Abstract
To assess the influence of zoogeographic factors and life-history parameters (effective population size, generation length, and dispersal) on the evolutionary genetic structure of marine fishes in the southeastern USA, phylogeographic patterns of mitochondrial DNA (mtDNA) were compared between disjunct Atlantic and Gulf of Mexico populations in three coastal marine fishes whose juveniles require an estuarine or freshwater habitat for development. Black sea bass (Centropristis striata), menhaden (Brevoortia tyrannus andB. patronus) and sturgeon (Acipenser oxyrhynchus) samples were collected between 1986 and 1988. All species showed significant haplotype frequency differences between the Atlantic and Gulf, but the magnitude and distribution of mtDNA variation differed greatly among these taxa: sea bass showed little within-region mtDNA polymorphism and a clear phylogenetic distinction between the Atlantic and Gulf; menhaden showed extensive within-region polymorphism and a paraphyletic relationship between Atlantic and Gulf populations; and sturgeon exhibited very low mtDNA diversity both within regions and overall. Evolutionary effective sizes of the female populations (N f (e)) estimated from the mtDNA data ranged fromN f (e) = 50 (Gulf of Mexico sturgeon) toN f (e) = 800 000 (Atlantic menhaden), and showed a strong rank-order agreement with the current-day census sizes of these species. The relationship betweenN f (e) and the estimated times of divergence (t) among mtDNA lineages (from conventional clock calibrations) predicts the observed phylogenetic distinction between Atlantic and Gulf sea bass, as well as the paraphyletic pattern in menhaden, provided the populations have been separated by the same long-standing zoogeographic barriers thought to have influenced other coastal taxa in the southeastern USA. However, vicariant scenarios alone cannot explain other phylogenetic aspects of the menhaden (and sturgeon) mtDNA data and, for these species, recent gene flow between the Atlantic and Gulf coasts is strongly implicated. These data are relevant to management and conservation issues for these species.
Highlights
The geographic structure of any species is a product of both historical and contemporary gene flow (Slatkin 1987), and is likely to have been affected by such factors as geographic or ecologic impediments to movement and inherent dispersal capability
In the unweighted pair-group method with arithmetic means (UPGMA) phenogram, the genotypes grouped into two distinct clusters whose members differed on average by about 0.9% sequence divergence (Fig. 1)
With the exception of the marine catfish, all these taxa exhibit a striking pattern of phylogeographic concordance involving a clear genetic distinction of most populations in the Gulf of Mexico from those along the remainder of the Atlantic coast
Summary
The geographic structure of any species is a product of both historical and contemporary gene flow (Slatkin 1987), and is likely to have been affected by such factors as geographic or ecologic impediments to movement and inherent dispersal capability. We examine mitochondrial DNA (mtDNA) population structure in three temperate coastal marine fishes that require an estuarine or freshwater habitat for juvenile development. Our major goals are to: (1) further assess the influence of historical geographic factors and life-history pattern on the genetic structure of the marine fauna of the southeastern US; (2) provide genetic data that may be of relevance to management and conservation decisions for these species; and (3) assess two additional, seldom-considered factors in phylogeographic outcomes - the significance of differing effective population sizes and generation lengths on the distributions of gene lineages within and among populations. Parsimony networks based on the qualitative restriction-fragment data were generated by the approach described in Aviseet al. (1979)
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