Differential spatio-temporal expression and functional diversification of the myogenic regulatory factors MyoD1 and MyoD2 in Atlantic halibut (Hippoglossus hippoglossus).

Development of the vertebrate skeletal muscle is orchestrated by the myogenic regulatory factors MyoD, Myf5, myogenin and MRF4, which likely arose from the duplications of a single ancestral gene early in vertebrate evolution. We have isolated two myod genes from Atlantic halibut and examined their differential expression during embryogenesis using quantitative PCR and in situ hybridization to address their functional roles in this asymmetrically organized flatfish. myod1 was initially maternally expressed, while myod2 mRNA was first detectable during gastrulation. The myod1 mRNA levels predominated throughout somitogenesis, and both slow and fast muscle precursor cells displayed the bilateral symmetric myod1 signal during the formation of the symmetric somite pairs. In contrast, myod2 was left-right asymmetrically expressed in the fast muscle precursors. The random expression of myod2 was not associated with the right-sided eye migration and the development of thicker fast skeletal muscle on the eyed side than on the blind side. The nucleotide substitution analysis indicated that the teleost MyoDs essentially have evolved under purifying selection, but a subset of amino acid sites under strong positive selection were identified in the MyoD2 branch. Altogether, halibut MyoD1 seems to have retained the central role of MyoD in driving skeletal myogenesis, whereas the function of MyoD2 is uncertain in this flatfish species.