Meeting Abstract
Amphibians acquire bones postembryonically, over larval growth and metamorphosis. They also show considerable interspecific variation in ossification sequence. Most postembryonic changes in metamorphosing amphibians are mediated by thyroid hormone (TH) and many frogs have developmental patterns that follow a universal (Gosner) staging system. This conservancy implies that most TH-mediated development remains tightly coordinated across frogs, and that interspecific variation in ossification sequence is more likely the result of changes in TH sensitivity than in TH production. This study quantifies the effects of TH type (T4 and T3), dose and larval stage on bone development in Xenopus laevis with the aim of comparing induced and natural ossification sequences in this and other species. We examined a developmental series of untreated specimens as well as specimens from a study of TH-induced cartilage development that were treated at early, mid and late larval stages with 1, 5, 10 and 50 nM T4 or T3. The thyroid inhibitors methimazole and iopanoic acid were additionally used to isolate the effect of each hormone. With the exception of T4 at the lowest dose and stage, both hormones only induced more ossification than in controls at late larval stages. TH-induced sequences were generally similar to natural sequences, with earlier appearing bones being induced at low stages and doses, and later ones being induced at higher stages and doses. The maxilla, nasal, premaxilla, and vomer were sometimes omitted, and the orbitosphenoid induced ahead of others. These results are consistent with tissue sensitivity accounting for variation in ossification sequence. This study emphasizes the value of interpreting amphibian ossification sequences in the context of the neuroendocrine mechanisms regulating developmental and life history stages.
