Patterns of Genetic Variation in Eastern Mosquitofish (Gambusia holbrooki Girard) from the Piedmont and Coastal Plain of Three Drainages

Abstract

Patterns of genetic variation at 21 loci are described for 88 populations of Gambusia holbrooki sampled from three drainages in Georgia, South Carolina and North Carolina. Genetic diversity within populations (76-92%) was higher than that among populations (6-23%), drainages (1-2%), geographic provinces (Piedmont vs Coastal Plain: 2.8%) and water flow types (still vs running: 0.1%). The level of differentiation among populations is one of the highest reported for fishes (Fs, = 0.175). Coastal Plain populations had lower differentiation within drainages (Gs, = 6.4%) than those in the Piedmont (Gs, = 22.4%), lower average genetic distances among populations (D = 0.100 vs 0.173) and drainages (D = 0.129 vs 0.207), higher heterozygosity (fH = 0.134 vs 0.115), and more alleles per locus (N, = 1.61 vs 1.51). There were statistically significant differences in allele frequencies among populations at 20 of 21 loci. Waccamaw populations were strikingly different from the other Coastal Plain populations. Differences among drainages and water flow types also were detected for several loci. No evidence was found for a previously proposed north-south differentiation of G. holbrooki into Types I and II. Most of the genetic divergence occurs among local populations and not among populations from different geographic regions or environments, which is in contrast to the situation found generally in other species.