Lack of interpopulation genetic structure in the genus Stegastes (Perciformes) with indication of local introgression.

The family Pomacentridae comprises about 326 species belonging to 28 genera. The genus Stegastes is composed of nearly 33 species, and 8 are endemic to the Brazilian Province, inhabiting the Brazilian coast (Stegastes fuscus, S. variabilis, S. leucosticus, S. uenfi, and S. pictus) or Western Atlantic oceanic islands (S. trindadensis, S. rocasensis and S. sanctipauli). Stegastes species play a major role in the reef ecosystem since they interfere significantly with the composition of benthonic organisms. Studies about population genetics and speciation of Neotropical ichthyofauna are scarce, particularly at insular areas from the Western Atlantic. Random amplified polymorphic DNA markers were used to analyze the population genetic structure of the continental species S. fuscus and S. variabilis (Northeastern Brazil) as well as the insular species S. sanctipauli (Saint Paul's Rocks). Analysis of population parameters revealed a high index of intrapopulation genetic variability in the species, except for S. sanctipauli, which showed low values. The phiST values in samples of S. fuscus and S. variabilis obtained at distinct collection sites 35 km apart from each other indicated a lack of population genetic structure. An intermediary profile of species-specific markers was detected in some individuals of S. fuscus and S. variabilis from Santa Rita, Rio Grande do Norte, suggesting a putative introgression event between the two species. The genetic profiles observed in Stegastes populations indicate a higher genetic variability along the shoreline than at oceanic sites, related to a reduced effective population size on islands. The lack of genetic differentiation among coastal populations suggests that, despite some biological features such as non-migratory behavior and territoriality, the pelagic larval phase of these species is able to promote an interpopulation homogeneity among sampled areas.