Abstract
Marginal populations are expected to provide the frontiers for adaptation, evolution and range shifts of plant species under the anticipated climate change conditions. Marginal populations are predicted to show genetic divergence from central populations due to their isolation, and divergent natural selection and genetic drift operating therein. Marginal populations are also expected to have lower genetic diversity and effective population size (N e) and higher genetic differentiation than central populations. We tested these hypotheses using eastern white pine (Pinus strobus) as a model for keystone, long-lived widely-distributed plants. All 614 eastern white pine trees, in a complete census of two populations each of marginal old-growth, central old-growth, and central second-growth, were genotyped at 11 microsatellite loci. The central populations had significantly higher allelic and genotypic diversity, latent genetic potential (LGP) and N e than the marginal populations. However, heterozygosity and fixation index were similar between them. The marginal populations were genetically diverged from the central populations. Model testing suggested predominant north to south gene flow in the study area with curtailed gene flow to northern marginal populations. Signatures of natural selection were detected at three loci in the marginal populations; two showing divergent selection with directional change in allele frequencies, and one balancing selection. Contrary to the general belief, no significant differences were observed in genetic diversity, differentiation, LGP, and N e between old-growth and second-growth populations. Our study provides information on the dynamics of migration, genetic drift and selection in central versus marginal populations of a keystone long-lived plant species and has broad evolutionary, conservation and adaptation significance.
Highlights
Marginal populations are expected to provide the frontiers for adaptation, evolution and range shifts of plant species under the anticipated climate change conditions
Genetic Diversity and Ne Our results demonstrate that Eastern white pine (EWP) central populations have significantly higher allelic diversity, genotypic diversity (GAe), Ne, and latent genetic potential (LGP) than the marginal populations but statistically similar heterozygosity and FIS
The marginal EWP populations are genetically distinct from the central populations
Summary
Marginal populations are expected to provide the frontiers for adaptation, evolution and range shifts of plant species under the anticipated climate change conditions. Organisms may experience a host of harsh environmental, climatic, edaphic, and nutrient conditions, and impediments to gene flow In such environments, selection regimes different from those at the abundant center may operate. It is crucial to understand patterns of genetic diversity, population structure and evolutionary processes such as natural selection, genetic drift and gene flow in central versus marginal populations, and genetic mechanisms underlying local adaptation in marginal populations, especially in long-lived, widely-distributed keystone plant species. This information is critical for forest trees as they are normally the keystone species in their ecosystems. Existence and survival of many flora and fauna in an ecosystem depend upon the existence of such keystone species
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