Genetic Variability and Systematics of Gambusia in the Southeastern United States

Mosquitofish were sampled from 76 locations in 19 drainages in the southeastern United States. Thirteen polymorphic loci were resolved for fish from each location. Populations from eastern drainages had significantly higher levels of heterozygosity (H = 0.113) than those in western drainages (H = 0.055). Abrupt changes in allele frequencies for several loci occurred in the area of Mobile Bay, which corresponds with discontinuities previously reported for chromosomal and morphological data. Based on geographic patterns of allele frequencies, abrupt differentiation in local genetic constituency (DR = 0.443 between eastern Gambusia affinis holbrooki and G. a. afinis) and previously reported data on the existence of a reproductive barrier between the forms, the original taxonomic designations recognizing G. holbrooki and G. affinis as separate species are preferred. Populations in drainages west of Mobile Bay should be considered G. affinis, with those east of this divide being G. holbrooki.