The functional gene diversity in natural populations over postglacial areas: the shaping mechanisms behind genetic composition of longnose dace (Rhinichthys cataractae) in northeastern North America.

The diversity of functional genes and the related processes are important issues for conservation biology. This is especially relevant for populations that have suffered from demographic reduction as a consequence of the processes of postglacial colonization. In this perspective, the aims of the present study are (1) to quantify the genetic diversity of functional genes and (2) to disentangle the long- and short-term effects of natural selection that shapes genetic diversity from those of drift, mutation, and allopatric fragmentation. This research was conducted using an extensive genetic polymorphism analysis of populations of longnose dace (Rhinichthys cataractae) living over an area once covered by Pleistocene glaciations. The sequence and diversity of one exon of three genes (MHC IIβ, growth hormone, and trypsin) were jointly analyzed with non-coding nuclear loci from 27 populations; these populations were sampled over four major basins of northeastern North America. The survey revealed a surprisingly low allelic richness, especially for the MHC gene, considering the number of individuals and populations sampled. The results suggest that there is a complex mixture of different evolutionary processes shaping the level of polymorphism among longnose dace. While our study underlines the importance of the short-term effects of neutral processes and the major impact of post-glacial colonization on gene diversity, locally dependent balancing selection was detected on MHC. From this perspective, our results support an understanding of the importance of drift on functional gene diversity but also highlight the transient effects of natural selection on allelic composition, even in populations that show drastic reduction of genetic diversity.