Thyroid hormone-dependent metamorphosis in a direct developing frog

Abstract

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 ontogeny. Expression patterns of the thyroid hormone receptors, TRalpha and TRbeta, were similar to those of indirect developers. TRbeta mRNA levels increased dramatically around the time of thyroid maturation, when remodeling events reminiscent of metamorphosis occur. Treatment with the goitrogen methimazole inhibited this remodeling, which was reinitiated on cotreatment with TH. Despite their radically altered ontogeny, direct developers still undergo a TH-dependent metamorphosis, which occurs before hatching. We propose a new model for the evolution of anuran direct development.

Figure 1

PCR specificity. (A) The TRα- and TRβ-specific primers do not amplify the paralogous receptors. Plasmids were used as templates. dCTP (1 μCi) was added to each reaction, and the gel was exposed to film. (B) The TRα- and TRβ-specific oligo probes do not crosshybridize with the paralogous receptor PCR products. TS12 embryo cDNA was used as template. Controls are water blanks.

Figure 2

Developmental expression of TRs, analyzed by RT-PCR (Left). 2wk ph, 2 wk posthatching. Drawings of selected stages are depicted over the relevant lanes. No products were detected in the absence of RT (data not shown). TR expression in full grown unmatured oocytes relative to TS12 levels (Right). L8 is the loading control.

Figure 3

Developmental effects of methimazole treatment. All E. coqui specimens are early posthatching stage. Scale bars represent 2 mm. (A) Control froglet with regressing tail. Adult skin characters include leg bands (black arrowhead), tubercles (white arrowhead), and raised dorsolateral ridges (white arrow). (B) Methimazole-treated sibling. (C) T₃-rescued froglet. (D) Control froglet, ventral view. Skin has sutured (arrowhead). Black line indicates “snout–upper jaw” length, and white line represents the “lower jaw–suture line” length, measured for morphometric analysis (Table 2). (E) Methimazole-treated embryo, with a gap between head and trunk skin (arrowhead). (F) T₃-rescued embryo. (G) Suture marks of newly metamorphosed Rana pipiens (arrowhead). (H–J) Meckel's cartilage (arrow) in control (H), methimazole-treated (I), and T₃-rescued animals (J). (K) Control froglet, exhibiting convergence of the rectus abdominis (*) on the midline (arrowhead) and posterior elongation of the pectoral muscles (white arrow), events inhibited by methimazole (L) and rescued by T₃ (M).

Figure 4

Models of direct development. A and B based on ref. . E, embryogenesis; M, metamorphosis.