Variation in thyroid hormone action and tissue content underlies species differences in the timing of metamorphosis in desert frogs.

Hormonal control of post-embryonic morphogenesis is well established, but it is not clear how differences in developmental endocrinology between species may underlie animal diversity. We studied this issue by comparing metamorphic thyroid hormone (TH) physiology and gonad development across spadefoot toad species divergent in metamorphic rate. Tissue TH content, in vitro tail tip sensitivity to TH, and rates of TH-induced tail tip shrinkage correlated with species differences in larval period duration. Gonad differentiation occurred before metamorphosis in species with long larval periods and after metamorphosis in the species with short larval periods. These differences in TH physiology and gonad development, informed by phylogeny and ecology of spadefoot metamorphosis, provide evidence that selection for the short larval periods in spadefoot toads acted via TH physiology and led to dramatic heterochronic shifts in metamorphic climax relative to gonad development.