Genetic Variability in Species with Extensive Gene Duplication: The Tetraploid Catostomid Fishes

Tetraploid catostomid fishes have been achieving genic diploidization at many loci for over 50 million yr. On the average, catostomids are exhibiting duplicate gene expression at 47% of their loci, thus displaying a greater diversity of isozymes than diploids. However, on a per-locus basis, the typical catostomid has the same level of genetic variability as diploid teleosts. Our observation of reduced heterozygosity at multilocus systems supports the hypothesis that some of the duplicate loci may be retained due to interactions between their gene products or perhaps because these duplicate loci are diverging to different tissue specificities. The finding that singly expressed loci tend to be more polymorphic than duplicate loci also supports the concept that the levels of genic diploidization are in part determined by the likelihood of the appearance and fixation of null alleles, which in turn is determined by the intrinsic mutation rate of the enzyme involved. The present analysis of genetic variability at a number of enzyme loci in many species of many genera within a tetraploid family helps provide a broad evolutionary picture of strategies of genetic variability found in different taxa, and we suggest a number of components which are probably contributing to both genetic variability and to the loss of duplicate gene expression.