The direct developing anuran. Eleutherodactylus coqui, lacks a tadpole, hatching as a tiny frog. we investigated the role of the metamorphic trigger, thyroid hormone (TH), in this unusual on- togeny. Expression patterns of the thyroid hormone receptors, TRo and TRp, were similar to those of indirect developers. TRp mRNA levels increased dramatically around the time of thyroid matura- tion, when remodeling events reminiscent of metamorphosis oc- cur. Treatment with the goitrogen methimazole inhibited this remodeling, which was reinitiated on cotreatment with TH. De- spite their radically altered ontogeny, direct developers still un- dergo a TH-dependent metamorphosis, which occurs before hatch- ing. We propose a new model for the evolution of anuran direct development.
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