Analysis of a hybrid zone in Fundulus majalis in a northeastern Florida ecotone

Abstract

Fundulus majalis and F. similis are small fish that inhabit the coastal waters of eastern North America. Fundulus majalis occurs from New Hampshire to northeastern Florida where it is replaced by F. similis. Past morphological studies of these species have suggested the existence of a hybrid zone in the Flagler County area of northeast Florida; this location also corresponds to an ecological transition from northern Juncus–Spartina marsh to southern mangrove marsh. We used protein electrophoresis to test the hypothesis that there is a significant barrier to gene flow between the two species in the area of Flagler County, FL. Twenty populations were surveyed electrophoretically for 26 protein producing loci. Significant allozyme variation was observed for five loci: adenosine deaminase, an esterase, isocitrate dehydrogenase, malate dehydrogenase and phosphoglucomutase. Allele frequencies at these loci vary clinally from a ‘majalis’-like distribution in more northern localities to a ‘similis’-like distribution in southern localities. The steepest portion of the cline occurs over a 30 km portion of intracoastal salt marsh bordered by the northern and southern boundaries of Flagler County, FL, which led us to examine eight populations from within this area. We speculate that the nominal F. majalis and F. similis were allopatric and in the process of speciation when they recontacted one another, and the process of differentiation broke down and secondary intergradation occurred. The width of the genotypic clines (only sMdh-A is fixed at both ends), Nm estimates, and the absence of significant linkage disequilibria support the hypothesis that the rapid shift in genotypes and morphology in Flagler County is a dispersal-independent cline maintained by selection across the rapid ecological transition of an ecotone.