Divergent evolution of the myosin heavy chain gene family in fish and tetrapods: evidence from comparative genomic analysis

Myosin heavy chain genes (MYHs) are the most important functional domains of myosins, which are highly conserved throughout evolution. The human genome contains 15 MYHs, whereas the corresponding number in teleost appears to be much higher. Although teleosts comprise more than one-half of all vertebrate species, our knowledge of MYHs in teleosts is rather limited. A comprehensive analysis of the torafugu (Takifugu rubripes) genome database enabled us to detect at least 28 MYHs, almost twice as many as in humans. RT-PCR revealed that at least 16 torafugu MYH representatives (5 fast skeletal, 3 cardiac, 2 slow skeletal, 1 superfast, 2 smooth, and 3 nonmuscle types) are actually transcribed. Among these, MYHM743-2 and MYHM5 of fast and slow skeletal types, respectively, are expressed during development of torafugu embryos. Syntenic analysis reveals that torafugu fast skeletal MYHs are distributed across five genomic regions, three of which form clusters. Interestingly, while human fast skeletal MYHs form one cluster, its syntenic region in torafugu is duplicated, although each locus contains just a single MYH in torafugu. The results of the syntenic analysis were further confirmed by corresponding analysis of MYHs based on databases from Tetraodon, zebrafish, and medaka genomes. Phylogenetic analysis suggests that fast skeletal MYHs evolved independently in teleosts and tetrapods after fast skeletal MYHs had diverged from four ancestral MYHs.