Highly divergent duplicate mannose-6-phosphate isomerase (Mpi) genes in the blue mussel, Mytilus edulis

Gene duplication has been hypothesized to play a major role in the evolution of genes and genomes and in generating phenotypic diversity among the proteins those genes encode. We have identified duplicate genes for the glycolytic enzyme mannose-6-phosphate isomerase (MPI: EC 5.3.1.8) in marine mussels in the genus Mytilus. Overall, there was only 52% sequence identity (72% similarity) between the proteins encoded by these two genes, designated as Mpi-A and Mpi-B. Based on a comparison of the rate of non-synonymous substitution between orthologous and paralogous Mpi coding sequences obtained from Mytilus edulis and the congener M. trossulus we estimate that the duplication of Mpi in mussels occurred ~ 170 MYA. We detected paralog-specific differences in the ratio of non-synonymous to synonymous substitutions (ka/ks) and in the predicted net charge for MPI-A and MPI-B. Using a real-time quantitative RT-PCR assay we observed substantial changes in Mpi-A and Mpi-B transcript levels between tissue types; the strongest expression of Mpi-A was observed in mantle tissue while Mpi-B expression exceeded that of Mpi-A in gill and hepatopancreas tissues. Taken together, these observations suggest that different functional roles have evolved for these two Mytilus Mpi genes subsequent to gene duplication.