Sexual dimorphism in the fibers of a "clasp" muscle of Xenopus laevis.

When antibodies specific to fast-twitch, slow-twitch, and slow-tonic myosins were used to stain the clasp muscle, m. sternoradialis, of Xenopus laevis, three predominant fiber types were identified in both males and females. Three fiber types can also be distinguished by the diameters and conduction velocities of their motor nerves. Significant differences in the numbers of slow-tonic fibers were identified between the genders and between control and castrated male animals. This finding suggests that these slow-tonic fibers, which probably dominate the clasp reflex during mating, may be under hormonal control.