Department of Biological Sciences, Sherman Fairchild Center for the Life Sciences, Columbia University, New York, New York 10027, USA.
The larynx, the vocal organ of Xenopus laevis, is sexually differentiated; male laryngeal muscle fibers are entirely fast twitch while female fibers are predominantly slow twitch. In adults, all male laryngeal muscle fibers express the mRNA for a laryngeal-specific myosin heavy chain (MHC), LM; female laryngeal muscle expresses LM in a subset of fast-twitch fibers. In juvenile females, LM expression is increased by exposure to exogenous androgen, suggesting that sexually differentiated expression of this laryngeal-specific MHC is regulated by exposure to male sex hormones. Here we examine hormonal regulation of LM expression in juvenile male and female frogs. Exposure to exogenous androgen increases LM expression in both sexes. In situ hybridization analyses of larynges reveal upregulation of LM expression within 8 hr of androgen treatment in males; upregulation is not evident until after 48 hr in females. The upregulation in juvenile females includes both an increase in the number of muscle fibers expressing LM and an increase in expression in fibers already expressing LM. In juvenile males, all fibers express LM from the end of metamorphosis on; levels of LM expression are increased in all fibers by androgen treatment. Androgen-induced increases in LM expression are prevented by treatment with cycloheximide and are thus dependent upon protein synthesis. Castration of juvenile males results in diminished LM expression relative to intact animals. We conclude that expression of LM is regulated by exposure to androgen and that this regulation accounts for the sexually differentiated phenotype.