GENETIC AND DEMOGRAPHIC RESPONSES OF MOSQUITOFISH (GAMBUSIA-HOLBROOKI GIRARD 1859) POPULATIONS STRESSED BY MERCURY

Genetic and demographic changes in mosquitofish populations are reported after chronic (111 d) exposure to mercury. Sex ratios, normally female-biased in field populations, were also female-biased in control mesocosms. However, the sex ratio was male-biased in the mercury treatments. Frequencies of glucosephosphate isomerase-2 (Gpi-2) allozymes for fish exposed to mercury differed from initial frequencies and from those of control fish. In a selection-component analysis, female sexual selection was statistically significant for the mercury-treated fish; the proportion of females that were gravid differed among Gpi-2 genotypes. The number of developing embryos per female also differed among Gpi-2 genotypes. Mercury had genotype-specific effects on mosquitofish reproduction in addition to genotype-specific effects on mortality reported earlier. These effects may reflect metabolic qualities of the Gpi-2 genotypes or loci closely linked to the Gpi-2 locus.