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Pollen fertility assessed with aceto-carmine staining on slides. a pollen grains of Dioscorea alata tetraploid (VU461) showing good fertility. b Pollen grains of D. nummularia male polyploidy showing poor stainability
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Controlled pollinations were conducted to develop Dioscorea alata L. hybrids resistant to anthracnose (Colletotrichum gloeosporioides (Penz.) Penz. & Sacc. Overall, 10,410 hand pollinations were carried out with Indian (IN) and Vanuatu (VU) clones in 53 different parental combinations, including diploids (2x) and tetraploids (4x) female and male pl...
Citations
... Fruit and seed setting rates in tetraploid and diploid varieties using lyophilized pollens were almost similar but we observed that they were lower than fresh pollens. Likewise, a previous study reported higher fruit and seed setting rates using fresh pollens under field conditions [48]. It is well known that a successful fruit and seed setting is highly dependent on the genotype, environment and requires a precise timing [49]. ...
Various biological constraints including erratic and asynchronous flowering between male and female plants hin- der successful hybrid development and genetic gains in greater yam breeding programs. Therefore, pollen storage has gained much attention to facilitate artificial pollinations and increase the genetic gains. This 4-year study aimed at developing a practical long-term pollen storage technique for the successful development of yam hybrids. Fresh pollens were collected from two Dioscorea alata males, then lyophilized (two lyophilization treat- ments were applied), followed by storage at room temperature (24°C–25°C) for 12 months. Moreover, the lyophi- lized and stored pollens were tested for viability by crossing with four female varieties. Our results showed that lyophilization is effective for achieving viable pollens after 12 months of storage. Treatment 1 (48 h drying) showed higher pollen germination and fertility rates than Treatment 2 (72 h drying). Although we observed a reduction in viability of lyophilized pollens after 12 months of storage, we generated hybrid seedlings with success rates from 12% to 21% compared to 21%–31% when using fresh pollens. Paternity testing based on molecular genotyping confirmed the hybrid status of the obtained seedlings, which grew well in a greenhouse. Lyophilization is a practical approach for a long-term storage of greater yam pollen samples. This protocol will positively impact yam breeding programs particularly in developing countries.
... Controlled crosses were also conducted between D. alata and D. nummularia (the closest related species) and vigorous hybrids were produced (Lebot et al., 2019). Unfortunately, these too showed oxidizing tuber flesh. ...
The Pacific Islands (Melanesia, Micronesia, Polynesia) were the last region in the world to be colonized by modern humans. In their migrations, they dispersed asexually propagated food plants that were first domesticated in Melanesia and exposed them to further selection in diverse insular environments. Later, staple food crops were introduced from South America. In our search for research on genetic resources characterization and breeding conducted between 1950 and 2022, we retrieved 338 references on 13 crop species. All studies were examined, and a review is presented here. Relatively few were on genetic improvement (92 references) compared to those on morphological diversity (246 references). The paradox is that the Pacific is considered a region exposed to the climatic crisis, perhaps more vulnerable than any other region in the world. This paradox is surprising when we consider that most people in the Pacific are growing staple food crops, for household use and/or for local markets, and occasionally for export. In some countries, it involves more than 75% of the population with a considerable contribution to gross domestic product (GDP). Plant breeding has not been considered a priority to mitigate against forthcoming changes in climate, which is likely to be detrimental to crops with a narrow genetic base. We suggest several possible reasons and that conventional plant breeding should be given high priority based on new approaches that include geographical distribution of allelic diversity.
... ; https://doi.org/10.1101/2023.03.12.532272 doi: bioRxiv preprint using lyophilized pollens were almost similar but we observed that they were lower than with fresh pollens. Likewise, a previous study reported higher fruit and seed setting rates using fresh pollens under field conditions [48]. It is well known that a successful fruit and seed setting is highly dependent on the genotype, environment and requires a precise timing [49]. ...
Background: Greater yam, Dioscorea alata L., is a significant food security crop in tropical areas. However, low genetic diversity and various biological constraints, including susceptibility to viruses, ploidy, erratic and low flowering intensity, and asynchronous flowering hinder successful hybrid development and genetic gains in greater yam breeding programs. Therefore, pollen storage has gained much attention to facilitate genetic material exchanges, artificial pollinations and to increase the genetic gains in breeding programs. This 4-year study aimed at developing a practical long-term pollen storage technique for the successful development of yam hybrids. Fresh pollens were collected from two D. alata males, then lyophilized (two lyophilization treatments were applied), followed by storage at room temperature (24-25 oC) for 12 months. Moreover, the lyophilized and stored pollens were tested for viability by crossing with four female varieties.
Results: Our results showed that lyophilization is effective for achieving viable pollens after 12 months of storage. Treatment 1 (48 h drying) showed higher pollen germination and fertility rates than Treatment 2 (72 h drying). Although we observed a reduction in viability of lyophilized pollens after 12 months of storage, we generated hybrid seedlings with success rates from 12 to 21% compared to 21-31% when using fresh pollens. Paternity testing based on molecular genotyping confirmed the hybrid status of the obtained seedlings, which grew well in a greenhouse.
Conclusions: The results signify the importance of pollen lyophilization for yam breeding programs.
... Hand pollination is used as an alternative solution; it is 2-3 times more efficient than natural pollination by insects [10,20]. Whether natural or artificial, the pollination success is associated with other factors such as pollen viability, stigma receptivity, cross compatibility, and the prevailing weather conditions [3,4,26]. ...
Hand pollination success rate is low in yam (Dioscorea spp.), due partly to suboptimal weather conditions. Thus, determining the most suitable time for pollination could improve the pollination success in yam breeding programs. We performed continuous hand pollination within flowering windows of D. rotundata and D. alata for two consecutive years to determine the most appropriate month, week, and hours of the day allowing maximum pollination success. In D. alata crossing block, we observed significant differences among crossing hours for pollination success (p = 0.003); morning hours (8-12 a.m.) being more conducive than afternoons (12-5 p.m.). No significant differences existed between crossing hours in D. rotun-data, though the midday seemed optimal. For both species, the time interval 11-12 a.m. was more appropriate for crossing while 4-5 p.m. was the poorest. However, in vitro pollen germination tests showed that midday pollen collection (12 noon-2 p.m.) had better results than both extremes, though there were strong genotypic effects on outcomes. Pollination success rates differed significantly among months for D. alata (p < 0.001) but not for D. rotundata (p > 0.05). Differences in pollination success existed across weeks within flowering windows of both D. alata (p < 0.001) and D. rotundata (p = 0.004). The seed production efficiency (SPE) had a similar trend as the pollination success rate. No clear pattern existed between the pollination time and the seed setting rate (SSR) or seed viability (SV), though their dynamics varied with weeks and months. This study provided an insight on the dynamics of pollination outcomes under the influence of pollination times and allows detecting months, weeks, and hours of the day when hybridization activities should be focused for better results.
... La resistencia genética es la estrategia en la que más se ha investigado, obteniéndose resultados promisorios con cruzamientos inter específicos, sin embargo, aún no se ha logrado una buena calidad de tubérculos (Lebot et al., 2019). El mejoramiento clásico se dificulta por la baja tasa de compactibilidad de la biología floral de la especie y entre especies, siendo importante abordar otras estrategias de mejoramiento (Mondo et al., 2020). ...
El ñame Dioscorea spp., es un cultivo de importancia económica en el Caribe colombiano, hace parte de la dieta alimenticia y del sustento para los productores. La antracnosis Colletotrichum gloeosporioides es una de las enfermedades más limitantes de la producción del ñame, reportándose en Colombia pérdidas entre el 50 y 100 %. Se evaluó la resistencia a la antracnosis en 84 accesiones de D. alata y 28 de D. rotundata, pertenecientes a la colección de ñame de la Universidad de Córdoba, a los 120, 150 y 180 días después de la siembra (DDS), empleando una escala de severidad de 0-6. Las accesiones fueron clasificadas por su resistencia, empleando el método de la suma de rangos del área bajo la curva de progreso ABCPE y el método de la severidad a los 120, 150 y 180 DDS. Se ajustó La severidad a los 180 DDS tuvo fuerte relación lineal r = 0,99 con la variable integral área bajo la curva de progreso de la enfermedad ABCPE y la tasa de progreso de la enfermedad TPE. La clasificación de la resistencia por el método de la suma de rangos fue la más eficiente identificando en D. alata 18 accesiones resistentes R, 24 moderadamente resistente MR, 26 moderadamente susceptible MS y 16 susceptibles S. En la especie D. rotundata se identificaron 5 accesiones R, 8 MR, 8 MS, 6 S. La colección de ñame de la Universidad de Córdoba tiene accesiones con resistencia a la antracnosis, siendo importante iniciar con estas accesiones trabajos de mejoramiento, para identificar sus características agronómicas.
... Anthracnose disease is characterized by leaf necrosis and shoot dieback, and can cause losses of over 80% of production [15][16][17][18] . Anthracnose disease affects greater yam more than other domesticated yams; moderate resistance to this disease is present, however, in greater yam landraces and breeder's lines 19,20 . High-quality genomic resources and tools can facilitate rapid breeding methods for greater yam improvement with huge potential to impact food and nutritional security, particularly in Africa. ...
The nutrient-rich tubers of the greater yam, Dioscorea alata L., provide food and income security for millions of people around the world. Despite its global importance, however, greater yam remains an orphan crop. Here, we address this resource gap by presenting a highly contiguous chromosome-scale genome assembly of D. alata combined with a dense genetic map derived from African breeding populations. The genome sequence reveals an ancient allotetraploidization in the Dioscorea lineage, followed by extensive genome-wide reorganization. Using the genomic tools, we find quantitative trait loci for resistance to anthracnose, a damaging fungal pathogen of yam, and several tuber quality traits. Genomic analysis of breeding lines reveals both extensive inbreeding as well as regions of extensive heterozygosity that may represent interspecific introgression during domestication. These tools and insights will enable yam breeders to unlock the potential of this staple crop and take full advantage of its adaptability to varied environments. While greater yam provides food and income security for millions of people around the world, there are limited genomic resources available. Here, the authors report a chromosome-scale assembly of the greater yam genome as well as quantitative trait loci associated with anthracnose resistance and tuber traits.
... Our results showed that male and female flowers of monoecious individuals were as fertile as the flowers of male and female individuals. Low pollen viability was often reported as a limitation to sexual reproduction in yam [43]. In addition to pollen viability, the timing and limited duration of opening of male flowers during the day is another factor that may limit the pollination process of D. rotundata yams [15,48]. ...
... Stigma receptivity averaged 40% in our study whatever the cultivar. Previous works also reported low stigma-receptivity rates within D. rotundata cultivars [29,43]. Our results showed that there was no particular male or female sterility problem in monoecious individuals compared to male or female individuals in the analyzed cultivars. ...
... However, a low tolerance to autogamy of 18% for fruit set and 11% for seed set was observed, showing that autogamy was not completely prevented. Autogamy has been observed in monoecious cross-pollinated plants such as cassava and taro [41][42][43]. The low rate of self-compatibility observed in D. rotundata suggests that there are prezygotic and/or postzygotic barriers directed against autogamy. ...
Cultivated yam (D. rotundata) is a staple tuber crop in West Africa whose sexual reproduction control remains largely unknown despite its importance for plant breeding programs. In this paper, we compared self-pollination, intracultivar cross-pollination and intercultivar cross-pollination in three monoecious cultivars (Amoula, Heapala and Yassi). Results showed that pollen viability (49%) and stigma receptivity (40%) were similar in monoecious and dioecious plants, suggesting that autogamy could occur in monoecious plants. However, fruit and seed sets were significantly lower after self-pollination compared to cross-pollination. Overall, autogamy reached 11% and pollen lability was almost zero (<1%). The low percentage of pollen grains germinating on the stigma (37%) and pollen tubes reaching the ovules (25%) after self-pollination partly explained the low seed set. Strong inbreeding depression was observed after self-pollination and almost all fruits and about 75% of the seeds resulting from self-pollination showed malformations. Seed germination was also 20 times lower after self-pollination compared to cross-pollination. Sexual reproduction remained low in D. rotundata even after cross-pollination as fruit and seed set did not exceed 18% and 13% respectively. Moreover, comparison between intracultivar cross-pollination and self-pollination revealed intravarietal genetic diversity inside the analyzed yam cultivars. Overall, our results showed that D. rotundata has a very low tolerance to autogamy in monoecious cultivars and has developed pre- and postzygotic mechanisms to limit selfing.
... Although some yam species cross naturally in controlled and wild environments 22 , efforts are necessary to understand the nature of crossing barriers and devise means that facilitate gene flow among yam species to achieve cross-breeding goals. Most reports associated the low cross-compatibility among yam species with differences in ploidy status 7,16,20,21 . However, the causes and extent of interspecific barriers in yam have been poorly investigated and documented. ...
... Since the genetic distance had no significant effect on yam cross-compatibility rates, it might be wise to cross genetically distant parents to exploit heterosis in yam breeding. Past studies supported that tuber yield is positively correlated with heterozygosity in yam 16,20,21 . It is, however, noteworthy that outbreeding over more considerable genetic distances could lead to a decline in the clonal component of fitness, as warned by Willi and Van Buskirk 28 . ...
Yam (Dioscorea spp.) is a staple crop for millions of people in the tropics and subtropics. Its genetic improvement through breeding is being challenged by pre-zygotic and post-zygotic cross-compatibility barriers within and among species. Studies dissecting hybridization barriers on yam for improving the crossability rates are limited. This study aimed to assess the cross-compatibility, which yielded fruit set, viable seeds and progeny plants in an extensive intraspecific and interspecific crossing combinations in a yam genetic improvement effort to understand the internal and exogenous factors influencing pollination success. Cross-compatability was analyzed at the individual genotype or family level using historical data from crossing blocks and seedling nurseries from 2010 to 2020 at the International Institute of Tropical Agriculture (IITA). The average crossability rate (ACR) was lower in interspecific crossing combinations (6.1%) than intraspecific ones (27.6%). The seed production efficiency (SPE) values were 1.1 and 9.3% for interspecific and intraspecific crosses, respectively. Weather conditions and pollinator's skills are the main contributors to the low success rate in the intraspecific cross combinations in yam breeding. At the same time, genetic distance and heterozygosity played little role. Interspecific cross barriers were both pre-zygotic and post-zygotic, resulting from the evolutionary divergence among the yam species. Dioscorea rotundata had higher interspecific cross-compatibility indices than D. alata. Distant parents produced intraspecific crossbred seeds with higher germination rates compared to closest parents (r = 0.21, p = 0.033). This work provided important insights into interspecific and intraspecific cross-compatibility in yam and suggested actions for improving hybridization practices in yam breeding programs.
... Genetic variability in progenies from interspecific crosses is always inevitable, although few of them are viable. With success achieved through interspecific hybridization between Dioscorea alata (water yam), and Dioscorea nummularia (yam), Lebot et al. (2019) encourage replication of this strategy in other orphan crop species and suggest further opportunities to introgress superior traits. On the one hand, ample genetic variation can be created while introgressing useful traits from one species into another. ...
Orphan crops are indigenous and invariably grown by small and marginal farmers under subsistence farming systems. These crops, which are common and widely accepted by local farmers, are highly rich in nutritional profile, good for medicinal purposes, and well adapted to suboptimal growing conditions. However, these crops have suffered neglect and abandonment from the scientific community because of very low or no investments in research and genetic improvement. A plausible reason for this is that these crops are not traded internationally at a rate comparable to that of the major food crops such as wheat, rice, and maize. Furthermore, marginal environments have poor soils and are characterized by extreme weather conditions such as heat, erratic rainfall, water deficit, and soil and water salinity, among others. With more frequent extreme climatic events and continued land degradation, orphan crops are beginning to receive renewed attention as alternative crops for dietary diversification in marginal environments and, by extension, across the globe. Increased awareness of good health is also a major contributor to the revived attention accorded to orphan crops. Thus, the introduction, evaluation, and adaptation of outstanding varieties of orphan crops for dietary diversification will contribute not only to sustained food production but also to improved nutrition in marginal environments. In this review article, the concept of orphan crops vis-à-vis marginality and food and nutritional security is defined for a few orphan crops. We also examined recent advances in research involving orphan crops and the potential of these crops for dietary diversification within the context of harsh marginal environments. Recent advances in genomics coupled with molecular breeding will play a pivotal role in improving the genetic potential of orphan crops and help in developing sustainable food systems. We concluded by presenting a potential roadmap to future research engagement and a policy framework with recommendations aimed at facilitating and enhancing the adoption and sustainable production of orphan crops under agriculturally marginal conditions.
... Genetic variability in progenies from interspecific crosses is always inevitable, although few of them are viable. With success achieved through interspecific hybridization between Dioscorea alata (water yam), and Dioscorea nummularia (yam), Lebot et al. (2019) encourage replication of this strategy in other orphan crop species and suggest further opportunities to introgress superior traits. On the one hand, ample genetic variation can be created while introgressing useful traits from one species into another. ...
Orphan crops are indigenous and invariably grown by small and marginal farmers under subsistence farming systems. These crops, which are common and widely accepted by local farmers, are highly rich in nutritional profile, good for medicinal purposes, and well adapted to suboptimal growing conditions. However, these crops have suffered neglect and abandonment from the scientific community because of very low or no investments in research and genetic improvement. A plausible reason for this is that these crops are not traded internationally at a rate comparable to that of the major food crops such as wheat, rice, and maize. Furthermore, marginal environments have poor soils and are characterized by extreme weather conditions such as heat, erratic rainfall, water deficit, and soil and water salinity, among others. With more frequent extreme climatic events and continued land degradation, orphan crops are beginning to receive renewed attention as alternative crops for dietary diversification in marginal environments and, by extension, across the globe. Increased awareness of good health is also a major contributor to the revived attention accorded to orphan crops. Thus, the introduction, evaluation, and adaptation of outstanding varieties of orphan crops for dietary diversification will contribute not only to sustained food production but also to improved nutrition in marginal environments. In this review article, the concept of orphan crops vis-à-vis marginality and food and nutritional security is defined for a few orphan crops. We also examined recent advances in research involving orphan crops and the potential of these crops for dietary diversification within the context of harsh marginal environments. Recent advances in genomics coupled with molecular breeding will play a pivotal role in improving the genetic potential of orphan crops and help in developing sustainable food systems. We concluded by presenting a potential roadmap to future research engagement and a policy framework with recommendations aimed at facilitating and enhancing the adoption and sustainable production of orphan crops under agriculturally marginal conditions.
