Diversity in Myosin Heavy Chain Composition of the Papionin Masseter Muscle Indicates the Importance of Hybrid Phenotypes for Feeding

The contractile performance of a muscle fiber is directly related to fiber type. Variation in fiber type within and among muscles leads to a diversity of phenotypes, providing the flexibility needed to perform an assortment of behaviors with varying functional demands. Previous analyses have demonstrated variation in fiber phenotype of the chewing muscles in a number of primates and nonprimate mammals. Some of this variation has been linked to species‐specific feeding behaviors. To further evaluate the relationship between fiber phenotype and feeding behavior, we compare the distribution of masseter muscle fiber types in four papionin species: the hard‐object feeding sooty mangabey ( Cercocebus atys ) and three papionin species that do not specialize on hard objects. Immunohistological staining was used to determine the presence of slow (MyHC‐1 and MyHC α –cardiac) and fast (MyHC‐2 and MyHC‐M) isoforms. Samples were taken from the anterior superficial masseter of C. atys (n=8), M. mulatta (n=2), M. fascicularis (n=2), and P. anubis (n=2). Histological sections were subsequently digitized to determine the relative proportions of MyHC types and fiber phenotype in each specimen. The presence of MyHC α‐cardiac was ubiquitous throughout the sample ( C. atys =99%; M. mulatta =76%; M. fascicularis =77%; P . anubis =79%). MyHC α‐cardiac maintains fatigue resistance while producing twitch tension more rapidly than MyHC‐1, thus providing an advantage for animals that require a fiber phenotype combining fatigue resistance and intermediate speed of contraction. Previously, we reported high proportions of MyHC α‐cardiac in C. atys , which we functionally linked to the need for sustained and repetitive high bite forces required for crushing hard seeds. Our finding that C. atys consistently express a higher percentage of MyHC α‐cardiac in comparison to the other papionins further supports this interpretation. The presence of hybrid fibers in all four papionin species was notable. Slow types MyHC‐1 and MyHC α‐cardiac were co‐expressed in nearly 100% of fibers sampled in each of the species. Hybrid fibers co‐expressing slow and fast‐type phenotypes were also prevalent. The abundance of hybrid fibers across papionins suggests a shared fiber phenotype that provides for versatility in the contractile properties of the anterior superficial masseter, a muscular region that must meet diverse performance requirements. This initial work is part of a larger ongoing study to assess jaw‐muscle fiber phenotype in non‐human primates. Future analyses will include larger samples and a greater diversity of species to enable us to test key functional hypotheses about fiber type diversity in primates. Support or Funding Information Funded in part by National Science Foundation Grants (BCS 1719743, BCS 0962677, BCS 0095522) and Yerkes National Primate Research Center Base Grant ORIP/OD P51OD011132 and the Comparative AIDS Core This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal .