More on myosin mutations

Melanie McCollum and colleagues have a short paper in JHE about the evolution of MYH16, the myosin gene associated with masticatory musculature.

Much of the article is devoted to debunking the connection of MYH16 to the expansion of the brain. There's not much to that story -- muscles beating down on the skull don't inhibit brain growth. And they discuss the implications of the revised date estimate for the deactivation of the gene by Perry and colleagues (2005).

But by far the most interesting part of this is their discussion of the effects of the evolution of masticatory myosin in other mammals:

With respect to interspecific variation in the expression of "masticatory" myosin, it is important to note that humans are not unique in their failure to express this particular isoform. "Masticatory" myosin is lacking in the jaw-closing muscles of a number of mammals, most notably ungulates, rodents, rabbits, and kangaroos (Kang et al., 1994, Sfondrini et al., 1996, Hoh, 2002 and Qin et al., 2002). Comparative genetic studies suggest that the masticatory MyHC gene originated through duplication of an ancestral striated MyHC gene expressed in the mandibular arch musculature of early gnathostomes, and that it has since been retained as the primitive phenotype in vertebrates (Qin et al., 2002). Functional loss of masticatory myosin in a number of non-carnivorous mammalian species is believed to have followed shifts in dietary strategies that ultimately freed these taxa from the need for powerful jaw closure. As a consequence, these taxa are believed to have replaced their masticatory myosin with functionally more appropriate myosin isoforms (e.g., slow/beta-cardiac fibers in ungulates, Kang et al., 1994; fast MyHCs in rodents, Sfondrini et al., 1996) (McCollum et al. 2006, references in original).

This would seem to support the hypothesis that changing masticatory function in hominids (caused by dietary changes) favored the gene's deactivation. But deactivation might have preceded the conversion of the gene into a pseudogene:

However, the very fact that muscle fibers readily change their myosin heavy chain expression suggests that masticatory myosin in hominids could very well have been significantly, if not totally downregulated prior to its conversion to a pseudogene. If this were the case, inactivation of the MYH16 gene would have had little impact on the muscles of mastication of early hominids and far less severe consequences for its carriers. In fact, the introduction of a nonsense mutation in the MYH16 gene may have been below the threshold of selection. If this alternative is correct, then the real question of interest is whether the change in masticatory function that occurred during hominid evolution and that led to MYH16 downregulation and inactivation was diet-related, as has been recently suggested (Hoh, 2002), or instead reflected changes in social behaviors that would have eliminated the need for an aggressive bite, as was suggested over 20 years ago (Rowlerson et al., 1983).

OK, so we have first the question of the date (2.4 million vs. 5.3 million years ago), and second the question of whether the functional change -- whatever it was -- preceded the date. So this could reflect anything from change associated with hominid origins up to the origin of Homo.

McCollum et al. make clear in their description of muscle fiber function that we may be looking not at a muscle decrease but a functional change. In which case, the deactivation might have accompanied greater masticatory force instead of less.

What a mess.

References:

McCollum MA, Sherwood CC, Vinyard CJ, Lovejoy CO, Schachat F. 2006. Of muscle-bound crania and human brain evolution: The story behind the MYH16 headlines. J Hum Evol (in press) DOI link