GRINDSTAFF, R.D.*; PYLES, R.A.: Thyroid and prolactin hormones in metamorphic and paedomorphic species of ambystomatid salamanders
The endocrine control of metamorphosis, long identified with circulating levels of thyroid hormone and prolactin, has been studied almost exclusively in anurans with little research on hormonal control of metamorphosis among urodeles. Comparisons of hormone levels in metamorphic and paedomorphic salamanders should clarify the roles of tetraiodothyronine (T4) and prolactin in regulating the structural and functional changes associated with metamorphic processes. This study provides measurements of circulating thyroid and prolactin hormones at different ontogenetic stages in three closely related species of ambystomatid salamanders with differing life histories—an obligate paedomorph, an obligate metamorph, and a facultative paedomorph. Quantification of thyroid hormone was obtained by fluorescein polarization immunoassay, and of prolactin by microparticle enzyme immunoassay. Among species comparisons show T4 levels are significantly different in all developmental stages except metamorphosing Ambystoma maculatum and A. talpoideum. Overall T4 hormone concentrations in A. mexicanum were higher than those of A. talpoideum in all developmental stages, contradicting the assumption that obligately paedomorphic species lack a functional thyroid gland. Higher levels of T4 in paedomorphic species suggests T4-induced metamorphic changes are probably regulated at target tissues, either by controlling receptor availability or preventing conversion of T4 to triiodothyronine in peripheral tissue. Our data also show prolactin secretion increases as metamorphosis continues in all species, indicating prolactin is unlikely as a primary inhibitor of metamorphosis.
