Mitochondrial DNA diversification and population structure in fishes from the Gulf of Mexico and western Atlantic

We examined variation in mitochondrial DNA (mtDNA) among geographic samples of seven species of marine fish sampled from the Gulf of Mexico (Gulf) and along the southeastern Atlantic coast of the United States (western Atlantic), Species included three sciaenids that are estuarine dependent during larval and juvenile stages, three reef-associated species that are typically found as adults on reefs, hard bottoms, or submerged structures, and one scombrid that is pelagic throughout its life history. All seven species are significant components of recreational and/or commercial fisheries, and the issue of population structure/gene flow is critical to their conservation and management. MtDNA variation in the three sciaenids is consistent with the hypothesis that dispersal/gene flow in ail three species follows an isolation-by-distance model, and that dispersal potential is consistent with expectations based on life history. No evidence of population structuring was observed in the three reef-associated species, in contrast to what might be expected on the basis of their adult life history Low levels of mtDNA variation in two of the reef-associated species is consistent with the occurrence of past or present-day bottleneck events. MtDNA variation in the scombrid was not structured geographically, consistent with expectations for a highly pelagic species, but was discordant with variation observed at an allozyme locus (PEPA-2). However, variation in mtDNA was independent of sex or age of Individuals, whereas variation at the allozyme locus was not, Genotypes at the allozyme locus thus appear to vary in part as a function of the sex and age composition of samples, This finding serves as a warning that genetic markers may not necessarily be independent with respect to life-history parameters such as sex and age.