Experiments on the metamorphosis of neotenous amphibians

Reduced effects of thyroid hormone on gene expression and metamorphosis in a paedomorphic plethodontid salamander

“…Previous hypotheses have suggested that larval paedomorphosis is a result of reduced activity of the thyroid gland and consequently lower endogenous thyroid hormone (or thyroxine) levels (reviewed in Norris and Platt, ). Other hypotheses propose that paedomorphosis may also have arisen from reduced sensitivity of ancestrally metamorphic tissues (e.g., external gills, tailfin, and hyobranchial apparatus) to thyroid hormone or other metamorphic stimulants (Swingle, ; Noble, ; Svob et al, ; Safi et al, ). Here we show that applying a potent dose of thyroid hormone (25 nM) to E. tynerensis larvae from recently derived paedomorphic populations does not recover metamorphosis at the same rate as larvae from metamorphic populations (Fig.…”

“…Two common hypotheses are: (1) lower thyroid activity reduces endogenous thyroid hormone levels (reviewed in Norris and Platt, ), or (2) target tissues evolve reduced sensitivity to thyroid hormone (Noble, ; Svob et al, ; Safi et al, ). The latter hypothesis is based on several obligatory paedomorphic lineages that do not show metamorphic changes (to gills or tailfin) in the presence of exogenous T 3 or T 4 , yet have functional thyroid glands (Swingle, ; Noble, ; Noble and Farris, ; Svob et al, ; Safi et al, ). At least one obligately paedomorphic species ( Necturus maculosus ) has functional TRs in target tissues (e.g., gills; Safi et al, ) that are regulated by TH, but the lack of tissue transformation may be due to a decoupling of TH regulation of downstream target genes that would otherwise result in metamorphosis (Safi et al, ; Vlaeminck‐Guillem et al, ).…”

Thyroid hormone receptor expression in the obligatory paedomorphic salamander Necturus maculosus

“…Necturus TRs also bind T3 and T4 with a high affinity (Kd in the nano molar range) and transactivates target genes in response to TH with EC50 values very similar to the ones found for Xenopus TRs (Safi et al, 2006). The fact that TR genes are expressed and functional led us to question the insensitivity of Necturus tissues treated with extremely high amounts of exogenous TH (Swingle, 1922). In fact we recently demonstrated that, when observed at the gene expression level Necturus tissues are sensitive to TH treatment since we observed an upregulation of TRα, TRβ and stromelysin 3 in gills.…”

“…Thyroid glands of Necturus are however TSH-responsive and capable of function. Implants of Necturus thyroid tissue have been shown to bring about accelerated metamorphosis in frog larvae (Swingle, 1922) and exogenous TSH administration induces hormone secretion by thyroid glands (Kerkof et al, 1963). In addition, thyroid glands of Necturus can respond to implants of anterior pituitaries from Rana pipiens (Grant, 1930).…”

“…Likewise, larvae of Rana clamitans implanted with a piece of an adult Necturus anterior pituitary showed metamorphic features (Charipper and Corey, 1930), suggesting that the Necturus pituitary gland is functional. In contrast, Necturus displays no evidence of metamorphosis when given T4, whole thyroid substance or thyroid implants (Swingle, 1922). In addition, when skin of a species capable of undergoing metamorphosis is transplanted to Necturus and the latter is given T4, the donor skin undergoes metamorphic changes, while that of the host is unaffected (Noble and Richards, 1931).…”