Understanding origins of present-day genetic structure in marine fish: biologically or historically driven patterns?

Determining the origin of genetic structure is of wide interest because of its use in stock discrimination in marine organisms. Schematically, genetic differentiation can result from historical patterns maintained over geological time or from present-day isolation attributable to biological characteristics of the species. We used a comparative approach to population genetic analysis based on allozyme polymorphism to determine the impact of reproductive strategy (i.e. biological origin) and habitat (i.e. historical origin) on the genetic structure of individuals sampled from five isolated islands in French Polynesia. Eight species of coral reef fishes from two families (Chaetodontidae and Pomacentridae) were selected to test the impact of sea-level change (historical origin) and reproductive strategy (biological origin) on genetic structure. Seven of the eight study species showed significant divergence in allelic frequencies computed over all sites. For these seven species, multilocus F_stvalues ranged from 0.0114 to 0.0287. None of the eight species showed a significant relationship between genetic divergence and geographical distance between sites. Significant divergence (difference in allozyme frequencies) between some pairs of sites occurred but was unrelated to distances between them. These results suggest that the genetic structure of coral reef fish in French Polynesia is likely to be driven according to an island model in which migrations between populations are rare and random in space and time. Overall, none of the species showed congruent genetic structures between sites sampled. Genetic structure of the eight species did not appear significantly related either to reproductive strategy or habitat preference. Genetic diversity (heterozygosity) was significantly correlated with these two factors, with species laying benthic eggs and/or inhabiting lagoons showing significantly higher multilocus heterozygosity than species laying pelagic eggs and/or living on the outer reef slope. Overall, the absence of differences according to habitat and/or reproductive strategy did not provide any conclusive pattern regarding the origin of the genetic structure, but the limited divergence in allelic frequencies suggests recent differentiations.