Polymorphic and monomorphic serum esterase heterogeneity in catostomid fish populations

A one-locus, two-allele serum esterase polymorphism is described in populations of Catostomus (Pantosteus) clarki in the connected and disconnected tributaries of the lower Colorado River system. A clinal distribution of allelic frequency is linearly correlated with latitude. This distribution of allelic frequencies, and other characteristics of the polymorphism in C. (P.) clarki populations, are indicative of the selective maintenance of the heterogeneity. Sympatric species of the subgenus Catostomus exhibit a monomorphic esterase phenotype indistinguishable from the heterozygote phenotype in polymorphic populations of C. (P.) clarki. Genetic mechanisms involved in the evolution of one polymorphic locus and/or two homozygous loci from an ancestral gene are discussed.