Contemporary nuclear and mitochondrial genetic clines in a north temperate estuarine fish reflect Pleistocene vicariance

Contemporary genetic spatial structure in north temperate marine species is likely the culmination of multiple vicariant and dispersive cycles. Here we evaluate spatial genetic structure in an estuarine fish, rainbow smelt Osmerus mordax, from coastal Newfoundland, Canada, using data from both mtDNA (ND5) sequences and nuclear loci (11 microsatellites). Sequence analysis of ND5 identified a previously unrecognized genetic discontinuity between 2 hypo thesized glacial clades in southeastern Newfoundland. Microsatellite based tests for directional selection identified a locus (Omo11, p <0.001) that mirrored mtDNA clades in the geographic distribution of its 2 common alleles but did not display elevated differentiation following correction for heterozygosity. Bayesian multilocus clustering of the remaining micro satellite loci supported the presence of 2 predominant groups, for which the spatial distribution was also largely consistent with those of the mtDNA and Omo11 clades. Taken together, the similarity in microsatellite and mtDNA clines supports the hypothesis that contemporary spatial structure in smelt reflects historical landscape isolation maintained by low dispersal and selective processes producing reinforcement between diverging populations. As genetic structure in northern marine and estuarine species may be largely determined by historical glacial cycles of vicariance, contemporary estimates of connectivity should be interpreted in the context of both past and present landscape structure.