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Characteristics and origin of a new birch species, Betula murrayana, from southeastern Michigan
Abstract
A new taxon, Betula murrayana, is described from southeastern Michigan. This birch appears to be an oetoploid (2n = 112) derivative of an unreduced gamete of Purpus birch (B. × purpusii Schneid. = B. alleghaniensis Britt. × B. pumila L., 2n = 70) and a reduced gamete of yellow birch (B. alleghaniensis, 2n = 84). Betula murrayana has relatively uniform, good, large pollen grains and leaf stomata larger than its putative ancestors; this multistemmed plant has larger leaves and fruits than the hybrid.
... Hybridization and introgression phenomena have been recognized as evolutionary processes that facilitate and enhance the adaptation of plant species to changing environments (Abbott et al., 2013;Stebbins, 1959 (Barnes & Dancik, 1985). In our study, observed patterns of nuclear hybridization, thereby pointing towards a syngameon. ...
Arctic and subarctic ecosystems are rapidly transforming due to global warming, emphasizing the need to understand the genetic diversity and adaptive strategies of northern plant species for effective conservation. This study focuses on Betula glandulosa, a native North American tundra shrub known as dwarf birch, which demonstrates an apparent capacity to adapt to changing climate conditions. To address the taxonomic challenges associated with shrub birches and logistical complexities of sampling in the northernmost areas where species' ranges overlap, we adopted a multicriteria approach. Incorporating molecular data, ploidy level assessment and leaf morphology, we aimed to distinguish B. glandulosa individuals from other shrub birch species sampled. Our results revealed three distinct species and their hybrids within the 537 collected samples, suggesting the existence of a shrub birch syngameon, a reproductive network of interconnected species. Additionally, we identified two discrete genetic clusters within the core species, B. glandulosa, that likely correspond to two different glacial lineages. A comparison between the nuclear and chloroplast SNP data emphasizes a long history of gene exchange between different birch species and genetic clusters. Furthermore, our results highlight the significance of incorporating interfertile congeneric species in conservation strategies and underscores the need for a holistic approach to conservation in the context of climate change, considering the complex dynamics of species interactions. While further research will be needed to describe this shrub birches syngameon and its constituents, this study is a first step in recognizing its existence and disseminating awareness among ecologists and conservation practitioners. This biological phenomenon, which offers evolutionary flexibility and resilience beyond what its constituent species can achieve individually, may have significant ecological implications.
... Occasional or localized interspecific gene flow may give rise to hybrids or, further along through introgression, blur morphological species boundaries, which further complicates species delimitation (Tovar-Sańchez and Oyama, 2004;Bardy et al., 2011;Castillo-Mendoza et al., 2019;Beeler et al., 2020;Wehenkel et al., 2020). Some hybrids possess a combination of parental morphological characters and sometimes were regarded as species (Barnes and Dancik, 1985), while hybridization can also lead to such true species by establishing reproductive barriers, e.g., though polyploidization (Ferguson and Sang, 2001;Rieseberg, 2001). In plants, diploid and tetraploid close relatives usually have a strong reproductive isolation as triploids are mostly sterile, impeding interspecific gene flow (Levin, 1975;Fowler and Levin, 1984;Rieseberg, 2001;Husband and Sabara, 2004;Roccaforte et al., 2015). ...
Taxa are traditionally identified using morphological proxies for groups of evolutionarily isolated populations. These proxies are common characters deemed by taxonomists as significant. However, there is no general rule on which character or sets of characters are appropriate to circumscribe taxa, leading to discussions and uncertainty. Birch species are notoriously hard to identify due to strong morphological variability and factors such as hybridization and the existence of several ploidy levels. Here, we present evidence for an evolutionarily isolated line of birches from China that are not distinguishable by traditionally assumed taxon recognition proxies, such as fruit or leaf characters. We have discovered that some wild material in China and some cultivated in the Royal Botanic Gardens Edinburgh, formerly recognized as Betula luminifera, differ from other individuals by having a peeling bark and a lack of cambial fragrance. We use restriction site-associated DNA sequencing and flow cytometry to study the evolutionary status of the unidentified Betula samples to assess the extent of hybridization between the unidentified Betula samples and typical B. luminifera in natural populations. Molecular analyses show the unidentified Betula samples as a distinct lineage and reveal very little genetic admixture between the unidentified samples and B. luminifera. This may also be facilitated by the finding that B. luminifera is tetraploid, while the unidentified samples turned out to be diploid. We therefore conclude that the samples represent a yet unrecognized species, which is here described as Betula mcallisteri.
... Betula × purpusii, is a hybrid between B. alleghaniensis and B. pumila var. glandulifera (Dancik and Barnes 1972) and B. murrayana is a back-cross between B. × purpusii and B. alleghaniensis (Barnes and Dancik 1985). Natural hybrids of B. alleghaniensis and papyrifera were also described (Barnes et al. 1974). ...
Key message
This review for the first time gathers the current state of knowledge on the role of plant and microbial methyl salicylate (MeSA) signaling processes in forest ecosystems. It aims to establish a basis for the use of high-MeSA-emitting trees as a silvicultural tool aiming to enhance stability and resilience in managed temperate forests affected by climate change.
Abstract
Methyl salicylate (MeSA) is a volatile plant and microbial signaling compound involved in systemic acquired resistance (SAR) and defense against pests and microbial pathogens, and antagonists. MeSA emitted by plants is also believed to trigger SAR in neighboring plant individuals, thus contributing to the resilience of the entire plant community. In this review, we discuss volatile plant-to-plant communication processes with a special focus on MeSA and provide an overview about the occurrence of MeSA in fungi and other microbes. We summarize present findings on the role of MeSA in plants and particularly in birches ( Betula spp.) and discuss the potential use of MeSA and MeSA-emitting plants in agriculture and forestry. MeSA levels in plant tissues are adjusted by methylation of salicylic acid to MeSA and the reverse process of demethylation. Some plant species possess constitutively high MeSA levels and thus are suitable for experiments of admixture of high MeSA plants, e.g., birches of the subgenera Betulenta and Acuminata in plant communities such as mixed forests. Furthermore, knowledge of candidate genes and the molecular pathways underlying high MeSA emission is expected to offer a basis for altering MeSA levels and/or the selection of high MeSA mutants.
... Another polyploid in section Lentae, B. insignis (10×), an endemic species to central and south China, has B. lenta as a close relative of one of its diploid parents, a species from North America, suggesting possible past range overlap between the two species. The octoploid species B. murrayana has previously been suggested as a recent allopolyploid derivative from B. x purpusii, an intersubgenus hybrid between B. alleghaniensis (6×) and B. pumila (4×) (Barnes and Dancik, 1985), consistent with our read mapping that both B. murrayana and B. alleghaniensis have the highest proportion of loci mapped to from B. lenta. In addition, B. murrayana has a higher proportion of loci mapped to from B. occidentalis and B. populifolia, similar to the read mapping pattern of B. pumila (Fig. 4). ...
Numerous plant genera have a history including frequent hybridisation and polyploidisation (allopolyploidisation), which means that their phylogeny is a network of reticulate evolution that cannot be accurately depicted as a bifurcating tree with a single tip per species. The genus Betula, which contains many ecologically important tree species, is a case in point. We generated genome-wide sequence reads for 27 diploid and 36 polyploid Betula species or subspecies using restriction site associated DNA (RAD) sequences. These reads were assembled into contigs with a mean length of 675 bp. We reconstructed the evolutionary relationships among diploid Betula species using both supermatrix (concatenation) and species tree methods. We identified the closest diploid relatives of the polyploids according to the relative rates at which reads from polyploids mapped to contigs from different diploid species within a concatenated reference sequence. By mapping reads from allopolyploids to their different putative diploid relatives we assembled contigs from the putative sub-genomes of allopolyploid taxa. We used these to build new phylogenies that included allopolyploid sub-genomes as separate tips. This approach yielded a highly evidenced phylogenetic hypothesis for the genus Betula, including the complex reticulate origins of the majority of its polyploid taxa. Our phylogeny divides the genus into two well supported clades, which, interestingly, differ in their seed-wing morphology. We therefore propose to split Betula into two subgenera.
... The study of Li et al. (2005) found a similar result that B. lenta and B. lenta f. uber formed a clade with B. medwediewii. It has previously been suggested (Barnes and Dancik, 1985) that the octoploid species B. murrayana is a recent allopolyploid derivative from B. Â purpusii, an inter-subgenus hybrid between B. alleghaniensis (8x) and B. pumila (4x). We find it to form a clade with B. alleghaniensis in the ITS tree, supporting this species as one of its parents (Ashburner and McAllister, 2013). ...
Background and aims:
BetulaL. (birch) is a genus of approx. 60 species, subspecies or varieties with a wide distribution in the northern hemisphere, of ecological and economic importance. A new classification ofBetulahas recently been proposed based on morphological characters. This classification differs somewhat from previously published molecular phylogenies, which may be due to factors such as convergent evolution, hybridization, incomplete taxon sampling or misidentification of samples. While chromosome counts have been made for many species, few have had their genome size measured. The aim of this study is to produce a new phylogenetic and genome size analysis of the genus.
Methods:
Internal transcribed spacer (ITS) regions of nuclear ribosomal DNA were sequenced for 76Betulasamples verified by taxonomic experts, representing approx. 60 taxa, of which approx. 24 taxa have not been included in previous phylogenetic analyses. A further 49 samples from other collections were also sequenced, and 108 ITS sequences were downloaded from GenBank. Phylogenetic trees were built for these sequences. The genome sizes of 103 accessions representing nearly all described species were estimated using flow cytometry.
Key results:
As expected for a gene tree of a genus where hybridization and allopolyploidy occur, the ITS tree shows clustering, but not resolved monophyly, for the morphological subgenera recently proposed. Most sections show some clustering, but species of the dwarf sectionApterocaryonare unusually scattered.Betula corylifolia(subgenusNipponobetula) unexpectedly clusters with species of subgenusAspera Unexpected placements are also found forB. maximowicziana,B. bomiensis,B. nigraandB. grossa Biogeographical disjunctions were found withinBetulabetween Europe and North America, and also disjunctions between North-east and South-west Asia. The 2C-values forBetularanged from 0·88 to 5·33 pg, and polyploids are scattered widely throughout the ITS phylogeny. Species with large genomes tend to have narrow ranges.
Conclusions:
Betula grossamay have formed via allopolyploidization between parents in subgenusBetulaand subgenusAspera. Betula bomiensismay also be a wide allopolyploid.Betula corylifoliamay be a parental species of allopolyploids in the subsectionChinenses Placements ofB. maximowicziana,B. michauxiiandB. nigraneed further investigation. This analysis, in line with previous studies, suggests that sectionApterocaryonis not monophyletic and thus dwarfism has evolved repeatedly in different lineages ofBetula Polyploidization has occurred many times independently in the evolution ofBetula.
Natural hybridisation is now recognised as pervasive in its occurrence across the Tree of Life. Resurgent interest in natural hybridisation fuelled by developments in genomics has led to an improved understanding of the genetic factors that promote or prevent species cross‐mating. Despite this body of work overturning many widely held assumptions about the genetic barriers to hybridisation, it is still widely thought that ploidy differences between species will be an absolute barrier to hybridisation and introgression. Here, we revisit this assumption, reviewing findings from surveys of polyploidy and hybridisation in the wild. In a case study in the British flora, 203 hybrids representing 35% of hybrids with suitable data have formed via cross‐ploidy matings, while a wider literature search revealed 59 studies (56 in plants and 3 in animals) in which cross‐ploidy hybridisation has been confirmed with genetic data. These results show cross‐ploidy hybridisation is readily overlooked, and potentially common in some groups. General findings from these studies include strong directionality of hybridisation, with introgression usually towards the higher ploidy parent, and cross‐ploidy hybridisation being more likely to involve allopolyploids than autopolyploids. Evidence for adaptive introgression across a ploidy barrier and cases of cross‐ploidy hybrid speciation shows the potential for important evolutionary outcomes.
Despite their evolutionary relevance, multispecies networks or syngameons are rarely reported in the literature. Discovering how syngameons form and how they are maintained can give insight into processes such as adaptive radiations, island colonizations, and the creation of new hybrid lineages. Understanding these complex hybridization networks is even more pressing with anthropogenic climate change, as syngameons may have unique synergistic properties that will allow participating species to persist. The formation of a syngameon is not insurmountable, as several ways for a syngameon to form have been proposed, depending mostly on the magnitude and frequency of gene flow events, as well as the relatedness of its participants. Episodic hybridization with small amounts of introgression may keep syngameons stable and protect their participants from any detrimental effects of gene flow. As genomic sequencing becomes cheaper and more species are included in studies, the number of known syngameons is expected to increase. Syngameons must be considered in conservation efforts as the extinction of one participating species may have detrimental effects on the survival of all other species in the network.
Hybridization is an important and creative process in plant evolution, but the factors that are associated with hybridization have not been systematically determined. In this study, we first quantify hybridization frequency in the “Field Manual of Michigan Flora” and then test multiple ecological and physiological variables for an association with hybridization using multiple logistic regression and other methods.
From our data, more than 17% of species produce hybrids with other species in the wild. There was no systematic association between phylogeny and hybridization rate, but taxonomic order was a significant independent variable in the regression model and the majority of hybrids were restricted to a handful of taxonomic groups. Life history, life form, and habitat disturbance were all significant explanatory variables for hybridization likelihood. Nearly 97% of hybridizing taxa were perennial with fern allies and woody groups found to hybridize the most. The data also indicate that the rate of hybridization increased with greater ecological disturbance. Interestingly, hybridization rate was positively correlated with the number of herbarium collections and this is potentially a confounding variable in quantifying the true frequency of hybridization.
Multivariate analyses of Betula cordifolia Regel, B. populifolia Marsh., and B. × caerulea Blanchard in a mixed population in northern Maine show that the latter is a hybrid between the former two species. Hybrid status of B. × caerulea is further supported by the fact that two putative hybrid individuals have the same chromosome number as reports for the two parental species and that all three taxa have overlapping flowering times at this site. Studies of pollen germination and stainability and seed germination show that the putative hybrids are fertile. Betula papyrifera Marsh., which is also present in this mixed population and was included in the analyses, flowers earlier than B. cordifolia and B. populifolia, and it apparently does not hybridize with the latter species at this site. Characters that have not been used prior to this study and that are among the best discriminators between B. cordifolia, B. papyrifera, and B. populifolia are number and width of preformed staminate catkins and length and diameter of overwintering vegetative buds.
Almost all polyploids arise by way of unreduced gametes; other mechanisms occur, but are negligible.
The most widespread and common event is (2n+n) reproduction.
Triploids so produced frequently yield 4X plants on backcrossing and 6X plants on selfing.
Spontaneous polyploids appear repeatedly, but their fate depends on vigor and competitive ability.
The polyploids most frequently produced are probably strict autoploids, but they often lack competitive ability and may not become successfully established.
Wide crosses occur in nature but not very often; if the hybrid is vigorous, it may yield a stable, fertile polyploid in the second or later generations via unreduced gametes.
Wide crosses by way of (2n+n) reproduction are probably more common and successful than the above (6) pathway.
The most likely pathway to a successful polyploid in nature involves neither wide crosses nor strict autoploidy but crosses between races, ecotypes and cytotypes within a biological species.
The more or less autoploids so produced may generate genuine alloploids by interspecific hybridization at polyploid levels.
The classic explanation of alloploidyvia “chromosome doubling” of a sterile diploid interspecific hybrid is misleading in most cases, and this route is probably negligible in the evolution of polyploid systems.
The objective of the study was to determine genetic differentiation of populations of yellow birch (Betula alleghaniensis Britton) along ecological gradients in Michigan and the Appalachian Mountains. Seeds were collected from populations in each of six physiographic regions of Michigan and in the northern and southern Appalachian Mountains. Seedlings were grown in a common garden in southeastern Michigan. First-year height of progeny, seed weight, initial germination, and germination percentage for 141 yellow birch trees in 30 populations were determined and compared.Significant differences were found among the six Michigan regions for all characters. Strong south–north clinal trends of decreasing height, increasing seed weight, earlier initial germination, and increasing total germination were revealed. Comparisons between physiographic regions showed that tall seedlings were produced by relatively light, late-germinating seeds. Within each region, however, seedlings from earlier germinating seeds tended to exhibit greater height growth. Significant differences were found among populations within a given region for specific characters. In the Appalachian Mountains, southern populations had taller progeny and lighter and poorer germinating seeds than northern populations.Regressions relating each character to 13 site variables were computed. Variation in total height and initial germination were interpreted as the result of differences in the length of the growing season associated with changes in elevation, terrain type, and aspect. Variation in seed weight was associated with differences in the length of growing season and soil moisture regimes.
The objective of the study was to elucidate the patterns of phenology of growth initiation and cessation among diverse sources of yellow birch (Betula alleghaniensis Britton) and sweet birch (B. lenta L.) along latitudinal and elevational gradients in the Appalachian Mountains. Fruit was collected from parent trees of both species, encompassing a range of 9° of latitude and 3000 ft (914 m) of elevation, and was sown in the greenhouse. The resulting seedling populations were grown in a uniform garden located in southeastern Michigan, where they were analyzed for differences in leaf flushing and growth cessation associated with latitude and elevation of the seed source. Under the conditions of the experiment, northern populations flushed earlier than southern populations. The absence of differences in leaf flushing among populations from different elevations suggests that environmental factors in addition to temperature regulate the initiation of growth. Populations of high latitudes and high elevations ceased growth earlier than populations from low latitudes and low elevations, indicating that populations from shorter growing seasons adapted by ceasing growth earlier. Sixty-seven percent of the variation in the phenology of shoot growth among seedling populations of both species was associated with latitude and elevation of the seed source; the rest was attributed to local variations in topography and resulting microclimates.Observations of parent trees in situ and of half-sib seedling progeny in the uniform garden revealed that individuals of yellow birch exhibit an earlier phenology than individuals of sweet birch from the same field sites. This appears to be an important feature of niche differentiation and a function of the adaptation of yellow birch to colder climates.
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