Stage-modified responses to thyroid hormone in anurans

Abstract

Tadpoles from a single clutch of Rana pipiens eggs were cultured at different temperatures in order to achieve at a single time a broad span of developmental stages (I+ through XII) and sizes (20–60 mm). They were placed in dl-thyroxine solutions at concentrations of 50 μg/liter, and maintained at 23°C, until death ensued (approximately 14 days for the largest, and 22 days for the smallest, animals). Initially all animals grew, the larger animals less than the smaller. Tail shortening occurred in all, with latent periods being least in the large and greatest in the small tadpoles. Also, changes in limb characteristics (a measure of metamorphic progress) were much greater in the small than in the large animals, before tail shortening began. After tail shortening began, changes in limb form and rate of tail loss were much alike for tadpoles of all sizes. The differences in latent period correlate inversely with stage and size, and thus compel the conclusion that capacity (i.e., sensitivity) of the tail to respond to thyroid hormone stimulation increases with developmental stage.