High migration rates shape the postglacial history of amphi-Atlantic bryophytes

Parallel patterns of genetic diversity and structure in circumboreal species of the Sphagnum capillifolium complex

PREMISE

Shared geographical patterns of population genetic variation among related species is a powerful means to identify the historical events that drive diversification. The Sphagnum capillifolium complex is a group of closely related peat mosses within the Sphagnum subgenus Acutifolia and contains several circumboreal species whose ranges encompass both glaciated and unglaciated regions across the northern hemisphere. In this paper, we (1) inferred the phylogeny of subg. Acutifolia and (2) investigated patterns of population structure and genetic diversity among five circumboreal species within the S. capillifolium complex.

METHODS

We generated RAD sequencing data from most species of the subg. Acutifolia and samples from across the distribution ranges of circumboreal species within the S. capillifolium complex.

RESULTS

We resolved at least 14 phylogenetic clusters within the S. capillifolium complex. Five circumboreal species show some common patterns: One population system comprises plants in eastern North America and Europe, and another comprises plants in the Pacific Northwest or around the Beringian and Arctic regions. Alaska appears to be a hotspot for genetic admixture, genetic diversity, and sometimes endemic subclades.

CONCLUSIONS

Our results support the hypothesis that populations of five circumboreal species within the S. capillifolium complex survived in multiple refugia during the last glacial maximum. Long-distance dispersal out of refugia, population bottlenecks, and possible adaptations to conditions unique to each refugium could have contributed to current geographic patterns. These results indicate the important role of historical events in shaping the complex population structure of plants with broad distribution ranges.

Life-history characteristics and historical factors are important to explain regional variation in reproductive traits and genetic diversity in perennial mosses

BACKGROUND AND AIMS

Plants have evolved an unrivalled diversity of reproductive strategies, including variation in the degree of sexual vs. clonal reproduction. This variation has important effects on the dynamics and genetic structure of populations. We examined the association between large-scale variation in reproductive patterns and intraspecific genetic diversity in two moss species where sex is manifested in the dominant haploid generation and sex expression is irregular. We predicted that in regions with more frequent realized sexual reproduction, populations should display less skewed sex ratios, should more often express sex and should have higher genetic diversity than in regions with largely clonal reproduction.

METHODS

We assessed reproductive status and phenotypic sex in the dioicous long-lived Drepanocladus trifarius and D. turgescens, in 248 and 438 samples across two regions in Scandinavia with frequent or rare realized sexual reproduction, respectively. In subsets of the samples, we analysed genetic diversity using nuclear and plastid sequence information and identified sex with a sex-specific molecular marker in non-reproductive samples.

KEY RESULTS

Contrary to our predictions, sex ratios did not differ between regions; genetic diversity did not differ in D. trifarius and it was higher in the region with rare sexual reproduction in D. turgescens. Supporting our predictions, relatively more samples expressed sex in D. trifarius in the region with frequent sexual reproduction. Overall, samples were mostly female. The degree of sex expression and genetic diversity differed between sexes.

CONCLUSIONS

Sex expression levels, regional sex ratios and genetic diversity were not directly associated with the regional frequency of realized sexual reproduction, and relationships and variation patterns differed between species. We conclude that a combination of species-specific life histories, such as longevity, overall degree of successful sexual reproduction and recruitment, and historical factors are important to explain this variation. Our data on haploid-dominated plants significantly complement plant reproductive biology.

Demographic history and gene flow in the peatmosses Sphagnum recurvum and Sphagnum flexuosum (Bryophyta: Sphagnaceae)

Population size changes and gene flow are processes that can have significant impacts on evolution. The aim of this study was to investigate the relationship of geography to patterns of gene flow and population size changes in a pair of closely related Sphagnum (peatmoss) species: S. recurvum and S. flexuosum. Both species occur in eastern North America, and S. flexuosum also occurs in Europe. Genetic data from restriction-site-associated DNA sequencing (RAD-seq) were used in this study. Analyses of gene flow were accomplished using coalescent simulations of site frequency spectra (SFSs). Signatures of gene flow were confirmed by f 4 statistics. For S. flexuosum, genetic diversity of plants in glaciated areas appeared to be lower than that in unglaciated areas, suggesting that glaciation can have an impact on effective population sizes. There is asymmetric gene flow from eastern North America to Europe, suggesting that Europe might have been colonized by plants from eastern North America after the last glacial maximum. The rate of gene flow between S. flexuosum and S. recurvum is lower than that between geographically disjunct S. flexuosum populations. The rate of gene flow between species is higher among sympatric plants of the two species than between currently allopatric S. flexuosum populations. There was also gene flow from S. recurvum to the ancestor S. flexuosum on both continents which occurred through secondary contact. These results illustrate a complex history of interspecific gene flow between S. flexuosum and S. recurvum, which occurred in at least two phases: between ancestral populations after secondary contact and between currently sympatric plants.

Genetic Diversity and Population Structure in Bryophyte With Facultative Nannandry

Among plants, gender dimorphism occurs in about 10% of all angiosperms and more than 50% of all moss taxa, with dwarf males (DM) found exclusively in some unisexual mosses. In this study, we explore the role of male dwarfism as a reproductive strategy in the widespread acrocarpous moss Dicranum scoparium, which has facultative male dwarfism, having both dwarf males (DMs) and normal-sized males (NMs). We retrieved 119 SNP markers from transcriptomes which were used to genotype 403 samples from 11 sites at seven localities in southern Sweden. Our aims were to compare the genetic variability and genetic structure of sexually reproducing populations at different geographic levels (cushion, site, and locality) and compare in particular the relative contribution of females, dwarf males and normal-sized males to the observed genetic diversity. The numbers of DMs differed strongly between sites, but when present, they usually outnumbered both females and NMs. Low genetic differentiation was found at locality level. Genetic differentiation was strongest between cushions for females and NMs and within cushions for DMs indicating small scale structuring and sometimes inbreeding. NMs were more clonal than either DMs or females. Genetic diversity was similar between females and DMs, but lower for NMs. Two haplotypes were shared between females and DMs and one haplotype was shared between a DM and a NM. In conclusion, our results show that DMs and NMs play different roles in reproduction, inbreeding may occur at cushion level, but gene flow is high enough to prevent substantial genetic drift.

Do mosses really exhibit so large distribution ranges? Insights from the integrative taxonomic study of the Lewinskya affinis complex (Orthotrichaceae, Bryopsida)

The strikingly lower number of bryophyte species, and in particular of endemic species, and their larger distribution ranges in comparison with angiosperms, have traditionally been interpreted in terms of their low diversification rates associated with a high long-distance dispersal capacity. This hypothesis is tested here with Lewinskya affinis (≡ Orthotrichum affine), a moss species widely spread across Europe, North and East Africa, southwestern Asia, and western North America. We tested competing taxonomic hypotheses derived from separate and combined analyses of multilocus sequence data, morphological characters, and geographical distributions. The best hypothesis, selected by a Bayes factor molecular delimitation analysis, established that L. affinis is a complex of no less than seven distinct species, including L. affinis s.str., L. fastigiata and L. leptocarpa, which were previously reduced into synonymy with L. affinis, and four new species. Discriminant analyses indicated that each of the seven species within L. affinis s.l. can be morphologically identified with a minimal error rate. None of these species exhibit a trans-oceanic range, suggesting that the broad distributions typically exhibited by moss species largely result from a taxonomic artefact. The presence of three sibling western North American species on the one hand, and four Old World sibling species on the other, suggests that there is a tendency for within-continent diversification rather than recurrent dispersal following speciation. The faster rate of diversification as compared to intercontinental migration reported here is in sharp contrast with earlier views of bryophyte species with wide ranges and low speciation rates.

Dispersal and genetic differentiation of Syntrichia caninervis populations across different desert regions in China

The moss Syntrichia caninervis is widely distributed in cool temperate and cold deserts where environmental pressures create a dependence on asexual reproduction (fragment reproduction). However, when compared to sporophyte-producing mosses, there is a lack of evidence to support the capacity of drought-tolerant mosses that predominantly fragment and produce protonema to disperse over long distances. We used 20 microsatellite loci to study genetic variation and structure in six populations (five natural populations and one population from a regeneration site) in three contrasting and widely separated regions of China. The genetic diversity and expected heterozygosity were lower in populations from the Tengger Desert than in populations from the other regions. Using PCoA, UPGMA and Structure analysis, the genetic grouping divided the three regions into three distinct groups. This may indicate that in regions where S. caninervis reproduces predominantly asexually, propagules are spread mainly by short-distance dispersal. The genetic diversity of the population from the regeneration site in the Tengger Desert was slightly higher than that of the nearby, naturally occurring population, and included some input from the Pamir Plateau almost 2,300 km to the west, suggesting long-distance dispersal of S. caninervis propagules across the region. Predominantly asexually reproducing populations of S. caninervis are mainly dependent on short-distance dispersal. Long-distance dispersal of S. caninervis propagules across the region is difficult. Establishment of populations with dominant asexual reproduction will eventually result in genetic differentiation.

No borders during the post-glacial assembly of European bryophytes

Climatic fluctuations during the Last Glacial Maximum (LGM) exerted a profound influence on biodiversity patterns, but their impact on bryophytes, the second most diverse group of land plants, has been poorly documented. Approximate Bayesian computations based on coalescent simulations showed that the post-glacial assembly of European bryophytes involves a complex history from multiple sources. The contribution of allochthonous migrants was 95-100% of expanding populations in about half of the 15 investigated species, which is consistent with the globally balanced genetic diversities and extremely low divergence observed among biogeographical regions. Such a substantial contribution of allochthonous migrants in the post-glacial assembly of Europe is unparalleled in other plants and animals. The limited role of northern micro-refugia, which was unexpected based on bryophyte life-history traits, and of southern refugia, is consistent with recent palaeontological evidence that LGM climates in Eurasia were much colder and drier than what palaeoclimatic models predict.

Multiple factors influence population sex ratios in the Mojave Desert moss Syntrichia caninervis

PREMISE OF RESEARCH

Natural populations of many mosses appear highly female-biased based on the presence of reproductive structures. This bias could be caused by increased male mortality, lower male growth rate, or a higher threshold for achieving sexual maturity in males. Here we test these hypotheses using samples from two populations of the Mojave Desert moss Syntrichia caninervis.

METHODS

We used double-digest restriction-site associated DNA (RAD) sequencing to identify candidate sex-associated loci in a panel of sex-expressing plants. Next, we used putative sex-associated markers to identify the sex of individuals without sex structures.

KEY RESULTS

We found a 17:1 patch-level phenotypic female to male sex ratio in the higher elevation site (Wrightwood) and no sex expression at the low elevation site (Phelan). In contrast, on the basis of genetic data, we found a 2:1 female bias at the Wrightwood site and only females at the Phelan site. The relative area occupied by male and female genets was indistinguishable, but males were less genetically diverse.

CONCLUSIONS

Our data suggest that both male-biased mortality and sexual dimorphism in thresholds for sex expression could explain genetic and phenotypic sex ratio biases and that phenotypic sex expression alone over-estimates the extent of actual sex ratio bias present in these two populations of S. caninervis.