AN ENDOCRINE-BASED MODEL FOR DEVELOPMENTAL AND MORPHOGENETIC DIVERSIFICATION IN METAMORPHIC AND PAEDOMORPHIC URODELES

Mechanistic interpretations of the diversity in urodele cranial ontoge nies have focused largely on the primary distinctions of metamorphic v ersus paedomorphic forms and obligate versus facultative expressions o f the latter. These distinctions, however, do not address the underlyi ng spectrum of developmental and morphogenetic patterns in thyroid hor mone (TH)-mediated tissues. This study integrates empirical and compar ative observations on TH-mediated remodelling to formulate a new endoc rine-based model to explain cranial diversification within and between metamorphic and paedomorphic urodeles. The dose-dependent remodelling induced by TH in a metamorphic urodele, the hemidactyliine plethodont id Eurycea bislineata, is compared against ontogenetic and phylogeneti c variation in the same remodelling across Urodela. Immersion of Euryc ea larvae in a T-4 concentration within the range of plasma T-4 levels found in natural Eurycea metamorphs results in rapid, synchronous. an d complete metamorphic tissue responses as in natural plethodontid dev elopment. In contrast, lower doses produce gradual, incomplete remodel ling patterns that bear greater resemblance to nonplethodontid develop ment. A large proportion of remodelling events shows a strong correspo ndence between their sensitivity to TH in Eurycea and both their range of occurrence and developmental sequence in nonplethodontids. Also, t he morphogenesis exhibited by certain tissues at low TH, although aber rant for plethodontids, is similar to natural development in nonpletho dontids. These findings suggest that the widespread dissociation evinc ed by urodele cranial ontogenies may owe more to variable thyroid acti vity than previously realized. In particular, the abruptly metamorphic ontogeny of plethodontids and the varying degrees of metamorphic remo delling exhibited by facultative and obligate paedomorphs would seem t o be explained more parsimoniously by specific changes in the profile of TH activity than by independent changes in individual tissue sensit ivities. The corollary, that tissue sensitivity is largely consented i n urodeles, raises important implications for understanding character evolution, homology, and dissociation in metamorphic systems.