with exceptionally one hexaploid (nuclear DNA range more than 600) with method 3. Across the different methods, maximum polyploidy induction efficiency (42.22%) was observed with method 2 and minimum was observed with method 1 (26.67%). This is the first report of in situ polyploidization in guava, providing valuable novel plant material for further breeding programmes.
... However, we do not expect that large hybrid breeding programs with well-established genomic selection pipelines represent the main field of application for our method. A recent review on hybrid breeding from the perspective of commercial breeders has pointed out that breeding targets shift and change over time, and that older cultivars and selection candidates form an important secondary breeding pool that might contain useful variation for new traits of interest (Steeg et al., 2022). One potential application of the GB-SE based on parentage information might be to pre-screen older field trials without ad-hoc available genotypic data and pre-select interesting candidates for such novel breeding targets for future testing. ...
Testcross factorials in newly established hybrid breeding programs are often highly unbalanced, incomplete, and characterized by predominance of special combining ability (SCA) over general combining ability (GCA). This results in a low efficiency of GCA-based selection. Machine learning algorithms might improve prediction of hybrid performance in such testcross factorials, as they have been successfully applied to find complex underlying patterns in sparse data. Our objective was to compare the prediction accuracy of machine learning algorithms to that of GCA-based prediction and genomic best linear unbiased prediction (GBLUP) in six unbalanced incomplete factorials from hybrid breeding programs of rapeseed, wheat, and corn. We investigated a range of machine learning algorithms with three different types of predictor variables: (a) information on parentage of hybrids, (b) in addition hybrid performance of crosses of the parental lines with other crossing partners, and (c) genotypic marker data. In two highly incomplete and unbalanced factorials from rapeseed, in which the SCA variance contributed considerably to the genetic variance, stacked ensembles of gradient boosting machines based on parentage information outperformed GCA prediction. The stacked ensembles increased prediction accuracy from 0.39 to 0.45, and from 0.48 to 0.54 compared to GCA prediction. The prediction accuracy reached by stacked ensembles without marker data reached values comparable to those of GBLUP that requires marker data. We conclude that hybrid prediction with stacked ensembles of gradient boosting machines based on parentage information is a promising approach that is worth further investigations with other data sets in which SCA variance is high.
... Hybrid breeding is applied to around 50 of ca. 200 crop species with substantial agricultural importance (ter Steeg et al. 2022). For many crops, hybrids are often the first choice of farmers because of their uniformity and higher yields, which is reflected by a substantial market share of these varieties. ...
Key message
Heterosis is already manifested early in root development. Consistent with the dominance model of heterosis, gene expression complementation is a general mechanism that contributes to phenotypic heterosis in maize hybrids.
Abstract
Highly heterozygous F1-hybrids outperform their parental inbred lines, a phenomenon known as heterosis. Utilization of heterosis is of paramount agricultural importance and has been widely applied to increase yield in many crop cultivars. Plant roots display heterosis for many traits and are an important target for further crop improvement. To explain the molecular basis of heterosis, several genetic hypotheses have been proposed. In recent years, high-throughput gene expression profiling techniques have been applied to investigate hybrid vigor. Consistent with the classical genetic dominance model, gene expression complementation has been demonstrated to be a general mechanism to contribute to phenotypic heterosis in diverse maize hybrids. Functional classification of these genes supported the notion that gene expression complementation can dynamically promote hybrid vigor under fluctuating environmental conditions. Hybrids tend to respond differently to available nutrients in the soil. It was hypothesized that hybrid vigor is promoted through a higher nutrient use efficiency which is linked to an improved root system performance of hybrids in comparison to their inbred parents. Recently, the interaction between soil microbes and their plant host was added as further dimension to disentangle heterosis in the belowground part of plants. Soil microbes influenced the performance of maize hybrids as illustrated in comparisons of sterile soil and soil inhabited by beneficial microorganisms.
... The deployment of the hybrid system has generated a steep gain already at the end of 1980s in most crops, and in some cases it has also resulted in better strategies for seed production (Ter Steeg et al., 2022). Unfortunately, several crucial factors determine its commercial feasibility, and its deployment in self-pollinated species is still minor. ...
At the turn of 2000 many authors envisioned future plant breeding. Twenty years after, which of those authors’ visions became reality or not, and which ones may become so in the years to come. After two decades of debates, climate change is a “certainty,” food systems shifted from maximizing farm production to reducing environmental impact, and hopes placed into GMOs are mitigated by their low appreciation by consumers. We revise herein how plant breeding may raise or reduce genetic gains based on the breeder's equation. “Accuracy of Selection” has significantly improved by many experimental‐scale field and laboratory implements, but also by vulgarizing statistical models, and integrating DNA markers into selection. Pre‐breeding has really promoted the increase of useful “Genetic Variance.” Shortening “Recycling Time” has seen great progression, to the point that achieving a denominator equal to “1” is becoming a possibility. Maintaining high “Selection Intensity” remains the biggest challenge, since adding any technology results in a higher cost per progeny, despite the steady reduction in cost per datapoint. Furthermore, the concepts of variety and seed enterprise might change with the advent of cheaper genomic tools to monitor their use and the promotion of participatory or citizen science. The technological and societal changes influence the new generation of plant breeders, moving them further away from field work, emphasizing instead the use of genomic‐based selection methods relying on big data. We envisage what skills plant breeders of tomorrow might need to address challenges, and whether their time in the field may dwindle.
... Breeding for resilience to both biotic and abiotic stresses and quality-and consumer-driven traits will target distinct markets and create demand for hybrid cultivars (see Bethke et al., 2019). Additionally, the hybrid breeding system in combination with the high seed multiplication rates lends itself useful for the targeted development of cultivars for specific conditions, while taking into account user and consumer demands, to be made available globally for growers (de Vries et al., 2023;ter Steeg et al., 2022). ...
The introduction of hybrid breeding in potato (Solanum tuberosum L.) requires novel and efficient cropping systems for potato production based on true potato seed (TPS). Such systems address the limitations of conventional seed tuber–based systems, including low multiplication rates, high degeneration rates, and high costs of transport and storage. Of the possible cultivation pathways of TPS, we introduce and discuss the potential of field transplanting nursery‐raised potato seedlings as an alternative system for seed and ware production. This review discusses the current knowledge available on field transplanting of potato seedlings, the key factors that influence the success or failure of the system, and some of the prospective factors that will influence the wide introduction and utilization of field transplanting of potato seedlings in diverse farming systems. A field transplanting system will require the successful production of seedlings in the nursery, a successful establishment of transplanted seedlings in the field, and successful crop management to attain a productive seed or ware crop. The contribution of various factors in the various phases of the system to the success of the transplanted crop is also discussed. The introduction and utilization of the field transplanting system will be accelerated when hybrid breeding focuses on the introgression of traits of interest into high yielding cultivars and when agronomic studies focus on defining factors influencing productivity in distinct phases of the system.
... Whether this will work as efficiently with potato, with its many important quantitative traits, as with some vegetables, is still to be experienced. Hybrids in other crops have the additional advantage for the breeder of a 'biological protection' against the use of farm-saved seed, which is one of the Impact of hybrid potato 55 reasons why investments in breeding of crops like maize, tomato, cabbage and rice have grown tremendously (Ter Steeg et al., 2022). However, given the possibility to multiply potato through tubers, this advantage will be much smaller for crops for which such farm-saved seed cannot be restricted. ...
... The inbred lines allow breeders to better control genetics and this results in more efficient breeding. Many food crops have been adapted to hybrid breeding (Ter Steeg et al., 2022). Potato has been recalcitrant as the tetraploid nature of commercial potato cultivars and inbreeding depression have hampered the development of pure inbred lines. ...
... transition from the vegetative to the generative seed system allows for much faster multiplication, reducing the number of multiplication generations from up to eight to a maximum of two (Ter Steeg et al., 2022). Less land is required for seed multiplication, risks of crop losses and quality deterioration are reduced. ...
... When considering the enabling environment of the seed potato sector, there are four main topics: import regulation, variety registration and release, plant variety protection (PVP) and quality assurance (Ter Steeg et al., 2022b). Import regulations are focused on the management of phytosanitary risks. ...
... Hybrid seeds represent another option to explore the heterosis found in good combinations. They began to be employed at the beginning of the 20th century with the cultivation of maize and later with numerous other annual species (Bernardo 2020;Andorf et al., 2019;Steeg et al., 2022). The use of hybrid seeds has decisively contributed to increasing grain production across the world. ...
Background
This review aims to discuss alternatives for obtaining new clones of eucalyptus ( Eucalyptus spp.) with greater efficiency than current methods, as well as the feasibility of obtaining hybrid seeds. The commercial-scale use of eucalyptus hybrid seeds may reduce implantation costs and bring other advantages for forest exploitation. The proposal focuses on using inbred progenies (S 1, S 2...S g) to improve the species. Self-fertilization releases a greater proportion of undesirable alleles hidden in heterozygotes, increasing selection success.
Results
From the best individuals of the inbred progenies, full-sibling (FS) progenies will be developed, allowing the selection of new clones and conduct recurrent selection programs. The hybrid seeds must be obtained, also from the FS evaluations, in each selfing generation (S g x S g). Simultaneously with the conduction of inbreeding generations, information must be gathered aiming to implement the strategy for obtaining hybrid seeds, such as verifying the feasibility of generating double haploid lines and looking for alternatives to facilitate the large-scale production of hybrid seeds.
Conclusion
this review of research results can serve as an initial basis for obtaining new clones of eucalyptus with greater efficiency than current methods, as well as the feasibility of obtaining hybrid seeds.
Keywords:
forest breeding; quantitative genetics; heterosis; inbreeding depression; recurrent selection
... The most effective technique for increasing wheat productivity has shown to be genetic manipulation (Govindaraj et al., 2015). To take advantage of heterosis and develop viable recombinants, genetic diversity is critical in plant breeding (Ali et al., 2013;Ali et al., 2014ab;Ter Steeg et al., 2022). ...
Wheat is highly self-pollinated crop and main staple food of the world. Yield is one of the main breeding objectives in the wheat crop. Plant breeders are focusing on the development of new lines with increased yield, resistant against biotic and abiotic factors and having more nutritional values. The goal of the current research was to identify acceptable crossings for further investigation by determining the type of gene action (genetic effects) and combining ability of parental genotypes for morphological features. This approach was used to evaluate three lines, namely Ujala-2016, Johar-2016, Galaxy-2013 and four testers’ viz. XJ22, XJ23, XJ24 and XJ25. Collected data were subjected to line × tester analysis. Among parents as lines genotype Johar-2016 found good general combiner for studied traits. While among testers XJ25 proved to be the best general combiner for studied traits. Similarly, cross combinations hybrid XJ25 × Galaxy-2013 performed best as specific combiner. It was noticed that SCA variance was greater than GCA variance for all factors studied in wheat except for grains/spike. The superior genotypes and crosses can be further tested in yield for development of improved wheat varieties.
Simple Summary
Conventional potato breeding has produced only limited genetic gain due to the polyploid nature of the crop. In recent years, hybrid potato breeding at the diploid level has been developed to overcome this limited genetic gain. In diploid potato breeding, homozygous inbred lines are developed by self-fertilization, enabling incremental improvements of the material in each generation. This type of breeding requires self-fertility, which makes hybridization of inbred lines labor-cumbersome and results in hybrids that produce many undesirable berries in the field. In many crop species, cytoplasmic male sterility is used to produce maternal inbred lines that are male sterile. In this study, we explore the antherless cytoplasmic male sterility system in potato. We identify a recessive locus that is required for sterility and we show that this trait is expressed in Phureja cytoplasm but not in Andigena or Tuberosum cytoplasm. We implemented this system in hybrid seed production and show that the resulting hybrids set far fewer berries in the field than male fertile controls.
Abstract
Recent advances in diploid F1 hybrid potato breeding rely on the production of inbred lines using the S-locus inhibitor (Sli) gene. As a result of this method, female parent lines are self-fertile and require emasculation before hybrid seed production. The resulting F1 hybrids are self-fertile as well and produce many undesirable berries in the field. Utilization of cytoplasmic male sterility would eliminate the need for emasculation, resulting in more efficient hybrid seed production and male sterile F1 hybrids. We observed plants that completely lacked anthers in an F2 population derived from an interspecific cross between diploid S. tuberosum and S. microdontum. We studied the antherless trait to determine its suitability for use in hybrid potato breeding. We mapped the causal locus to the short arm of Chromosome 6, developed KASP markers for the antherless (al) locus and introduced it into lines with T and A cytoplasm. We found that antherless type male sterility is not expressed in T and A cytoplasm, proving that it is a form of CMS. We hybridized male sterile al/al plants with P cytoplasm with pollen from al/al plants with T and A cytoplasm and we show that the resulting hybrids set significantly fewer berries in the field. Here, we show that the antherless CMS system can be readily deployed in diploid F1 hybrid potato breeding to improve hybridization efficiency and reduce berry set in the field.
Hybridization plays an indispensable role in creating the diversity associated with plant evolution and genetic improvement of crops. Production of hybrids requires control of pollination and avoidance of self-pollination for species that are predominantly autogamous. Hand emasculation, male sterility genes or male gametocides have been used in several plant species to induce pollen sterility. However, in cowpea (Vigna unguiculata (L.) Walp), a self-pollinated cleistogamous dryland crop, only hand emasculation is used, but it is tedious and time-consuming. In this study, male sterility was effectively induced in cowpea and two dicotyledonous model species (Arabidopsis thaliana (L.) Heynh. and Nicotiana benthamiana Domin) using trifluoromethanesulfonamide (TFMSA). Pollen viability assays using Alexander staining showed that 30 ml of 1000 mg/l TFMSA with two-time treatments of one-week interval at the early stage of the reproductive phase under field or greenhouse conditions induced 99% pollen sterility in cowpea. TFMSA treatment induced non-functional pollen in diploid A. thaliana at two-time treatment of 10 ml of 125–250 mg/l per plant and N. benthamiana at two-time treatment of 10 ml of 250–1000 mg/l per plant. TFMSA-treated cowpea plants produced hybrid seeds when used as the female parent in crosses with non-treated plants used as male parents, suggesting that TFMSA had no effect on female functionality in cowpea. The ease of TFMSA treatment and its effectiveness to induce pollen sterility in a wide range of cowpea genotypes, and in the two model plant species tested in this study, may expand the scope of techniques for rapid pollination control in self-pollinated species, with potential applications in plant breeding and plant reproduction science.
In this article we reflect on the discussions as to whether breeding and seed system development should proceed along its current well established route of developing varieties with a higher agricultural productivity or if the diversity of farmers, their contexts and rationales requires broader approaches. We make use of data from a recently held survey (2018) in West Kenya. The data show that some 80% of the households in the survey planted both local and hybrid maize varieties. The choices that people make about which variety to plant are many. Apart from rainfall, the availability of cash, the promise of a good yield, the presence of projects and programs and the culture of seed also influences these choices. We argue that an inclusive demand-oriented maize breeding and seed system needs to include a range of varieties and seed sources and to develop and support different delivery pathways to fit farmers’ diverse use of seeds and varieties. Our findings also indicate the need for more systematic study of the diversity of farmers’ rationales and the performance of crop varieties. This would provide useful information for all the actors involved.
Societal Impact Statement
The widespread use of patents on plants and plant parts in low‐ and middle‐income countries demonstrates the increasing privatisation of crop genetic resources and potentially limits the use of these resources in farmer breeding, increasing the dependence of smallholder farmers on the private seed sector. Use of genetically modified traits in farmer breeding poses biosafety issues. Adaptation of patent legislation to the benefit of smallholder breeding and development of alternative seed sources from the public breeding sector could contain these negative impacts on farmer‐breeder efforts and ultimately on food and nutrition security.
Summary
This paper explores the potential impact of increased use of digital sequence information (DSI) through new breeding technologies (NBTs) and its associated patent and biosafety strategies and policies on smallholder agriculture and breeding in low‐ and middle‐income countries.
We performed a case study in the southern Philippines, involving multiple field visits and interviews, where smallholder farmers deliberately and successfully incorporated a genetically modified, patent‐protected trait into popular open‐pollinated varieties (OPVs) of both yellow and white maize, resulting in the wide‐spread dissemination of glyphosate‐tolerant open‐pollinated varieties (OPVs) called sige‐sige.
The particular case poses a suite of questions regarding farmer producer health, biosafety and access to plant genetic resources protected by patent rights. Considering current trends, it is predicted that the rise of NBTs such as gene editing will lead to more patents on breeding processes and on gene‐edited crop plants, including in low‐ and middle‐income countries and emerging economies. Since in many jurisdictions NBTs may be regulated as genetic engineering processes and products, biosafety regulations and related management requirements need to be considered as well.
By recognising and supporting the breeding efforts of farmers, governments can tap into the potential for creating a wider portfolio of varieties that are better adjusted to the needs of smallholder farmers. If the spread of GMOs in smallholder agriculture should be contained, alternative seed sources for farmer‐breeding from the public breeding sector will be highly needed.
Reinventing potato from a clonally propagated tetraploid into a seed-propagated diploid, hybrid potato, is an important innovation in agriculture. Due to deleterious mutations, it has remained a challenge to develop highly homozygous inbred lines, a prerequisite to breed hybrid potato. Here, we employed genome design to develop a generation of pure and fertile potato lines and thereby the uniform, vigorous F1s. The metrics we applied in genome design included the percentage of genome homozygosity and the number of deleterious mutations in the starting material, the number of segregation distortions in the S1 population, the haplotype information to infer the break of tight linkage between beneficial and deleterious alleles, and the genome complementarity of the parental lines. This study transforms potato breeding from a slow, non-accumulative mode into a fast-iterative one, thereby potentiating a broad spectrum of benefits to farmers and consumers.
The potential of big data to support businesses has been demonstrated in financial services, manufacturing, and telecommunications. Here, we report on efforts to enter a new data era in plant breeding by collecting genomic and phenotypic information from 12,858 wheat genotypes representing 6575 single-cross hybrids and 6283 inbred lines that were evaluated in six experimental series for yield in field trials encompassing ~125,000 plots. Integrating data resulted in twofold higher prediction ability compared with cases in which hybrid performance was predicted across individual experimental series. Our results suggest that combining data across breeding programs is a particularly appropriate strategy to exploit the potential of big data for predictive plant breeding. This paradigm shift can contribute to increasing yield and resilience, which is needed to feed the growing world population.
Background
Introgression of a quantitative trait locus (QTL) by successive backcrosses is used to improve elite lines (recurrent parent) by introducing alleles from exotic material (donor parent). In the absence of selection, the proportion of the donor genome decreases by half at each generation. However, since selection is for the donor allele at the QTL, elimination of the donor genome around that QTL will be much slower than in the rest of the genome (i.e. linkage drag). Using markers to monitor the genome around the QTL and in the genetic background can accelerate the return to the recurrent parent genome. Successful introgression of a locus depends partly on the occurrence of crossovers at favorable positions. However, the number of crossovers per generation is limited and their distribution along the genome is heterogeneous. Recently, techniques have been developed to modify these two recombination parameters.
Results
In this paper, we assess, by simulations in the context of Brassicaceae , the effect of increased recombination on the efficiency of introgression programs by studying the decrease in linkage drag and the recovery of the recurrent genome. The simulated selection schemes begin by two generations of foreground selection and continue with one or more generations of background selection. Our results show that, when the QTL is in a region that initially lacked crossovers, an increase in recombination rate can decrease linkage drag by nearly ten-fold after the foreground selection and improves the return to the recurrent parent. However, if the QTL is in a region that is already rich in crossovers, an increase in recombination rate is detrimental.
Conclusions
Depending on the recombination rate in the region targeted for introgression, increasing it can be beneficial or detrimental. Thus, the simulations analysed in this paper help us understand how an increase in recombination rate can be beneficial. They also highlight the best methods that can be used to increase recombination rate, depending on the situation.
Although hybrid crop varieties are among the most popular agricultural innovations, the rationale for hybrid crop breeding is sometimes misunderstood. Hybrid breeding is slower and more resource-intensive than inbred breeding, but it allows systematic improvement of a population by recurrent selection and exploitation of heterosis simultaneously. Inbred parental lines can identically reproduce both themselves and their F1 progeny indefinitely, whereas outbred lines cannot, so uniform outbred lines must be bred indirectly through their inbred parents to harness heterosis. Heterosis is an expected consequence of whole-genome non-additive effects at the population level over evolutionary time. Understanding heterosis from the perspective of molecular genetic mechanisms alone may be elusive, because heterosis is likely an emergent property of populations. Hybrid breeding is a process of recurrent population improvement to maximize hybrid performance. Hybrid breeding is not maximization of heterosis per se, nor testing random combinations of individuals to find an exceptional hybrid, nor using heterosis in place of population improvement. Though there are methods to harness heterosis other than hybrid breeding, such as use of open-pollinated varieties or clonal propagation, they are not currently suitable for all crops or production environments. The use of genomic selection can decrease cycle time and costs in hybrid breeding, particularly by rapidly establishing heterotic pools, reducing testcrossing, and limiting the loss of genetic variance. Open questions in optimal use of genomic selection in hybrid crop breeding programs remain, such as how to choose founders of heterotic pools, the importance of dominance effects in genomic prediction, the necessary frequency of updating the training set with phenotypic information, and how to maintain genetic variance and prevent fixation of deleterious alleles.
Transgressive segregation and heterosis are the reasons that plant breeding works. Molecular explanations for both phenomena have been suggested and play a contributing role. However, it is often overlooked by molecular genetic researchers that transgressive segregation and heterosis are most simply explained by dispersion of favorable alleles. Therefore, advances in molecular biology will deliver the most impact on plant breeding when integrated with sources of heritable trait variation – and this will be best achieved within a quantitative genetics framework. An example of the power of quantitative approaches is the implementation of genomic selection, which has recently revolutionized animal breeding. Genomic selection is now being applied to both hybrid and inbred crops and is likely to be the major source of improvement in plant breeding practice over the next decade. Breeders’ ability to efficiently apply genomic selection methodologies is due to recent technology advances in genotyping and sequencing. Furthermore, targeted integration of additional molecular data (such as gene expression, gene copy number and methylation status) into genomic prediction models may increase their performance. In this review, we discuss and contextualize a suite of established quantitative genetics themes relating to hybrid vigour, transgressive segregation and their central relevance to plant breeding, with the aim of informing crop researchers outside of the quantitative genetics discipline of their relevance and importance to crop improvement. Better understanding between molecular and quantitative disciplines will increase the potential for further improvements in plant breeding methodologies and so help underpin future food security.
Recent innovations in breeding technologies have reduced the timeframe to develop improved plant varieties compared to conventional breeding processes. Technologies like speed breeding, or rapid generation advancement, may also accelerate the process of statutory variety registration. Within this procedure, improved varieties are required to satisfy distinctness, uniformity and stability (DUS) criteria to establish the unique identity of a given submission during the variety registration process. The DUS standard also provides a solid basis for seed certification, plant breeders’ rights, as well as variety maintenance throughout commercial lifespan of varieties. Currently, the overall timeline of variety registration may vary from 2 to 4 years, depending on crop type and country. In this article, we propose the concept of ‘speed DUS testing’: a rapid phenotype-based method, which could be integrated with approaches that take advantage of DNA markers. We compare methods and discuss how DUS testing could be modernized to fast-track variety registration.
The goals of quantitative genetics differ according to its field of application. In plant breeding, the main focus of quantitative genetics is on identifying candidates with the best genotypic value for a target population of environments. Keeping quantitative genetics current requires keeping old concepts that remain useful, letting go of what has become archaic, and introducing new concepts and methods that support contemporary breeding. The core concept of continuous variation being due to multiple Mendelian loci remains unchanged. Because the entirety of germplasm available in a breeding program is not in Hardy–Weinberg equilibrium, classical concepts that assume random mating, such as the average effect of an allele and additive variance, need to be retired in plant breeding. Doing so is feasible because with molecular markers, mixed-model approaches that require minimal genetic assumptions can be used for best linear unbiased estimation (BLUE) and prediction. Plant breeding would benefit from borrowing approaches found useful in other disciplines. Examples include reliability as a new measure of the influence of genetic versus nongenetic effects, and operations research and simulation approaches for designing breeding programs. The genetic entities in such simulations should not be generic but should be represented by the pedigrees, marker data, and phenotypic data for the actual germplasm in a breeding program. Over the years, quantitative genetics in plant breeding has become increasingly empirical and computational and less grounded in theory. This trend will continue as the amount and types of data available in a breeding program increase.
Recently, a hybrid breeding system was developed for diploid potato. We compared performance of diploid hybrids with commercially available tetraploid cultivars. Therefore, seedling tubers were produced from true hybrid seeds in field conditions. In the subsequent year, diploid hybrids grown from seedling tubers showed a yield potential comparable with commercial tetraploid cultivars: the highest yielding diploid hybrids showed a yield comparable with the lower yielding tetraploid cultivars. Yields of hybrids and commercial tetraploid cultivars were broken down into different yield components and the interactions with growing conditions were quantified. The stability of yield and other relevant traits in different growing conditions was similar between hybrids and commercial cultivars. The contribution of the different yield components to total yield over different environments was compared between diploid hybrids and tetraploid cultivars. In diploid hybrids as well as tetraploid cultivars, more tubers per stem resulted in the highest yield gain, while an increase in tuber size resulted in a relatively smaller increase of total yield.
Potato is playing an increasingly important role in food production. The development of new varieties is slow due to the genetic complexity of potato and the inefficient breeding process. Modern techniques, such as hybrid breeding and the introduction of new traits in specific, existing elite material, have not been reported in the development of new and improved potato varieties. This paper describes the first example of marker-assisted introgression of four different Phytophthora infestans resistance genes in selected highly homozygous, diploid potato lines. After two backcrosses and one selfing, the original line can be recovered with Phytophthora resistance, thus providing added value. After crossing two diploid lines, each with a different resistance gene, hybrids were obtained and tested for resistance to Phytophthora in small field trials. In these experiments, the hybrids with two resistance genes were more resistant than the plants with only one of the two resistance genes.
Testing inbred lines for their combining ability is, due to high numbers of line to line testing needed for determination of hybrid performance, the most limiting factor in the F1 hybrid breeding procedure. We propose a novel method of F1 hybrid breeding that enables evaluation of large number of line to line crosses for their hybrid performance. Inbred lines (preferably doubled haploid - DH) are produced from heterozygous populations, genotyped and maintained. A group of lines is inter-pollinated randomly and their progeny examined. To identify elite F1 hybrids, these individual plants are selected by their superior phenotypic characteristics. Finally using paternity testing only of selected hybrids, the origin of paternal lines is revealed. To predict the number of F1 offspring needed in relation to the number of inbred lines being inter-pollinated, a mathematical formula was developed. For instance, using this formula for the inter-pollination of 60 distinct lines, the probability of obtaining all descendants of paternal-parent lines in a maternal-parent row represented at least once is achieved with 420 F1 plants in a row (p = 0.95). In a practical experiment with white cabbage, DH lines were produced using microspore culture; plants were grown to maturity and genotyped at eight polymorphic SSR loci. Two groups of lines (36 and 33 lines per group) were inter-pollinated by two methods, either using cage pollination with bumblebees or using open pollination in isolated field. A total of 9,858 F1 plants were planted and based on their phenotypic characteristics 213 were selected as elite phenotypes. 99 of them were genetically diverse and 5 of them were selected as super elite. Selected plants were analysed by the same SSR markers and the paternal origin of selected F1 plants was determined. Out of 213 selected elite plants 48 were reciprocals thus exhibiting power of selection based on single plant. We demonstrate that this new approach to hybrid development is efficient in white cabbage and we propose breeders to test it in various vegetable and crop species. Moreover, some other aspects of the proposed technique need to be tested and verified both for practical and economic criteria.
Key message
The review outlines past failures, present status and future prospects of hybrid wheat, and includes information on CMS/CHA/transgenic approaches for male sterility, heterotic groups and cost-effective hybrid seed production.
Abstract
Hybrid varieties give increased yield and improved grain quality in both cross- and self-pollinated crops. However, hybrid varieties in self-pollinated crops (particularly cereals) have not been very successful, except for hybrid rice in China. In case of hybrid wheat, despite the earlier failures, renewed efforts in recent years have been made and hybrid varieties with desirable attributes have been produced and marketed in some European countries. This review builds upon previous reviews, with a new outlook and improved knowledge base, not covered in earlier reviews. New technologies have been described, which include the Hordeum chilense-based CMS–fertility restorer system, chromosomal XYZ-4E-ms system and the following transgenic technologies: (1) conditional male sterility involving use of tapetum-specific expression of a gene that converts a pro-toxin into a phytotoxin causing male sterility; (2) barnase-barstar SeedLink system of Bayer CropScience; (3) split-barnase system that obviates the need of a barstar-based male restorer line; and (4) seed production technology of DuPont-Pioneer that makes use of transgenes in production of male-sterile lines, but gives hybrid seed with no transgenes. This review also includes a brief account of studies for discovery of heterotic QTL, genomic prediction of hybrid vigour and the development of heterotic groups/patterns and their importance in hybrid wheat production. The problem of high cost of hybrid seed due to required high seed rate in wheat relative to hybrid rice has also been addressed. The review concludes with a brief account of the current efforts and future possibilities in making hybrid wheat a commercial success.
The world cropping area for wheat exceeds that of any other crop, and high grain yields in intensive wheat cropping systems are essential for global food security. Breeding has raised yields dramatically in high-input production systems; however, selection under optimal growth conditions is widely believed to diminish the adaptive capacity of cultivars to less optimal cropping environments. Here, we demonstrate, in a large-scale study spanning five decades of wheat breeding progress in western Europe, where grain yields are among the highest worldwide, that breeding for high performance in fact enhances cultivar performance not only under optimal production conditions but also in production systems with reduced agrochemical inputs. New cultivars incrementally accumulated genetic variants conferring favourable effects on key yield parameters, disease resistance, nutrient use efficiency, photosynthetic efficiency and grain quality. Combining beneficial, genome-wide haplotypes could help breeders to more efficiently exploit available genetic variation, optimizing future yield potential in more sustainable production systems.
The present investigation was made to generate information on the heterotic pools amongst pearl millet hybrid parents. A set of 17 representative parents was selected from a diverse set of 147 hybrid parents using SSR based genetic distance (GD) and clustering pattern; 136 hybrids were developed in diallel fashion and evaluated at two locations in India. Moderate positive significant correlation (r = 0.37, p<0.01) and (r = 0.33, p<0.01) was found between GD and mid-parent heterosis (MPH) and better-parent heterosis (BPH), respectively, for grain yield for all the hybrids. Higher correlation between genetically closer individuals was observed for grain yield heterosis when the parents of B- and R- crosses had lesser genetic distance (<0.68 GD) in comparison to those parental combinations having GD higher than 0.68, indicating that the GD based predictions for grain yield are better when the parents are genetically related than when they are genetically diverse. In this study, all the pearl millet hybrid parents seems to exist in two broad-based heterotic pools; one each represented by seed and restorer parents as B × R hybrids showed highest mean heterosis for grain yield than either of B × B or R × R crosses. Further, four heterotic pools have been identified in this diverse set of hybrid parents of pearl millet, two each for seed parents (HPB1 and HPB2) and for restorer parents (HPR3 and HPR4). Among these, HPB1 × HPR3 was identified having the highest heterotic level, and could be further used to develop higher yielding pearl millet hybrids.
Societal Impact Statement
Plant breeding is crucial for improving agricultural crops for human use. However, an urgent rethink is needed to ensure the next generation of plant breeders have the necessary breadth of skills to provide ever more efficient, nutritious, profitable, and environmentally sustainable crops. Plant breeding is a multifaceted endeavor, which intersects with many other disciplines and professions. To help ensure that future plant breeding efforts are sustainable and relevant to the needs of society, it is vital that the interdisciplinary nature of the plant breeding profession is adequately reflected in student training and development.
Summary
Breeders need to have many faces to understand not only genetics but also environmental, social, and economic factors that are relevant for maintaining or improving crops for human use. In the United States, there is a long history of public involvement in agriculture and plant breeding. However, recent changes in the social systems underpinning public agriculture (i.e., funding structure) necessitate a rethinking of how agriculture education, specifically plant breeding education, should be facilitated. To provide viable plant breeding programs, it is necessary to explicitly acknowledge that breeding has been an interdisciplinary, long‐standing public endeavor to increase food system stability. Acknowledging this complexity has important pedagogical implications: the core of plant breeding resides in genetics, but the changing nature of this profession requires breeders to embrace a much broader training. Here, we suggest specific curricular objectives for plant breeders.
Cabbage hybrids, which clearly present heterosis vigor, are widely used in agricultural production. We compared two S5 haplotype (Class II) cabbage inbred-lines 87–534 and 94–182: the former is highly SC while the latter is highly SI; sequence analysis of SI-related genes including SCR, SRK, ARC1, THL1, and MLPK indicates the some SNPs in ARC1 and SRK of 87–534; semi-quantitative analysis indicated that the SI-related genes were transcribed normally from DNA to mRNA. To unravel the genetic basis of SC, we performed whole-genome mapping of the quantitative trait loci (QTLs) governing self-compatibility using an F2 population derived from 87–534 × 96–100. Eight QTLs were detected, and high contribution rates (CRs) were observed for three QTLs: qSC7.2 (54.8%), qSC9.1 (14.1%) and qSC5.1 (11.2%). 06–88 (CB201 × 96–100) yielded an excellent hybrid. However, F1 seeds cannot be produced at the anthesis stage because the parents share the same S-haplotype (S57, class I). To overcome crossing incompatibility, we performed rapid introgression of the self-compatibility trait from 87–534 to 96–100 using two self-compatibility-QTL-specific markers, BoID0709 and BoID0992, as well as 36 genome-wide markers that were evenly distributed along nine chromosomes for background analysis in recurrent back-crossing (BC). The transfer process showed that the proportion of recurrent parent genome (PRPG) in BC4F1 was greater than 94%, and the ratio of individual SC plants in BC4F1 reached 100%. The newly created line, which was designated SC96–100 and exhibited both agronomic traits that were similar to those of 96–100 and a compatibility index (CI) greater than 5.0, was successfully used in the production of the commercial hybrid 06–88. The study herein provides new insight into the genetic basis of self-compatibility in cabbage and facilitates cabbage breeding using SC lines in the male-sterile (MS) system.
Maize has for many decades been both one of the most important crops worldwide and one of the primary genetic model organisms. More recently, maize breeding has been impacted by rapid technological advances in sequencing and genotyping technology, transformation including genome editing, doubled haploid technology, parallelled by progress in data sciences and the development of novel breeding approaches utilizing genomic information. Herein, we report on past, current and future developments relevant for maize breeding with regard to (1) genome analysis, (2) germplasm diversity characterization and utilization, (3) manipulation of genetic diversity by transformation and genome editing, (4) inbred line development and hybrid seed production, (5) understanding and prediction of hybrid performance, (6) breeding methodology and (7) synthesis of opportunities and challenges for future maize breeding.
How the growing world population can feed itself is a crucial, multi-dimensional problem that goes beyond sustainable development. Crop production will be affected by many changes in its climatic, agronomic, economic, and societal contexts. Therefore, breeders are challenged to produce cultivars that strengthen both ecological and societal resilience by striving for six international sustainability targets: food security, safety and quality; food and seed sovereignty; social justice; agrobiodiversity; ecosystem services; and climate robustness. Against this background, we review the state of the art in plant breeding by distinguishing four paradigmatic orientations that currently co-exist: community-based breeding, ecosystem-based breeding, trait-based breeding, and corporate-based breeding, analyzing differences among these orientations. Our main findings are: (1) all four orientations have significant value but none alone will achieve all six sustainability targets; (2) therefore, an overarching approach is needed: “systems-based breeding,” an orientation with the potential to synergize the strengths of the ways of thinking in the current paradigmatic orientations; (3) achieving that requires specific knowledge development and integration, a multitude of suitable breeding strategies and tools, and entrepreneurship, but also a change in attitude based on corporate responsibility, circular economy and true-cost accounting, and fair and green policies. We conclude that systems-based breeding can create strong interactions between all system components. While seeds are part of the common good and the basis of agrobiodiversity, a diversity in breeding approaches, based on different entrepreneurial approaches, can also be considered part of the required agrobiodiversity. To enable systems-based breeding to play a major role in creating sustainable agriculture, a shared sense of urgency is needed to realize the required changes in breeding approaches, institutions, regulations and protocols. Based on this concept of systems-based breeding, there are opportunities for breeders to play an active role in the development of an ecologically and societally resilient, sustainable agriculture.
Efficient hybrid wheat breeding requires the redesign of the wheat floral architecure to enhance cross-pollination. Several studies evaluated the phenotypic variation and the genetic architecture of male floral traits, but their contribution to the most important trait, hybrid seed set on the female parent, has not yet been considered. To bridge this gap, we employed 31 male lines and evaluated the hybrid seed set on two female tester lines in crossing blocks. Hybrid seed set showed large genetic variance and high heritability, which demonstrates the potential for the improvement of this trait. However, the assessment of hybrid seed set is difficult as secondary traits like plant height and especially flowering time, as well as the environment largely influence the hybrid seed set. Nevertheless, a moderately high correlation between visual anther extrusion and hybrid seed set opens up the possibility to use visual anther extrusion as an indirect trait for preliminary male screenings. Further research evaluating traits influencing female receptivity coupled with genomics-assisted approaches are highly recommended to develop an improved selection portfolio for maximizing hybrid seed set.
Most flowering plants are hermaphroditic, yet the proportion of seeds fertilized by self and outcross pollen varies widely among species, ranging from predominant self-fertilization to exclusive outcrossing. A population's rate of outcrossing has important evolutionary outcomes as it influences genetic structure, effective population size, and offspring fitness. Because most mating system studies have quantified outcrossing rates for just one or two populations, past reviews of mating system diversity have not been able to characterize the extent of variation among populations. Here we present a new database of more than 30 years of mating system studies that report outcrossing rates for three or more populations per species. This survey, which includes 741 populations from 105 species, illustrates substantial and prevalent among-population variation in the mating system. Intermediate outcrossing rates (mixed mating) are common; 63% of species had at least one mixed mating population. The variance among populations and within species was not significantly correlated with pollination mode or phylogeny. Our review underscores the need for studies exploring variation in the relative influence of ecological and genetic factors on the mating system, and how this varies among populations. We conclude that estimates of outcrossing rates from single populations are often highly unreliable indicators of the mating system of an entire species.
Improved maize seed is instrumental to deliver an Asian-style 'green revolution' for Africa. The paper reviews and makes a comparative analysis of the maize (corn) seed sector and its evolution in Kenya, Tanzania, Uganda and Ethiopia drawing from seed sector surveys and secondary data. Enhancing farmers' access to and use of new maize varieties still presents a number of challenges in eastern Africa - not least due to a number of policy and institutional impediments to the development of the seed sector. The regional seed sectors also show some remarkable contrasts: they have evolved at different speeds and in different directions, driven by diverging agricultural growth opportunities and varying degrees of regulation, liberalization and restructuring. The paper reiterates calls for an enabling environment for private seed companies to evolve in order to serve the diverse farmer communities so that they benefit from existing and future improved maize seed opportunities.
Increasing wheat yield is a key global challenge to producing sufficient food for a growing human population. Wheat grain yield can be boosted by exploiting heterosis, the superior performance of hybrids compared with midparents. Here we present a tailored quantitative genetic framework to study the genetic basis of midparent heterosis in hybrid populations derived from crosses among diverse parents. We applied this framework to an extensive data set assembled for winter wheat. Grain yield was assessed for 1 ,604 hybrids and their 135 parental elite breeding lines in 11 environments. The hybrids outperformed the midparents by 10% on average, representing approximately 15 years of breeding progress in wheat, thus further substantiating the remarkable potential of hybrid-wheat breeding. Genome-wide prediction and association mapping implemented through the
developed quantitative genetic framework showed that dominance effects played a less prominent role than epistatic effects in grain-yield heterosis in wheat.
The third most important food crop worldwide, potato (Solanum tuberosum L.) is a tetraploid outcrossing species propagated from tubers. Breeders have long been challenged by polyploidy, heterozygosity, and asexual reproduction. It has been assumed that tetraploidy is essential for high yield, that the creation of inbred potato is not feasible, and that propagation by seed tubers is ideal. In this paper, we question those assumptions and propose to convert potato into a diploid inbred line-based crop propagated by true seed. Although a conversion of this magnitude is unprecedented, the possible genetic gains from a breeding system based on inbred lines and the seed production benefits from a sexual propagation system are too large to ignore. We call on leaders of public and private organizations to come together to explore the feasibility of this radical and exciting new strategy in potato breeding. © Crop Science Society of America | 5585 Guilford Rd., Madison, WI 53711 USA All rights reserved.
Key message:
Global wheat genetic diversity can be used in a unified framework to support and accelerate hybrid breeding and the development of heterotic groups in wheat. Hybrid wheat breeding has great potential to increase the global wheat grain yield level particularly in view of the increasing abiotic and biotic stress challenges as well as variable climatic conditions. For the long-term success of hybrid wheat breeding and the maximum exploitation of heterosis, high-yielding heterotic patterns must be established. Here, we propose a unified framework for hybrid breeding and the establishment of heterotic groups in autogamous crops and exemplify it for hybrid wheat breeding in Germany. A key component is the establishment of genetic distance between heterotic groups and in this context, we assessed genetic diversity in a global collection of 1110 winter wheat varieties released during the past decades in 35 countries but with a focus on European origin. Our analyses revealed the absence of major population structure but nevertheless suggest genetically distinct subgroups with potential for hybrid wheat breeding. Taking our molecular results and additional phenotypic data together, we propose how global genetic diversity can be used to accelerate and support reciprocal recurrent selection for the development of genetically distinct heterotic groups in hybrid wheat breeding.
In plants, male sterility can be caused either by mitochondrial genes with coupled nuclear genes or by nuclear genes alone; the resulting conditions are known as cytoplasmic male sterility (CMS) and genic male sterility (GMS), respectively. CMS and GMS facilitate hybrid seed production for many crops and thus allow breeders to harness yield gains associated with hybrid vigor (heterosis). In CMS, layers of interaction between mitochondrial and nuclear genes control its male specificity, occurrence, and restoration of fertility. Environment-sensitive GMS (EGMS) mutants may involve epigenetic control by noncoding RNAs and can revert to fertility under different growth conditions, making them useful breeding materials in the hybrid seed industry. Here, we review recent research on CMS and EGMS systems in crops, summarize general models of male sterility and fertility restoration, and discuss the evolutionary significance of these reproductive systems. Expected final online publication date for the Annual Review of Plant Biology Volume 65 is April 29, 2014. Please see http://www.annualreviews.org/catalog/pubdates.aspx for revised estimates.
Heterosis is the increase in vigor that is observed in progenies of matings of diverse individuals from different species, isolated populations, or selected strains within species or populations. Heterosis has been of immense economic value in agriculture and has important implications regarding the fitness and fecundity of individuals in natural populations. Genetic models based on complementation of deleterious alleles, especially in the context of linkage and epistasis, are consistent with many observed manifestations of heterosis. The search for the genes and alleles that underlie heterosis, as well as for broader allele-independent, genomewide mechanisms, has encompassed many species and systems. Common themes across these studies indicate that sequence diversity is necessary but not sufficient to produce heterotic phenotypes, and that the molecular pathways that produce heterosis involve chromatin modification, transcriptional control, translation and protein processing, and interactions between and within developmental and biochemical pathways. Taken together, there are many and diverse molecular mechanisms that translate DNA into phenotype, and it is the combination of all these mechanisms across many genes that produce heterosis in complex traits.
The reconciliation between Mendelian inheritance of discrete traits and the genetically based correlation between relatives for quantitative traits was Fisher’s infinitesimal model of a large number of genetic variants, each with very small effects, whose causal effects could not be individually identified. The development of genome-wide genetic association studies (GWAS) raised the hope that it would be possible to identify single polymorphic variants with identifiable functional effects on complex traits. It soon became clear that, with larger and larger GWAS on more and more complex traits, most of the significant associations had such small effects, that identifying their individual functional effects was essentially hopeless. Polygenic risk scores that provide an overall estimate of the genetic propensity to a trait at the individual level have been developed using GWAS data. These provide useful identification of groups of individuals with substantially increased risks, which can lead to recommendations of medical treatments or behavioral modifications to reduce risks. However, each such claim will require extensive investigation to justify its practical application. The challenge now is to use limited genetic association studies to find individually identifiable variants of significant functional effect that can help to understand the molecular basis of complex diseases and traits, and so lead to improved disease prevention and treatment. This can best be achieved by 1) the study of rare variants, often chosen by careful candidate assessment, and 2) the careful choice of phenotypes, often extremes of a quantitative variable, or traits with relatively high heritability.
This book offers a detailed overview of both conventional and modern approaches to plant breeding. In 25 chapters, it explores various aspects of conventional and modern means of plant breeding, including: history, objective, activities, centres of origin, plant introduction, reproduction, incompatibility, sterility, biometrics, selection, hybridization, methods of breeding both self- and cross- pollinated crops, heterosis, synthetic varieties, induced mutations and polyploidy, distant hybridization, quality breeding, ideotype breeding, resistance breeding, breeding for stress resistance, G x E interactions, tissue culture, genetic engineering, molecular breeding, genomics, gene action and varietal release.
The book’s content addresses the needs of students worldwide. Modern methods like molecular breeding and genomics are dealt with extensively so as to provide a firm foundation and equip readers to read further advanced books.
Each chapter discusses the respective subject as comprehensively as possible, and includes a section on further reading at the end. Info-boxes highlight the latest advances, and care has been taken to include nearly all topics required under the curricula of MS programs. As such, the book provides a much-needed reference guide for MS students around the globe.
Rye is a robust and stress-tolerant cereal, grown on 4.4 million hectares, mainly in Northeastern Europe. Grain yields range, on average, from 2.0 to 5.8 mt ha⁻¹ on farm level depending on the country, but reached >10 mt ha⁻¹ in multi-locational official trials in Germany. Rye grain is used for bread making, distilling, homegrown feed and bioenergy production. Hybrid breeding has gained much attention caused by higher grain yields and a higher gain from selection compared to open-pollinated cultivars. Prerequisites are self-fertility, cytoplasmic-male sterility (CMS) with effective nuclear encoded genes to restore fertility (Rf) and distinct heterotic pools. Elaborated breeding plans are available. Commercial rye hybrids are crosses between a CMS single cross as seed parent and a restorer synthetic as pollinator. Molecular breeding was promoted in the last decade by the availability of PCR-based markers and the production of medium- to high-density single nucleotide polymorphism (SNP) assays. Markers are used for introgressing monogenic traits, developing landscapes of quantitative trait loci (QTL), and genomic selection. In the future, disease resistances to snow mold, stem rust, and Fusarium head blight, resilience to drought and heat stress, optimized feeding quality and yield improvement by broadening the genetic basis of hybrid breeding are important goals.
Sugar beet is a recent crop developed solely for the extraction of the sweetener, sucrose. Breeding and improvement of Beta vulgaris for sugar has a rich historical record. Sugar beet originated from fodder beet in the 1800s, and selection increased its sugar content from 4 to 6% then to over 18% today. Development of vegetable beets—namely table beet and leaf beet (chard)—predates the creation of sugar beet. Each of these likely shares a common ancestor in the wild relative B. vulgaris spp. maritima. Beets of all crop types share common disease pressures. Germplasm for breeding and improvement, mostly for disease resistance, is accessible from each of the crop types and wild relatives, as there are no barriers to sexual hybridization. All cultivated types are biennial, with a basic chromosome number of 9, and most new cultivars are diploid. The majority of sugar beets are hybrids, facilitated by a complex system of cytoplasmic male sterility (CMS). Hybrids are typically monogerm, which reduces the labor required for thinning. Genomics and molecular markers are rapidly improving our understanding of the genetic characters controlling sugar beet phenotypes, particularly with regard to bolting. Such understanding may allow an expansion of the range of sugar beet cultivation, and may help improve yield through earlier planting. Developing beets for new uses, as an energy resource, and for bio‐based industrial feedstocks, for instance, may further expand the range of beet production for human uses.
Estimates of inbreeding depression obtained from the literature were used to evaluate the association between inbreeding depression and the degree of self-fertilization in natural plant populations. Theoretical models predict that the magnitude of inbreeding depression will decrease with inbreeding as deleterious recessive alleles are expressed and purged through selection. If selection acts differentially among life history stages and deleterious effects are uncorrelated among stages, then the timing of inbreeding depression may also evolve with inbreeding. Estimates of cumulative inbreeding depression and stage-specific inbreeding depression (four stages: seed production of parent, germination, juvenile survival, and growth/reproduction) were compiled for 79 populations (using means of replicates, N = 62) comprising 54 species from 23 families of vascular plants. Where available, data on the mating system also were collected and used as a measure of inbreeding history. A significant negative correlation was found between cumulative inbreeding depression and the primary selfing rate for the combined sample of angiosperms (N = 35) and gymnosperms (N = 9); the correlation was significant for angiosperms but not gymnosperms examined separately. The average inbreeding depression in predominantly selfing species (δ = 0.23) was significantly less (43%) than that in predominantly outcrossing species (δ = 0.53). These results support the theoretical prediction that selfing reduces the magnitude of inbreeding depression. Most self-fertilizing species expressed the majority of their inbreeding depression late in the life cycle, at the stage of growth/reproduction (14 of 18 species), whereas outcrossing species expressed much of their inbreeding depression either early, at seed production (17 of 40 species), or late (19 species). For species with four life stages examined, selfing and outcrossing species differed in the magnitude of inbreeding depression at the stage of seed production (selfing δ = 0.05, N = 11; outcrossing δ = 0.32, N = 31), germination (selfing δ = 0.02, outcrossing δ = 0.12), and survival to reproduction (selfing δ = 0.04, outcrossing δ = 0.15), but not at growth and reproduction (selfing δ = 0.21, outcrossing δ = 0.27); inbreeding depression in selfers relative to outcrossers increased from early to late life stages. These results support the hypothesis that most early acting inbreeding depression is due to recessive lethals and can be purged through inbreeding, whereas much of the late-acting inbreeding depression is due to weakly deleterious mutations and is very difficult to purge, even under extreme inbreeding.
The amounts of inbreeding depression upon selfing and of heterosis upon outcrossing determine the strength of selection on the selfing rate in a population when this evolves polygenically by small steps. Genetic models are constructed which allow inbreeding depression to change with the mean selfing rate in a population by incorporating both mutation to recessive and partially dominant lethal and sublethal alleles at many loci and mutation in quantitative characters under stabilizing selection. The models help to explain observations of high inbreeding depression (> 50%) upon selfing in primarily outcrossing populations, as well as considerable heterosis upon outcrossing in primarily selfing populations. Predominant selfing and predominant outcrossing are found to be alternative stable states of the mating system in most plant populations. Which of these stable states a species approaches depends on the history of its population structure and the magnitude of effect of genes influencing the selfing rate.
This multiauthor book contains 10 chapters written by internationally renowned experts with extensive knowledge of their subjects. Following an introduction to the tomato crop and the industry, subsequent chapters cover genetics and breeding; developmental processes; crop growth and yield; fruit ripening and fruit quality; irrigation and fertilization; crop protection (control of weeds, nematodes, insect and mite pests, and diseases caused by fungi, bacteria and viruses) of field and greenhouse crops; production in the open field; greenhouse tomato production; and postharvest biology and handling. The book is intended for students and professionals working in horticulture.
The next generation of successful cultivars in many self-pollinated crops will be F1 hybrids developed through a more systematic exploitation of the phenomenon of heterosis. Longer term, marketing and varietal protection issues aside, the most successful cultivars in terms of superior genotypes in self-pollinated crops may be inbred lines. Presently, commercial exploitation of hybrid vigor is limited to a relatively small number of vegetables despite evidence that heterosis above high-parent exists within many species (Table 1). Commercial development is limited to those crops in which the added value of heterosis is sufficient to justify the cost of hybrid seed production and development. The formula for successful application of heterosis is: added value > added cost of seed production. Much research has focused on the second half of the equation, i.e. the development of efficient pollination mechanisms for hybrid seed production (George, 1985). The objective of this paper is to focus an examination of the first half of the equation, i.e. maximizing the value added, by examining the genetic information relevant in the development of F1 hybrid cultivars in self-pollinated species.
The methods of formal taxonomy have not been very satisfactory for the classification of cultivated plants. As a result, the people who deal with cultivated plants the most have developed their own informal and intuitive classifications based on experience as to what constitutes useful groupings. An attempt is made to provide a framework in which both systems can operate with a minimum of confusion. The structure of the total available gene pool is characterized by assigning taxa to primary, secondary and tertiary gene pools. At the infraspecific level, cultivars are grouped into races and subraces in an informal way without rigid rules for the use of terms.
The amounts of inbreeding depression upon selfing and of heterosis upon outcrossing determine the strength of selection on the selfing rate in a population when this evolves polygenically by small steps. Genetic models are constructed which allow inbreeding depression to change with the mean selfing rate in a population by incorporating both mutation to recessive and partially dominant lethal and sublethal alleles at many loci and mutation in quantitative characters under stabilizing selection. The models help to explain observations of high inbreeding depression (>50%) upon selfing in primarily outcrossing populations, as well as considerable heterosis upon outcrossing in primarily selfing populations. Predominant selfing and predominant outcrossing are found to be alternative stable states of the mating system in most plant populations. Which of these stable states a species approaches depends on the history of its population structure and the magnitude of effect of genes influencing the selfing rate.
Key message:
The predicted future yield potential of hybrids was competitive with lines in the near future, but on a long term the competitiveness of hybrids depends on a number of factors. The change from line to hybrid breeding in autogamous crops is a recent controversial discussion among scientists and breeders. Our objectives were to employ wheat as a model to: (1) deliver a theoretical framework for the comparison of the selection gain of hybrid versus line breeding; (2) elaborate key parameters affecting selection gain in this comparison; (3) and evaluate the potential to modify these parameters in applied breeding programs. We developed a prediction model for future yield potential in both breeding methods as the sum of the population mean and the expected selection gain. The expected selection gain was smaller in hybrid than in line breeding and depended strongly on the hybrid seed production costs and the genetic variance available in hybrid versus line breeding. Owing to heterosis, the predicted future yield potential of hybrids was competitive with lines in the near future. On a long term, however, the competitiveness of hybrid compared to line breeding is questionable and depends on a number of factors. However, market specifications and political reasons might justify the current high interest in hybrid wheat breeding.
Two-line hybrid rice technology based on photoperiod-sensitive genic male /thermosensitive genic male sterility (PGMS/TGMS) originated from Chinese scientists. The technology has already been used to study various topics ranging from the mechanism of fertility alteration to breeding technology. The system has the following components: (1) the rationale based on characteristic of fertility alteration as a basic content; (2) selection of P(T)GMS rice as a core content; (3) stock seed production and purity maintenance of core seed for P(T)GMS rice as an essential link; (4) spatial and temporal adjustment and irrigation with cold water as a basic link; and (5) anagement regulations for production and release of P(T)GMS rice. Since 1996, many japonica and indica P(T)GMS lines have been registered and more than 30 hybrids have been released for commercial production in China. The sown area under two-line hybrid rice was 2.67 million ha in 2002. In Chin, development of two-line hybrid rice has entered the fast phase.
Crop yields are a result of interactions between genetics, environment and management (G × E × M). As in the Netherlands differences between potential yield and actual farm yields (yield gaps) are relatively small, progress in genetic potential is essential to further increase farm yields. In this paper we study the progress in yields of newly released varieties in official Dutch variety trials as a proxy for the progress in genetic yield potential, i.e., yield under absence of water and nitrogen limitation and pests and diseases. The use of yields from variety trials as a proxy for genetic yield potential is justified as these are well managed and because water is hardly limiting under Dutch climate–soil conditions. We compared the genetic yield progress of winter wheat (Triticum aestivum L.), spring barley (Hordeum vulgare L.), ware and starch potato (Solanum tuberosum L.) and sugar beet (Beta vulgaris L.) over the past ca. 30 years (ca. 1980–2010) with the developments in on-farm yields over the same period. GenStat 14th edition was used to perform modified joint regression analyses (mjra) and residual (or restricted) maximum likelihood (reml) analyses of yields in order to separate year (i.e., climate and/or management) effects from variety effects. Genetic progress in yield has been linear with ca. 100 and 60 kg ha−1 year−1 (15% moisture), respectively, for winter wheat and spring barley, 580 kg ha−1 year−1 payment weight for starch potato, and, partly non-linear for sugar beet, i.e., 80–170 kg sugar ha−1 year−1 depending on resistance type of the varieties. We also analyzed significant year effects (corrected for genetic progress) for most crops in the variety trials, which point at an effect of climate (environment) and/or management in addition to the genetic effect. Farm yields of winter wheat, spring barley and starch potato have increased linearly over the last decades, with ca. 90, 70 and 320 kg ha−1 year−1 (in the same units as above). Increase in sugar yields on farms was concave (20–230 kg ha−1 year−1) and spectacular over the last 10 years. For ware potatoes the genetic yield increase was only 20 kg dry matter ha−1 year−1 and reliable farm statistics for dry matter yields are lacking. We conclude that for the main crops in the Netherlands genetic progress in yield potential of varieties newly released over the past three decades has been substantial and largely linear. Farm yields for these crops also continued to increase, at approximately the same rates, but could not always keep pace with the combined genetic and year effects (G × E × M) in variety trials, suggesting a widening yield gap.
The origin of U.S. Corn Belt corn (Zea mays L.), heterotic groups, and heterotic patterns becomes less obvious with more cycles of breeding. Heterosis is poorly understood; simple curiosity cries out for more information. I endeavor to shed light on the effect of adapted- ness and heterosis on U.S. Corn Belt corn. I relate pertinent hap- penings in the phenomenal increase in U.S. corn production. I briefly review the origins of Northern Flint and Southern Dent races of corn and two major, persistent open-pollinated cultivars; and how corn hybridization was preceded and eased by hybrid species of the horse (Equus spp.). I discuss heterotic groups and patterns. The objective of U.S. corn breeding has been to adapt a tropical crop to a tempe- rate climate. Adaptedness is important. Open-pollinated cultivars emphasized local adaptation, but some cultivars were more popular, widely grown, and better adapted over a broad geographic region. Hybrid seed corn companies grew larger by selling more widely adapted hybrids that favored germplasm from the more popular, widely grown, better adapted open-pollinated cultivars containing more genes for adaptedness. I examine morphological differences between inbred parents of a widely adapted hybrid. Relatively cons- tant percentage of heterosis of well-adapted hybrids over years is due to seasonal climate affecting hybrids and their parent inbreds in a like manner because of their selection for adaptedness. Adaptedness has been more important than heterosis in the U.S. corn yield and production increases. Adaptedness in analogous heterotic species hybrids of the genus Equus, where body size is female dominant, apparently discriminates for body size between mules and hinnies that have virtually identical genotypes—adaptedness determines superior- ity over and above heterosis.
Potato (Solanum tuberosum subsp. tuberosum) production has increased six-fold (per unit area) in the USA since the 1920s. Direct comparison of potato cultivars released during the past century can help us understand how potato breeding has contributed to these production improvements and to other important traits associated with marketing and utilization. Our objective was to study trends in potato genetic improvement during four subjective breeding periods (BP) (pre-1900 = BP I; 1930 to 1949 = BP II; 1950 to 1969 = BP III; 1970 to present = BP IV), and also to compare performance between and within the three major cultivar types (round-white, long, and red-skinned). In field trials conducted from 1990 to 1992, under best management practices (with scheduled irrigation) in Michigan, the greatest total yield potential was observed in several cultivars released during BP I and II. These cultivars also had late vine maturities. On average, BP II had the greatest marketable yield. Cultivars released in BP III had the lowest total yield, earliest vine maturity. highest scab resistance and must favorable tuber appearance. General trends over periods were for earlier maturity and improved tuber appearance. Round-white cultivars improved for chip-processing ability and dry matter content over breeding periods, while long types increased in percent marketable yield only in BP IV. No trends were observed for scab resistance. When cultivars were grouped according to tuber type, there were no differences in total yield; however, the long types had the lowest marketable yield and the red-skinned types had lowest dry matter content.