Molecular Evolution of Teleost Neural Isozymes

Isozymes, homologous enzymes coded by separate loci within a genome, present interesting systems for examining molecular and functional divergence through natural selection. Isozyme pairs for a number of metabolic enzymes, including Triosephosphate isomerase (Tpi), Malate dehydrogenase (Mdh), Phosphoglucose isomerase (Pgi), and Guanylate kinase (Guk), appear to all result from a single, large duplication event early in teleost evolution. These small gene families include two forms, a generally expressed form with no apparent charge and a neurally expressed form with a pronounced negative charge although the canalization of expression of the second form varies across families. Using ancestral sequence reconstructions and standard comparisons of rates of nonsynonymous and synonymous change, combined with the examination of the specific amino acid changes observed and predicted we examined the evolution of the Tpi and Guk families using all available vertebrate sequences and all four families using a smaller, common, dataset. We find that post-duplication, the neural Tpi and Guk isozymes evolved through similar periods of positive selection as evidenced by elevated rates of nonsynonymous change and accumulation of negative amino acids. Over the same evolutionary period our analysis suggests that Mdh and Pgi isozymes appear to have evolved under a less divergent pattern of selection. These distinct results likely reflect functional differences between the isozymes, possibly a result of differences in expression patterns.