Mechanistic basis of life history evolution in anuran amphibians: thyroid gland development in the direct-developing...

Direct development is a widespread, alternative life history in Recent amphibians. There is no free-living, aquatic larva; adult features form in the embryo and are present at hatching. The mechanistic bases of direct development remain relatively unexplored. The current study describes the embryonic ontogeny of the thyroid gland in the direct-developing frog Eleutherodactylus coqui (Leptodactylidae) and quantifies histological changes that occur in the gland after its initial appearance. The thyroid gland of E. coqui is first apparent at Townsend-Stewart stage 10, approximately two-thirds of the way through embryogenesis. Soon after this the thyroid begins to accumulate follicular colloid. Quantitative analyses of thyroid histology reveal embryonic peaks in two measures, follicle number and follicle volume, which are followed by declines in these measures prior to hatching. These peaks in thyroid activity in E. coqui are correlated with morphological changes that are directly comparable to metamorphic changes in frogs that retain the ancestral, biphasic life history. In metamorphic taxa, a histologically identifiable thyroid gland does not form until the larval period, well after hatching. Nevertheless, measures of thyroid histology observed in E. coqui follow the pattern reported for metamorphosing amphibians. The present results support the hypothesis that the evolution of direct development in anurans is associated with precocious development and activity of the thyroid axis.