Expression of myosin heavy chain isoforms mRNA transcripts in the temporalis muscle of common chimpanzees ( Pan troglodytes )

Abstract

The differential expression patterns of the mRNA transcripts of the MyHC isoforms in the temporalis muscle in P. troglodytes may be related to the functional differences that have been observed in electromyographic studies in other species of primates. Our findings can be applicable to the fields of comparative anatomy, evolutionary anatomy, and anthropology.

mRNA Expression of Myosin Heavy Chain Isoforms in the Sphenomandibularis Portion of the Temporalis Muscle

“…In our study, we observed that this deactivation of the MYH16 gene not only causes the disappearance of the MHC-M isoform at the protein level but also produces a marked reduction in its expression at the mRNA level. In contrast to chimpanzees (Ciurana et al, 2017), not all the human specimens in our study expressed the MHC-M isoform at the mRNA level. Only 60 % of individuals expressed it in the anterior temporalis and only 40 % expressed it in the posterior temporalis and the sphenomandibularis.…”

“…Our analysis of MM indicates that the human temporalis muscle is much smaller than that of other primates of the superfamily Hominoidea. If we compare the average MM in our study (26.8 ± 10.0 grams) with that of chimpanzees (120.5 ± 45.9 grams) (Ciurana et al, 2017), the hominoid primates most closely related phylogenetically to humans, we can see that the MM of the temporalis in humans is greatly reduced, with a ratio of 4.5:1 between the two species. However, this reduction in MM does not affect the proportion of the sphenomandibularis MM relative to the total MM of the temporalis, which was 9.6 % ± 2.3 % in humans in our sample and 9.9 % ± 3.5 % in chimpanzees (Ciurana et al, 2017).…”

“…If we compare the average MM in our study (26.8 ± 10.0 grams) with that of chimpanzees (120.5 ± 45.9 grams) (Ciurana et al, 2017), the hominoid primates most closely related phylogenetically to humans, we can see that the MM of the temporalis in humans is greatly reduced, with a ratio of 4.5:1 between the two species. However, this reduction in MM does not affect the proportion of the sphenomandibularis MM relative to the total MM of the temporalis, which was 9.6 % ± 2.3 % in humans in our sample and 9.9 % ± 3.5 % in chimpanzees (Ciurana et al, 2017). This small size of the human masticatory muscles in general (Aiello & Dean, 1990;Oxnard & Franklin, 2008) has been linked to the coding deactivation of the MYH16 gene of the MHC-M isoform, which allows this isoform to be expressed at the mRNA level but not at the protein level (Perry et al, 2004;Stedman et al, 2004).…”

“…Only 60 % of individuals expressed it in the anterior temporalis and only 40 % expressed it in the posterior temporalis and the sphenomandibularis. Moreover, in those individuals who did express the MHC-M isoform, the %MHC-M was clearly reduced compared to that of chimpanzees (Ciurana et al, 2017): 22.2 % ± 6.1 % vs 43.0 % ± 4.9 % in the anterior temporalis; 26.6 % ± 9.4 % vs 43.3 % ± 10.9 % in the posterior temporalis; and 14.0 % ± 8.3 % and 33.6 ± 3.2 % in the sphenomandibularis, respectively. This smaller MM and the decreased capacity for force generation in the masticatory muscles of humans, as a consequence of the loss of MHC-M expression at the protein level (Rowlerson et al, 1983;Hoh, 2002;Toniolo et al, 2008), has been linked to their evolutionary adaptation to a relatively soft diet compared to that of chimpanzees (Oxnard & Franklin, 2008).…”

“…Some authors consider the sphenomandibularis to be a separate muscle of mastication that is differentiated from the temporalis muscle (Dunn et al, 1996), while others argue that it is a deep bundle of the temporalis itself because there is no fascia between the two structures nor is there a specific vascularization or innervation of the sphenomandibularis (Türp et al, 1997;Ybarra & Bauer, 2001;Geers et al, 2005;Akita et al, 2019). The sphenomandibularis has also been identified in other primate species of the superfamily Hominoidea, such as chimpanzees (Pan troglodytes) (Ciurana et al, 2017).…”