Salinity-related fitness differences amongst GPI genotypes in the mosquitofish Gambusia holbrooki (Poeciliidae: Teleostei)

Salinity-induced mortality was examined in the mosquitofish Gambusia holbrooki to determine if natural selection associated with the tidal inundation of lower reach populations could potentially maintain glucose-6-phosphate isomerase ( GPI -2; E.C. 5.3.1.9) allele frequency variation observed at field sites. In vitro experiments established if survival was independent of GPI -2 genotype, and/or standard body length (S.L.). Bioassay results at 17%₀ were consistent with lower survival of GPI -2*88/88 homozygote genotypes, GPI -2*100/100 genotypes in juvenile size classes, and greater survival of GPI -2*109/100 genotypes; 100% mortality was observed at salinities of 30%₀. In the early stages of the 30%₀ bioassay, juvenile fish carrying each GPI -2 allele suffered similar, high rates of mortality. Following 19 weeks exposure to 4–8%₀ NaCl the frequency of GPI -2*88 alleles decreased in adult G. holbrooki (> 17 mm SL). Salinity-induced mortality was found to be a selective influence capable of creating and maintaining GPI -2 allele frequency differences among G. holbrooki subpopulations, and/or between sex/size cohorts within subpopulations, but only at intermediate salinities. Whether saltwater inundation at field sites maintains inter-population diversity and produces local adaptation will be determined by the age structure of the population during periods of exposure, fluctuations in salinity as determined by the relationship between rainfall and tidal regime, and the level of immigration from non-affected sites.