From inland to the coast: Spatial and environmental signatures on the genetic diversity in the colonization of the South Atlantic Coastal Plain

Abstract

The process of colonization and range expansion to novel environments involves both demographic and selective processes that can be traceable in the genetic diversity of the organisms. Nevertheless, it can be difficult to disentangle the signatures of the demographic and selective in the current genetic diversity of the populations. In this work, we use a landscape genetics framework to investigate the association of spatial and climatic variables with the genetic diversity and differentiation patterns of wild Petunia populations involved in a process of coastal colonization during the last 400 thousand years. Over 300 individuals from 17 populations were genotyped using ten microsatellite loci. Our results suggest that the genetic diversity is higher in populations located at the center of the species range with a decline toward the edges, and that gene flow follows an inland-to-coastal and central-to-peripheral dynamic that parallels the colonization history of this coastal lineage. We identify high levels of genetic differentiation between inland and coastal populations. As part of the inland-coastal genetic differentiation, we find signals of isolation by environment associated with differences in extreme temperature regimes. The differentiation of edge populations along the coast is associated with precipitation seasonality. Our results are robust to controls for spatial and historical divergence in the observed genetic differentiation. We conclude that the ecological differentiation process during the colonization of the South Atlantic Coastal Plain (SACP) was likely facilitated by genetic enrichment resulting from gene flow from central to marginal populations, as well as by rapid allele fixation resulting from serial founder effects during the range expansion along the coast.