Biochemical Genetics of Mosquitofish. II. Demographic Differentiation of Populations in a Thermally Altered Reservoir

Abstract

Mosquitofish (N = 7,580) from five sites along a thermal gradient in a reservoir were analyzed using electrophoretic techniques. Each sample was genetically different from the other four and none was in Hardy-Weinberg equilibrium for all loci. The populations also differed in their sex ratios and size structures. Juveniles tended to be less heterozygous than adults and heterozygotes tended to have longer body lengths than homozygotes within sex-age classes as indicated by single locus tests. Overall heterozygosity was highest in the population from the hottest water and lowest in those populations from the coldest water. Distributions of fish with different numbers of heterozygous loci were similar among sex-age classes within locations but significantly different for males and juveniles across locations. While stochastic processes are important in determining some of the genetic variation of the mosquitofish populations, selection is the probable explanation for the body length-genotype correlations and for the differences in genotypic proportions between males, females and juveniles.