Adaptive strategies and driving factors of a montane riparian tree: Trait-specific mechanisms across latitude.

Investigating the drivers of phenotypic and genetic divergence can reveal the underlying processes and strategies that species adopt in rapidly changing environments. However, knowledge of adaptive strategies and the underlying mechanisms is lacking for the majority of taxa, especially those living in habitats sensitive to climate change. Here, we investigated 20 populations of a Tertiary-relict tree species, Euptelea pleiospermum (Eupteleaceae), scattered in a mountain riparian habitat in China. We integrated genetic, growth, and reproductive traits, and evaluated the relative contributions of climatic and soil factors on genetic and functional trait divergence. The E. pleiospermum populations were divided into south and north genetic clusters, and there were significant differences in leaf density and seed mass of adult trees between the two. The spatial pattern of genetic divergence resulted from effects of both isolation by distance (IBD) and isolation by environment (IBE), whereas the divergence of growth and reproductive traits resulted solely from IBE effects. Spatial distance and selection by temperature and soils played dominant roles in genetic divergence. Precipitation drove the spatial divergence of sprouting. Both divergence of leaf density and seed mass were prominently induced by genetic divergence, and the influences might be enhanced by temperature and soil nutrients. We infer that E. pleiospermum populations adopt a resource-conservative strategy with low growth rates and higher sprouting under flooding disturbance, with larger seeds for improved seedling recruitment at lower latitudes. In contrast, high growth rate and sexual reproduction with small seeds are strategies adopted by populations at higher latitudes. We conclude that sprouting reflects a plastic response to precipitation, and leaf density and seed mass reflect local adaption under selection by temperature and soil factors. The underlying mechanisms of species adaptation strategies were trait-specific. Temperature and soil conditions are likely the main ecological factors shaping plant divergence in montane riparian regions.

Meta-analysis of genetic representativeness of plant populations under ex situ conservation in contrast to wild source populations.

Ex-situ conservation is widely used to protect wild plant species from extinction. However, it remains unclear how genetic variation of ex-situ plant collections reflects wild source population diversity. Here, we conduct a global meta-analysis of the genetic representativeness of ex-situ populations by comparing genetic diversity (i.e. AR, allelic richness; HE , expected heterozygosity; PPB, percentage polymorphic bands; and SWI, Shannon-Winner index), inbreeding coefficient (FIS ), and genetic differentiation between ex-situ plant collections and their wild source populations. Genetic diversity (i.e., HE , PPB, and SWI) was significantly lower in ex-situ populations than their wild source populations, while genetic differentiation between ex-situ and wild populations (ex-situ-wild FST ), but not that among ex-situ populations, was significantly higher than among wild populations. Outcrossing species, but not those with mixed mating system, had significantly lower genetic diversity in ex-situ populations, and significantly higher ex-situ-wild FST . When the collection size for ex-situ conservation was ≥30 or 50, PPB, HE , and ex-situ-wild FST were not significantly different between ex-situ and wild populations, indicating a relatively high genetic representativeness. Collecting from the entire natural distribution range and mixing collections from different sources could significantly increase the genetic representativeness of ex-situ populations. Type of ex-situ conservation (i.e., planting or seed bank) had no effect on genetic representativeness. The effect size of HE decreased and the effect size of ex-situ-wild FST increased with the duration of ex-situ conservation. Our results suggest that current ex-situ plant collections do not effectively capture the genetic variation of wild populations. Low genetic representativeness of ex-situ populations was caused by both initial incomplete sampling from wild populations and genetic erosion during ex-situ conservation. We emphasize that it is necessary to employ more thorough sampling strategies in future collecting efforts and to add new individuals where needed. Article impact statement: Low genetic representativeness of living plant collections is a worldwide problem in ex situ conservation. This article is protected by copyright. All rights reserved.

RADseq data reveal ancient, but not pervasive, introgression between Californian tree and scrub oak species (Quercus sect. Quercus: Fagaceae).

A long-term debate in evolutionary biology is the extent to which reproductive isolation is a necessary element of speciation. Hybridizing plants in general are cited as evidence against this notion, and oaks specifically have been used as the classic example of species maintenance without reproductive isolation. Here, we use thousands of SNPs generated by RAD sequencing to describe the phylogeny of a set of sympatric white oak species in California and then test whether these species exhibit pervasive interspecific gene exchange. Using RAD sequencing, we first constructed a phylogeny of ten oak species found in California. Our phylogeny revealed that seven scrub oak taxa occur within one clade that diverged from a common ancestor with Q. lobata, that they comprise two subclades, and they are not monophyletic but include the widespread tree oak Q. douglasii. Next, we searched for genomic patterns of allele sharing consistent with gene flow between long-divergent tree oaks with scrub oaks. Specifically, we utilized the D-statistic as well as model-based inference to compare the signature of shared alleles between two focal tree species (Q. lobata and Q. engelmannii) with multiple scrub species within the two subclades. We found that introgression is not equally pervasive between sympatric tree and scrub oak species. Instead, gene flow commonly occurs from scrub oaks to recently sympatric Q. engelmannii, but less so from scrub oaks to long-sympatric Q. lobata. This case study illustrates the influence of ancient introgression and impact of reproductive isolating mechanisms in preventing indiscriminate interspecific gene exchange.

Landscape genetic structure of a Streamside tree species Euptelea pleiospermum (Eupteleaceae): contrasting roles of river valley and mountain ridge.

We used landscape genetics and statistical models to test how landscape features influence connectivity or create barriers to dispersal for a mountain riparian tree species, Euptelea pleiospermum. Young leaves from 1078 individuals belonging to 36 populations at elevations of 900-2000 m along upper reaches of four rivers were genotyped using eight nuclear microsatellite markers. We found no evidence for the unidirectional dispersal hypothesis in E. pleiospermum within each river. The linear dispersal pattern along each river valley is mostly consistent with the "classical metapopulaton" model. Mountain ridges separating rivers were genetic barriers for this wind-pollinated tree species with anemochorous seeds, whereas river valleys provided important corridors for dispersal. Gene flow among populations along elevational gradients within each river prevails over gene flow among populations at similar elevations but from different rivers. This pattern of gene flow is likely to promote elevational range shifts of plant populations and to hinder local adaptation along elevational gradients. This study provides a paradigm to determine which of the two strategies (migration or adaptation) will be adopted by mountain riparian plants under climate warming.

Contrasting relationships between species diversity and genetic diversity in natural and disturbed forest tree communities.

• This study aimed to reveal species-genetic diversity correlations (SGDCs) and their underlying mechanisms in natural and disturbed forests. • A community survey and molecular analyses were carried out to compare species diversity (SD), the genetic diversity of the dominant tree species Euptelea pleiospermum (GD), the altitudinal patterns of SD and GD, SGDC, genetic differentiation (F(ST) ), community divergence (F(ST) -C), effective population size (N(e) ), and recent migration rate between mountain riparian forests along the Yandu (natural) and Nan (disturbed) rivers. • In natural forests, both SD and GD showed a unimodal altitudinal pattern and GD was positively correlated with SD, whereas a unimodal pattern and positive SGDC were not found in the disturbed forests. SD and F(ST) at the natural sites were higher than those at the disturbed sites. However, there were no significant differences in GD, F(ST) -C, N(e) or recent migration rate between the natural and disturbed sites. • A correlation between the patterns of SD and GD along a geographical gradient (e.g. altitude) is an important driver of positive SGDC. The absence of positive SGDC in the disturbed forests may result from reduced SD but unaffected GD, indicating nonparallel changes in SD and GD. This study furthermore cautions against generalizations about changes in SD and GD following disturbance.