Abstract
We have shown that the sarcoplasmic myosin heavy-chain (MyHC) isoform xtMyHC-101d is highly and specifically expressed in the larynx of the aquatic anuran, Xenopus tropicalis. In male larynges, the predominant MyHC isoform is xtMyHC-101d, while in females, another isoform, xtMyHC-270c, predominates. The X. tropicalis genome has been sequenced in its entirety, and xtMyHC-101d is part of a specific array of xtMyHC genes expressed otherwise in embryonic muscles (Nasipak and Kelley, Dev Genes Evol, in press, 2008). The administration of the androgen dihydrotestosterone increases the expression of xtMyHC-101d in juvenile larynges of both sexes. Using ATPase histochemistry, we found that in adults, X. tropicalis male laryngeal muscle contains only fast-twitch fibers, while the female laryngeal muscle contains a mix of fast- and slow-twitch fibers. Juvenile larynges are female-like in fiber type composition (44% slow twitch, 56% fast twitch); androgen treatment increases the percentage of fast-twitch fibers to 86%. xtMyHC-101d predominates in larynges of dihydrotestosterone-treated juveniles but not in larynges of untreated juveniles. We compared the larynx-specific expression of xtMyHC genes in X. tropicalis to the MyHC gene expressed in X. laevis larynx (xlMyHC-LM) by sequencing the entire xlMyHC-LM gene. The androgen-regulated xtMyHC that predominates in the male larynx of X. tropicalis is not the gene phylogenetically most similar to xlMyHC-LM at the nucleotide level but is instead a similar isoform found in the same MyHC array and expressed in the embryonic muscle.