Esterase Enzyme Function and Genetical Structure of Populations of the Freshwater Fish, Notropis stramineus

Abstract

Electrophoresis on starch gel of muscle homogenates of the freshwater fish, Notropis stramineus, indicated two regions of esterase heterogeneity. The most anodal group (Es-I) consisted of several closely associated minor bands while the more cathodal group (Es-II) exhibited a clear pattern of variation attributable to three codominant alleles segregating at an autosomal locus. A two-allele polymorphism was also detected at a lactate dehydrogenase locus (Ldh). Temperature-dependent activity of the esterase isozymes separated by column chromatography indicated: (1) there is a correlation of Es-I activity at various temperatures with Es-II genotype, and (2) a difference in temperature-dependent activity between the homozygotes and the heterozygote. The Es-I isozymes from Es-II heterozygotes exhibited a maximum activity at temperatures higher than those from Es-II homozygotes. Population samples during 3 years, ranging over more than 300 miles of the Kansas River mainstream, indicated both temporal and spatial stability of the Es-II polymorphism (i.e., no significant variation in p). A regular pattern of heterozygote deficiency was observed among samples when each was compared with expected Hard-Weinberg frequencies. Although positive assortative mating cannot be excluded as a cause of the observed deficiency, the latter cannot be due to the Wahlund effect, the presence of a null allele, or inbreeding. The last was excluded on the basis of slight heterozygote excesses observed at the Ldh locus in the same population samples. There were no differences in p between two age classes. Although males and females did not differ from one another in p, males exhibited a slight excess of heterozygotes while females exhibited a marked deficiency. Considering the potential relationship between enzyme function and adaptation, the observed pattern of functional diversity allows a prediction of intergeneration selection or intrageneration differential mortality, where heterozygotes are at an advantage at higher environmental temperatures and homozygotes at lower environmental temperatures. Magnitudes of heterozygote deficiency fluctuate with season in all population samples examined through time in a manner predicted by functional characteristics of the esterase isozymes. The rather large changes in zygotic frequencies occurring within a single generation suggest a significant amount of differential genetic mortality occurring within populations.