Phylogeography and population genetics of the estuarine and reef fish species Cyprinodon variegatus, Fundulus heteroclitus, and Thalassoma bifasciatum

The use of gene sequence data as a molecular marker allows the study of evolutionary processes in natural populations, and allows inferences to be made as to the relative contributions of forces such as drift, gene flow, historical demography and selection on the distribution of genetic variation. Most reef fish species have a planktonic larval phase that allows for potentially wide dispersal, which can be indirectly estimated using molecular marker based techniques. In the Caribbean wrasse, Thalassoma bifasciatum, previous studies have found substantial levels of local recruitment based on otolith microchemistry. Yet in spite of documented local recruitment, results from mitochondrial control region sequence and microsatellite data indicate a lack of genetic differentiation at both small and large spatial scales, consistent with an open population structure for this reef fish species. Historical processes, such as changes in population size, may have affected the current distribution of genetic variation to some extent. In contrast to the dispersal regime of tropical reef species, numerous small estuarine tidal marsh fishes of the U.S. Atlantic coast have low-dispersal life histories. Results from mitochondrial DNA in Cyprinodon variegatus are consistent with this life-history strategy, as populations from sites across its range are highly structured. Population structure also appears to be highly correlated with presumed barriers to gene flow, such as major biogeographic boundaries, or stretches of open ocean. Multiple range expansions, and founder effects near the limits of these expansions, including into formerly glaciated areas, have also affected the distribution of genetic variation not only at mitochondrial DNA, but also at three nuclear loci, which also show evidence for low gene flow in this species. A second estuarine species, Fundulus heteroclitus , also shows strongly structured populations as well as evidence for range expansions related to Pleistocene glaciation. However, unlike C. variegatus, this species was able to persist in formerly glaciated areas during multiple glacial advances.