Developmental Genetics of Fishes: Isozymic Analyses of Differential Gene Expression

Abstract

SYNOPSIS. The fundamental evolutionary position of the fishes and the plasticity of their gene expression render them well suited for developmental genetic analyses. Isozymes, encoded in related loci, have proven to be effective probes of differential gene expression in the fishes. The advanced bony fishes exhibit a higher temporal and spatial specificity of isozyme locus expression than the primitive fishes. Some isozyme loci are limited in their expression to specific developmental periods such as hatching. The more phylogenetically recently derived loci have a more restricted tissue expression, and tend to be expressed later in development than more ancient loci exhibiting a more generalized tissue expression. More closely related isozyme loci tend to overlap more in their cellular expressions than do more distantly related loci. The role of gene and genome relatedness in regulating gene expression also has been studied by the investigation of preferential allele expression in interspecific hybrids. The schedules of gene expression are often perturbed in interspecific fish hybrids. A progressive increase in the severity of developmental abnormalities, at the morphological and enzymatic levels, occurs during embryogenesis of hybrids formed from progressively more distantly related species. In most instances of allelic repression, the paternal allele is preferentially inhibited but occasionally the maternal allele is selectively repressed. A non-reciprocal developmental success is often observed for hybrid embryos derived from reciprocal crosses. These aberrant nucleocytoplasmic interactions have been related to models of gene regulation and the evolutionary divergence of gene regulatory mechanisms.