Downstream gene flow and genetic structure of Gambusia holbrooki (eastern mosquitofish) populations

Twelve populations of Gambusia holbrooki were sampled in three drainages of Florida and southern Georgia and their genetic structure was determined from the electrophoretic variation of 13 loci. There was less genetic variation (H̄ = 0.199 vs. 0.242) and more differentiation within (D̄ = 0.108 vs. 0.097) and among (D̄ = 0.192 vs. 0.148) drainages in upstream than in downstream populations. Estimated levels of gene flow were lower (Nm̄ = 1.2 vs. 2.5) among upstream than among downstream populations. Florida populations were genetically differentiated from those in Georgia (D̄ = 0.184). This differentiation could result from the isolation of peninsular Florida and downstream gene flow after it rejoined the mainland.