Contemporary and historical influences on the genetic structure of the estuarine-dependent Gulf killifish Fundulus grandis

In comparison to species living in open marine environments, estuarine-dependent species are expected to exhibit stronger genetic population structure due to dispersal limitations. Estuarine habitats are relatively transitory on geological time scales; thus, populations may not be at migration-drift equilibrium, which could confound estimates of current day gene flow or selection. We used 8 nuclear microsatellite loci to investigate the genetic structure of the estuarine Gulf killifish Fundulus grandis across 10 populations along the northwestern and northeastern Gulf of Mexico. Patterns of isolation by distance, spatial autocorrelation, and assignment tests indicate that dispersal is limited and occurs primarily between neighboring sites. Principal component analysis and Bayesian clustering revealed evidence for genetic discontinuities located in Mobile Bay and western Florida which are near hypothesized biogeographical boundaries. There was also a significant negative relationship between genetic diversity and latitude, a pattern consistent with the presence of hypothesized refugia in the southern Gulf regions during the Pleistocene that later recolonized the northern Gulf. Results suggest that populations may be at or near migration-drift equilibrium at a regional scale (e.g the western Gulf), but that dispersal barriers and potential historical signatures on population structure will need to be taken into consideration at larger spatial scales.