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A common pattern observed in temperate tree clades is that species are often morphologically distinct and partially interfertile but maintain species cohesion despite ongoing hybridization where ranges overlap. Although closely related species commonly occur in sympatry in tropical ecosystems, little is known about patterns of hybridization within a clade over time, and the implications of this hybridization for the maintenance of species boundaries. In this study, we focused on a clade of sympatric trees in the genus Diospyros in the Mascarene islands and investigated whether species are genetically distinct, whether they hybridize, and how patterns of hybridization are related to the time since divergence among species. We sampled multiple populations from each of 12 Mascarene Diospyros species, generated genome-wide single nucleotide polymorphism data using 2bRADseq, and conducted population genomic and phylogenomic analyses. We found that Mascarene Diospyros species diverged millions of years ago and are today largely genetically distinct from one another. Although hybridization was observed between closely related species belonging to the same subclade, more distantly related species showed little evidence of interspecific hybridization. Phylogenomic analyses also suggested that introgression has occurred during the evolutionary history of the clade. This suggests that, as diversification progressed, interspecific hybridization occurred among species, but became infrequent as lineages diverged from one another and evolved reproductive barriers. Species now coexist in partial sympatry, and experience limited hybridization between close relatives. Additional research is needed to better understand the role that introgression may have played in adaptation and diversification of Mascarene Diospyros, and its relevance for conservation., (© 2020 John Wiley & Sons Ltd.)

Increasing genetic diversity and maintaining evolutionary processes are primary goals of conservation translocations, which involve the intentional movement of an at-risk species to establish new populations or augment existing populations, with the ultimate goal of reversing declines. Much debate has focused on how to select source material for plant translocations, with early approaches focusing primarily on maintaining the genetic uniqueness of populations. However, recent strategies often advocate mixing population sources during translocation to increase genetic diversity and re-establish connectivity. Yet, despite hundreds of translocations programmes with at-risk plant species presently underway (e.g. Silcock et al., 2019), few studies have conducted thorough assessments of the effects of mixing population sources on both the genetic diversity and fitness of translocated populations. The study by Van Rossum et al. (2020) in this issue of Molecular Ecology uses detailed assessments of genetic parameters and fitness to understand the outcomes of mixing two genetically differentiated source populations in translocations of the rare, self-incompatible perennial herb, Arnica montana, whose populations are declining at low elevations in Western Europe. They examine genetic changes throughout the translocation process (source populations to F1 offspring) and demonstrate the maintenance of high genetic diversity in successive generations for all three translocations. Translocated populations exhibited high contemporary pollen flow, substantial admixture between source populations and low inbreeding in F1 offspring. Importantly, they found no evidence of outbreeding depression in F1 offspring. This work shows that genetically mixing source populations can result in optimal genetic outcomes in translocations of declining plant species and exemplifies how multigenerational genetic monitoring and fitness assessments can be used to evaluate the success of experimental translocations., (© 2020 John Wiley & Sons Ltd.)

Life history theory predicts the evolution of trait combinations that enhance fitness, and the occurrence of trade-offs depends in part on the magnitude of variation in growth rate or acquisition. Using recombinant inbred lines, we examined the genetic architecture of age and size at reproduction across abiotic conditions encountered by cultivars and naturalized populations of Brassica rapa. We found that genotypes are plastic to seasonal setting, such that reproduction was accelerated under conditions encountered by summer annual populations and genetic variances for age at reproduction varied across simulated seasonal settings. Using an acquisition-allocation model, we predicted the likelihood of trade-offs. Consistent with predicted relationships, we observed a trade-off where early maturity is associated with small size at maturity under simulated summer and fall annual conditions but not under winter annual conditions. The trade-off in the summer annual setting was observed despite significant genotypic variation in growth rate, which is often expected to decouple age and size at reproduction because rapidly growing genotypes could mature early and attain a larger size relative to slowly growing genotypes that mature later. The absence of a trade-off in the winter setting is presumably attributable to the absence of genotypic differences in age at reproduction. We observed QTL for age at reproduction that jointly regulated size at reproduction in both the summer and fall annual settings, but these QTL were environment-specific (i.e. different QTL contributed to the trade-off in the fall vs. summer annual settings). Thus, at least some of the genetic mechanisms underlying observed trade-offs differed across environments., (© 2011 Blackwell Publishing Ltd.)

Hybridization and/or incomplete sorting of ancestral polymorphism are commonly implicated to explain discordant phylogenetic analyses of closely related species complexes. One genus in which these phenomena have been suggested to have played major roles based on phylogenetic data is Conradina, a genus of mints (Lamiaceae) endemic to the southeastern USA containing several endangered species. The goals of this study were to use microsatellite data to better understand patterns of genetic structure in Conradina, to test hypotheses of recent or ancient hybridization and incomplete lineage sorting, and to clarify species boundaries. Individuals from 55 populations representing all Conradina species were genotyped using 10 microsatellite loci. Analyses of the patterns of genetic structure in Conradina revealed a clear differentiation of populations following recognized species boundaries, indicating that species have diverged from one another genetically and interspecific hybridization has not occurred recently. Neither ancient hybridization nor incomplete lineage sorting is supported as the sole cause of species nonmonophyly, suggesting that both may have contributed to patterns found in phylogenetic trees; however, analyses of other types of data may be more appropriate to distinguish between these two hypotheses. Because all described species appear to be valid entities, the current listing status of most endangered species of Conradina is appropriate; however, populations of Conradina canescens are genetically differentiated into three groups, each of which may merit species status, and several recently discovered populations of Conradina in Dunn's Creek State Park in Florida are highly differentiated genetically and also appear to represent a new species.