Abstract
Rear-edge populations are of significant scientific interest because they can contain allelic variation not found in core-range populations. However, such populations can differ in their level of genetic diversity and divergence reflecting variation in life-history traits, demographic histories and human impacts. Using 13 EST-microsatellites, we investigated the genetic diversity and differentiation of rear-edge populations of the Japanese endemic conifer Thuja standishii (Gordon) Carr. in southwest Japan from the core-range in northeast Japan. Range-wide genetic differentiation was moderate (Fst = 0.087), with northeast populations weakly differentiated (Fst = 0.047), but harboring high genetic diversity (average population-level Ar = 4.76 and Ho = 0.59). In contrast, rear-edge populations were genetically diverged (Fst = 0.168), but contained few unique alleles with lower genetic diversity (Ar = 3.73, Ho = 0.49). The divergence between rear-edge populations exceeding levels observed in the core-range and results from ABC analysis and species distribution modelling suggest that these populations are most likely relicts of the Last Glacial Maximum. However, despite long term persistence, low effective population size, low migration between populations and genetic drift have worked to promote the genetic differentiation of southwest Japan populations of T. standishii without the accumulation of unique alleles.
Highlights
Many plant species have distributions fitting a central-marginal model characterized by a core range surrounded by smaller edge populations
As populations differed in spatial extent, with most populations in southwest Japan and some in northeast Japan (e.g., NYUG, KUB and KUR) being small (Figure 1; for full names of populations see Table S1), we examined whether average spatial sampling distance between individuals in each population may be correlated with population-level genetic diversity
We investigated the importance of two major factors that may contribute to the genetic divergence of populations of T. standishii; namely isolation by distance (IBD: [47]), whereby geographic distance and landscape barriers cause restricted gene flow [48], and isolation by environment (IBE: [49]), in which gene flow is inhibited between populations occupying different environments [48]
Summary
Many plant species have distributions fitting a central-marginal model characterized by a core range surrounded by smaller edge populations. At the margins of species’ ranges, populations usually become smaller and increasingly geographically isolated confined to narrow areas of sub-optimal microclimates within or even outside the climatic envelope of the species core range [2] These populations are usually characterized by high genetic divergence and low effective population sizes [3]. Rear-edge populations, that is, those that are distributed at the low-latitude margin of species’ ranges [4], are often considered to be disproportionately important in terms of their contribution to species overall genetic diversity and ecological and evolutionary processes [5,6] This is because rear-edge populations have often experienced long-term isolation from the core range promoting genetic drift [7] and local adaptation [4]. For these reasons and the fact that rearedge populations of keystone species like large trees can form crucial “islands” of habitat in an otherwise inhospitable climate for various other organisms [8], such populations are of immense scientific and conservation interest
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