Salinity and hydrological barriers have little influence on genetic structure of the mosquitofish in a coastal landscape shaped by climate change

Gradients in salinity and vegetation have been dramatically altered along shoreline of the northern Gulf of Mexico by both natural (i.e., erosion, fluctuating sea levels) and anthropogenic (i.e., dredging, habitat restoration) drivers. We gauged the impacts of salinity gradients and open water barriers on the genetic structure of a common marsh resident, western mosquitofish (Gambusia affinis) by sampling 15 populations along 440 km of Texas/Louisiana coast. We characterized 602 individuals (similar to 40/population) using seven microsatellite loci and our results reflected significant isolation by distance (IBD) among populations, but without the hypothesized genetic substructure from local adaptation. Large tracts of open water, thought to inhibit mosquitofish movement, were apparently not significant deterrents of dispersal. There was no evidence of a significant relationship between salinity gradients and genetic divergence. Although mosquitofish dispersal is sufficiently limited to result in a strong pattern of IBD, it is high enough to maintain the connectedness of populations. Our results suggest that limited gene flow, combined with large effective size, creates conditions suitable to adaptation to local environment, suggesting that mosquitofish and other marsh residents with similar life histories will be able to adapt to changes occurring in coastal environments.