Gill Resorption Research Articles

What frog gill resorption brings: loss of function, cell death, and metabolic reorganization

BackgroundAnuran metamorphosis, which is driven by thyroid hormone (TH)-mediated processes, orchestrates intricate morphological and functional transformations for the transition from aquatic tadpoles to terrestrial life, providing a valuable model for studying organ functionalization, remodeling, and regression. Larva-specific organ regression is one of the most striking phenomena observed during the anuran metamorphic climax. While previous studies extensively analyzed the regression mechanisms of the tail, the molecular processes governing gill resorption remain elusive.ResultsWe employed Microhyla fissipes as a model, and utilized a comprehensive approach involving histological analysis, transmission electron microscopy, and transcriptomics to unravel gill development and resorption. The pro-metamorphic stages revealed highly developed gill structures, emphasizing their crucial role as the primary respiratory organ for tadpoles. The transcriptomic analysis highlighted the upregulation of genes associated with enhanced respiratory efficiency, such as hemoglobin and mucins. However, as metamorphosis progressed, gill filaments underwent shrinkage, decreases in blood vessel density, and structural changes that signified a decline in respiratory function. The molecular mechanisms driving gill resorption involved the TH pathway—in particular, the upregulation of thyroid hormone receptor (TR) β, genes associated with the tumor necrosis factor pathway and matrix metalloproteinases. Two distinct pathways orchestrate gill resorption, involving apoptosis directly induced by TH and cell death through the degradation of the extracellular matrix. In addition, metabolic reorganization during metamorphosis is a complex process, with tadpoles adapting their feeding behavior and mobilizing energy storage organs. The gills, which were previously overlooked, have been unveiled as potential energy storage organs that undergo metabolic reorganization. The transcriptomic analysis revealed dynamic changes in metabolism-related genes, indicating decreased protein synthesis and energy production and enhanced substrate transport and metabolism during metamorphic climax.ConclusionThis study sheds light on the structural, molecular, and metabolic dynamics during gill development and resorption in M. fissipes. The findings deepen our understanding of the intricate mechanisms governing organ regression and underscore the pivotal role of the gills in facilitating the transition from aquatic to terrestrial habitats.

Glucocorticoid receptor mediates corticosterone-thyroid hormone synergy essential for metamorphosis in Xenopus tropicalis tadpoles

In all vertebrates, thyroid hormone (TH) is critical for normal growth and development. In amphibians, corticosterone (CORT) has no action to advance development by itself but can accelerate development induced by TH. CORT accomplishes this acceleration by increasing tissue sensitivity and responsivity to TH. However, the receptor through which CORT acts to affect TH signaling is not known. To examine the role of the glucocorticoid receptor (GR), GR knockout tadpoles and wild-type tadpoles treated with the GR antagonist, RU486, were exposed to exogenous TH and/or CORT then assayed for gene expression and morphology. We found that levels of the response genes klf9 and thrb induced by TH and associated changes in morphology were decreased in GR knockout tadpoles compared to wild-type tadpoles, suggesting that GR signaling contributes to tissue responsivity to TH. To directly examine the role of GR in TH signaling, we co-treated tadpoles with TH and CORT and found that the TH response gene, thrb, was induced significantly beyond the level induced by TH alone in wild-type tadpoles but not in GR knockout tadpoles or wild-type tadpoles treated with RU486. Similarly, tail and gill resorption was greater in tadpoles treated with CORT plus TH compared to TH alone in wild-type tadpoles but not in tadpoles with impaired GR signaling. Surprisingly, even though GR knockout tadpoles die at metamorphosis, treatment with TH alone enabled their survival. These results demonstrate that signaling through GR is responsible for enhancing TH signaling and is essential for the completion of metamorphosis.

Functional analysis of thyroid hormone receptor beta in Xenopus tropicalis founders using CRISPR-Cas.

ABSTRACTAmphibians provide an ideal model to study the actions of thyroid hormone (TH) in animal development because TH signaling via two TH receptors, TRα and TRβ, is indispensable for amphibian metamorphosis. However, specific roles for the TRβ isoform in metamorphosis are poorly understood. To address this issue, we generated trβ-disrupted Xenopus tropicalis tadpoles using the CRISPR-Cas system. We first established a highly efficient and rapid workflow for gene disruption in the founder generation (F0) by injecting sgRNA and Cas9 ribonucleoprotein. Most embryos showed severe mutant phenotypes carrying high somatic mutation rates. Utilizing this founder analysis system, we examined the role of trβ in metamorphosis. trβ-disrupted pre-metamorphic tadpoles exhibited mixed responsiveness to exogenous TH. Specifically, gill resorption and activation of several TH-response genes, including trβ itself and two protease genes, were impaired. However, hind limb outgrowth and induction of the TH-response genes, klf9 and fra-2, were not affected by loss of trβ. Surprisingly, trβ-disrupted tadpoles were able to undergo spontaneous metamorphosis normally, except for a slight delay in tail resorption. These results indicate TRβ is not required but contributes to the timing of resorptive events of metamorphosis.

Characterization of a novel type I keratin gene and generation of transgenic lines with fluorescent reporter genes driven by its promoter/enhancer in Xenopus laevis

We investigated the characteristics of a novel type I keratin gene in Xenopus laevis during ontogenesis. The transcript was first detected in the posterior region at the late neurula stage, and then restricted to the fin and external gill during embryogenesis. To examine the transcriptional regulation of the keratin gene in vivo, we generated transgenic lines with fluorescent reporter genes driven by its 4.2-kb upstream sequence. The promoter/enhancer activity recapitulated the endogenous gene expression during embryogenesis. Sequential deletion analyses revealed that the regions proximal to the promoter were essential for fin-specific expression. Reporter expression was detected in various organs, including the fin and gill. In particular, robust expression was observed in the developing limbs and gill. The reporter fluorescence rapidly decreased with internal gill resorption during metamorphosis. The transgenic lines carrying the promoter/enhancer should represent valuable tools for elucidating the formation, development and resorption of various organs, especially the gill.

Low submetamorphic doses of dexamethasone and thyroxine induce complete metamorphosis in the axolotl (Ambystoma mexicanum) when injected together

Entanglement of functions between the adrenal (or interrenal) and thyroid axis has been well described for all vertebrates and can be tracked down up to the level of gene expression. Both thyroid hormones and corticosteroids may induce morphological changes leading to metamorphosis climax in the neotenic Mexican axolotl (Ambystoma mexicanum). In a first series of experiments, metamorphosis was induced with an injection of 25 μg T4 on three alternate days as judged by a decrease in body weight and tail height together with complete gill resorption. This injection also resulted in elevated plasma concentrations of T3 and corticosterone. Previous results have indicated that the same dose of dexamethasone (DEX) is ineffective in this regard (Gen. Comp. Endocrinol. 127 (2002) 157). In a second series of experiments low doses of T4 (0.5 μg) or DEX (5 μg) were ineffective to induce morphological changes. However, when these submetamorphic doses were injected together, morphological changes were observed within one week leading to complete metamorphosis. It is concluded that thyroid hormones combined with corticosteroids are essential for metamorphosis in the axolotl and that only high doses of either thyroid hormone or corticosteroid can induce morphological changes when injected separately.

Low submetamorphic doses of dexamethasone and thyroxine induce complete metamorphosis in the axolotl ( Ambystoma mexicanum) when injected together

Entanglement of functions between the adrenal (or interrenal) and thyroid axis has been well described for all vertebrates and can be tracked down up to the level of gene expression. Both thyroid hormones and corticosteroids may induce morphological changes leading to metamorphosis climax in the neotenic Mexican axolotl ( Ambystoma mexicanum). In a first series of experiments, metamorphosis was induced with an injection of 25 μg T 4 on three alternate days as judged by a decrease in body weight and tail height together with complete gill resorption. This injection also resulted in elevated plasma concentrations of T 3 and corticosterone. Previous results have indicated that the same dose of dexamethasone (DEX) is ineffective in this regard (Gen. Comp. Endocrinol. 127 (2002) 157). In a second series of experiments low doses of T 4 (0.5 μg) or DEX (5 μg) were ineffective to induce morphological changes. However, when these submetamorphic doses were injected together, morphological changes were observed within one week leading to complete metamorphosis. It is concluded that thyroid hormones combined with corticosteroids are essential for metamorphosis in the axolotl and that only high doses of either thyroid hormone or corticosteroid can induce morphological changes when injected separately.

Effects of dexamethasone treatment on iodothyronine deiodinase activities and on metamorphosis-related morphological changes in the axolotl ( Ambystoma mexicanum)

In amphibians, there is a close interaction between the interrenal and the thyroidal axes. Hypothalamic corticotropin-releasing hormone or related peptides stimulate thyroidal activity by increasing thyrotropin synthesis and release, while corticosterone accelerates both spontaneous and thyroid hormone-induced metamorphosis. One of the mechanisms that is thought to contribute to this acceleration is a corticosterone-induced change in peripheral deiodinating activity. The present experiments were designed to investigate further the effects of glucocorticoid treatment on amphibian deiodinase activities and to explore the possible role of these effects in metamorphosis. Neotenic axolotls ( Ambystoma mexicanum) were treated either acutely or chronically with dexamethasone (DEX) and changes in type II and type III iodothyronine deiodinase (D2 and D3) activities were studied in liver, kidney, and brain. In addition, gill length, tail height, and body weight were measured at regular intervals in the chronically treated animals in search of metamorphosis-related changes. A single injection of 50 μ g DEX decreased hepatic D3 activity (6–48 h) while it increased D2 activity in brain (6–48 h) and to a lesser extent in kidney (24 h). These changes were accompanied by an increase in plasma T 3 levels (48 h). Samples taken during chronic treatment with 20 or 100 μ g DEX showed that both hepatic D2 and D3 activities were decreased on day 26, while renal D3 activity was decreased but only in the 20 μ g dose group. All other deiodinase activities were not different from those in control animals. At 25 days, all DEX-treated axolotls showed a clear reduction in gill length, tail height, and body weight, changes typical of metamorphosis. Prolongation of the treatment up to 48 days resulted in complete gill resorption by days 44–60. Although probably several mechanisms contribute to these DEX-induced metamorphic changes, the interaction with thyroid function via a sustained downregulation of hepatic D3 may be one of them.

Acceleration of Ambystoma tigrinum metamorphosis by corticotropin-releasing hormone.

Previous work of others and ours has shown that corticotropin-releasing hormone (CRH) is a positive stimulus for thyroid and interrenal hormone secretion in amphibian larvae and that activation of CRH neurons may mediate environmental effects on the timing of metamorphosis. These studies have investigated CRH actions in anurans (frogs and toads), whereas there is currently no information regarding the actions of CRH on metamorphosis of urodeles (salamanders and newts). We tested the hypothesis that CRH can accelerate metamorphosis of tiger salamander (Ambystoma tigrinum) larvae. We injected tiger salamander larvae with ovine CRH (oCRH; 1 microg/day; i.p.) and monitored effects on metamorphosis by measuring the rate of gill resorption. oCRH-injected larvae completed metamorphosis earlier than saline-injected larvae. There was no significant difference between uninjected and saline-injected larvae. Mean time to reach 50% reduction in initial gill length was 6.9 days for oCRH-injected animals, 11.9 days for saline-injected animals, and 14.1 days for uninjected controls. At the conclusion of the experiment (day 15), all oCRH-injected animals had completed metamorphosis, whereas by day 15, only 50% of saline-injected animals and 33% of uninjected animals had metamorphosed. Our results show that exogenous oCRH can accelerate metamorphosis in urodele larvae as it does in anurans. These findings suggest that the neuroendocrine mechanisms controlling metamorphosis are evolutionarily conserved across amphibian taxa.

A posthatching developmental staging table for the long-toed salamander, Ambystoma macrodactylum krausei

AbstractA posthatching developmental staging table for the larval phase of an ambystomatid salamander is provided for the first time. Stage boundaries are based upon limb development prior to the metamorphic transition and upon gill resorption during metamorphosis. Comparison to other published staging tables for salamanders of other families and anecdotal accounts of other ambystomatids is made, and similarities and differences are noted. These comparative data suggest that limb development may differ between species of Ambystoma in terms of the relative timing of hindlimb and forelimb morphogenesis.

Effects of hatching age on development and hatchling morphology in the red-eyed tree frog, Agalychnis callidryas

The red-eyed treefrog, Agalychnis callidryas, lays eggs on leaves overhanging ponds. Tadpoles hatch and enter the water at different ages, and late-hatched tadpoles survive aquatic predators better than do early-hatched tadpoles. Here I assess developmental consequences of hatching age through: (1) a morphological study of embryos and tadpoles through the plastic hatching period; (2) a behavioural assay for an effect of hatching age on feeding; and (3) a field experiment testing the effect of hatching age on growth to metamorphosis. Substantial development of feeding, digestive, respiratory and locomotor structures occurs in embryos over the plastic hatching period. Hatchling morphology thus varies with age, with consequences for behaviour and predation risk. Hatched tadpoles develop faster than embryos, and early-hatched tadpoles feed before late-hatched tadpoles. After all tadpoles have hatched, the effect of hatching age on size decreases. I found no evidence for an effect of hatching age on size at metamorphosis and only weak evidence for an effect on larval period. Hatching age affects the sequence of developmental change: early-hatched tadpoles lose external gills while otherwise more developed embryos maintain them. Plasticity in external gill resorption may be adaptive given differences in the respiratory environments of embryos and tadpoles. Early-hatched tadpoles also diverge from embryos in shape, growing relatively smaller tails. The study of functional morphology and developmental plasticity will contribute to understanding hatching as an ontogenetic niche shift.

Effects of hatching age on development and hatchling morphology in the red-eyed treefrog, Agalychnis callidryas

The red-eyed treefrog, Agalychnis callidryas, lays eggs on leaves overhanging ponds. Tadpoles hatch and enter the water at different ages, and late-hatched tadpoles survive aquatic predators better than do early-hatched tadpoles. Here I assess developmental consequences of hatching age through: (1) a morphological study of embryos and tadpoles through the plastic hatching period; (2) a behavioural assay for an effect of hatching age on feeding; and (3) a field experiment testing the effect of hatching age on growth to metamorphosis. Substantial development of feeding, digestive, respiratory and locomotor structures occurs in embryos over the plastic hatching period. Hatchling morphology thus varies with age, with consequences for behaviour and predation risk. Hatched tadpoles develop faster than embryos, and early-hatched tadpoles feed before late-hatched tadpoles. After all tadpoles have hatched, the effect of hatching age on size decreases. I found no evidence for an effect of hatching age on size at metamorphosis and only weak evidence for an effect on larval period. Hatching age affects the sequence of developmental change: early-hatched tadpoles lose external gills while otherwise more developed embryos maintain them. Plasticity in external gill resorption may be adaptive given differences in the respiratory environments of embryos and tadpoles. Early-hatched tadpoles also diverge from embryos in shape, growing relatively smaller tails. The study of functional morphology and developmental plasticity will contribute to understanding hatching as an ontogenetic niche shift.

Effects of metamorphosis and captivity on the in vitro sensitivity of thyroid glands from the tiger salamander, Ambystoma tigrinum, to bovine thyrotropin

The sensitivity of thyroid glands from the tiger salamander, Ambystoma tigrinum, to bovine thyrotropin (bTSH) was tested in vitro. Thyroids were taken from subjects representing metamorphic stages I (premetamorphic larvae), II (onset of climax), and VII (completion of gill resorption), as well as from captivity control larvae. Exogenous TSH reduced the cumulative uptake of 125I in vitro by thyroids from stage I larvae after 24 and 48 hr. The capacity of thyroids to release thyroxine (T 4) in vitro was used subsequently as a measure of their responsiveness to TSH. Baseline levels of T 4 release in vitro were variable but did not differ significantly among developmental stages. A low dose of bTSH (5 × 10 −6 IU/ml) did not increase in vitro T 4 release compared with that of controls. A larger dose (5 × 10 −4 IU/ml) caused greater increases in T 4 release from thyroids of stage II and VII subjects than from those of controls. This dose produced only a small response by thyroids from captivity-control subjects. The results suggest that the thyroids of Ambystoma increase in their capacity to respond to TSH during the process of metamorphosis.

Adenohypophysial-thyroid activity of the tiger salamander, Ambystoma tigrinum, as a function of metamorphosis and captivity.

The aim of this study was to determine the timing of adenohypophysial activation during metamorphosis of the tiger salamander, Ambystoma tigrinum. It consisted of two parts: 1) determination of plasma thyroid hormone concentrations and analysis of thyroid gland histology as a function of metamorphic stage and 2) analysis of the time-course of uptake of 125I by the thyroids during metamorphosis as an indicator of endogenous thyrotropin (TSH) levels. Significant increases in both triiodothyronine (T3) and thyroxine (T4) first were evident at the onset of metamorphic climax (stage II). Maximum levels of both hormones were not observed, however, until the completion of gill resorption (stage VII). No changes in thyroid histology were observed that could be unambiguously related to metamorphic transformation. The thyroids accumulated 125I in a slow but linear fashion in premetamorphic larvae (stage I). However, uptake exhibited a rapid peak during early climax (stage II), before maximum concentrations of thyroid hormones were observed. In addition, uptake was maintained above premetamorphic levels at stage VII, in conjunction with maximum levels of T4 and T3. Captivity alone produced a small but significant increase in plasma concentrations of T3. It produced no significant effect on either thyroid histology or uptake of 125I. These results indicate that adenohypophysial activation occurs rapidly and is maximal at the onset of metamorphic climax.

Effects of delaying injections of prolactin and actinomycin D on metamorphic changes in thyroxine‐treated Ambystoma tigrinum

AbstractAs a part of our continuing effort to understand the nature of the inhibition of thyroxine‐induced metamorphosis by prolactin in the tiger salamander, we examined the effects of delaying the initiation of prolactin treatment for progressively longer time intervals following the injection of a metamorphosing dose of thyroxine. Prolactin continued to inhibit gill resorption when given either 2 or 4 days following thyroxine, but the magnitude of the inhibition appeared to diminish. Delay of prolactin treatment until day 6 abolished the inhibition. In contrast, tail fin regression appeared to be inhibited in an undiminished fashion by prolactin given 2, 4, or 6 days following thyroxine. Body weight loss was inhibited in a similar manner. In a second experiment, actinomycin D, a transcription inhibitor, was observed to inhibit metamorphic changes. A delay of 5 days in the administration of this drug relative to thyroxine treatment produced a significant reduction in its inhibition of gill resorption, tail fin regression, and body weight loss. Surprisingly, late (day 5 following thyroxine) treatments with prolactin still produced a significant inhibition of fin regression and weight loss even when the inhibitory dose of actinomycin D was given on day 5 along with the prolactin. These results are discussed with respect to their implications concerning our understanding of the mechanism of prolactin action during amphibian metamorphosis.

Inhibition of the antimetamorphic action of prolactin in larval Ambystoma tigrinum by lysine vasopressin, arginine vasotocin, and aldosterone.

Abstract In an earlier study it was reported that oxytocin was able to block the antimetamorphic action of prolactin (PRL) in larval tiger salamanders, Ambystoma tigrinum ( Platt and LiCause, 1980 ). In this study, we have examined the effects of lysine vasopressin (LVP), arginine vasotocin (AVT), and aldosterone on induced metamorphosis in larval A. tigrinum and on the inhibition of this process by PRL. Both LVP and AVT were found to interfere with the inhibition of gill resorption, tail fin regression, and body weight loss by PRL. An anti-PRL effect of aldosterone was observed but appeared to be limited to the gill. AVT alone had a significant acceleratory effect on body weight loss during metamorphosis. A “water balance” response was observed in only one case. Twenty-four hours after treatment with LVP and PRL, this group of animals did show a significant increase in body weight. These results are discussed with regard to the problem posed by neurohypophysial hormone contamination of PRL preparations and in context with our hypothesis that the antimetamorphic action of PRL is at least partially osmoregulatory in nature.

Effects of oxytocin in larval Ambystoma tigrinum: Acceleration of induced metamorphosis and inhibition of the antimetamorphic action of prolactin

Larval tiger salamanders induced to metamorphose with thyroxine were injected with oxytocin (100 mU/day) in order to assess the potential effect of neurohypophyseal contamination on the antimetamorphic activity of mammalian prolactin preparations. Unexpectedly, oxytocin caused a marked acceleration of tail fin regression and body weight loss and a slight acceleration of gill resorption. An additional experiment revealed that the decreases in tail height and body weight observed in the initial experiment were induced by oxytocin in animals that were not treated with thyroxine. Prolactin was only partially effective in blocking these effects of oxytocin. In animals induced to metamorphose and treated with oxytocin, prolactin, or both hormones, tail fin regression and weight loss were accelerated by oxytocin and antagonized by prolactin. Prolactin partially reversed the effects of oxytocin on tail height and body weight. In this experiment, oxytocin failed to accelerate gill resorption while prolactin inhibited this process. Oxytocin was without effect on prolactin inhibition of gill resorption. Incubation of tail tissue with oxytocin in vitro induced water loss compared to paired control tissue pieces and the addition of prolactin to the incubation medium caused only a slight inhibition of the oxytocin-induced water loss. These findings are discussed in relation to the hypothesis that the antimetamorphic action of prolactin is osmoregulatory in nature.

Metamorphosis and responses of the skin and thyroid gland in Pleurodeles submitted to an experimental aeroionization. (author's transl)

Abstract Metamorphosis and responses of the skin and thyroid gland in Pleurodeles submitted to an experimental aeroionization. When Pleurodeles (Amphibian, Urodela) larvae (age: 10 weeks) are submitted to aeroionization for 18 days (once and then twice a day, ion concentration: 500,000/ml), the percentage of animals completing their metamorphosis is affected: compared with control animals, this percentage increases significantly with a negative aeroionization (AE(−)) and decreases with a positive air ionization (AE(+)), according to external criteria (gill resorption, skin texture). These morphological observations were confirmed by a histological study of the skin: the disappearance of the Leydig cells and the skin keratinization occur more often in Pleurodeles after AE(−) than in controls and less frequently after AE(+). In control animals, a high thyroid activity is recorded at the beginning of metamorphosis and tends to decline at the end, according to histological criteria. An AE(−) does not seem to affect thyroid activity in animals not yet having completed their metamorphosis, but maintains a higher activity in animals reaching the last stage of metamorphosis than in controls at the same stage. Conversely, an AE(+) prevents the initial activation of the thyroid gland; later, this hypoactivity is much less evident. Air ions of opposite polarities seem to have opposite effects on thyroid activity: negative ions would be stimulating and positive air ions depressing. Some behavioral changes are noted simultaneously. In addition to thyroid hormones, various factors may be involved in inducing or inhibiting metamophosis. The possible interference of prolactin secretion is discussed.

The induction of collagenase and acid phosphatase by thyroxine in resorbing tadpole gills in vitro

AbstractExplants of tadpole gills were cultured in Hanks balanced salt solution or in Hanks supplemented with thyroid hormone. Gills in control media maintain their integrity in vitro while those treated with thyroxine undergo resorption. The activity of collagenase in the media increases in parallel with thyroxine induced resorption and is accompanied by a decrease in tissue collagen levels. T4 induced gill resorption is also associated with an increase in the specific activity of acid phosphatase. However, this increase is not the result of a significant increase in total enzyme activity but rather the result of a differential loss in total protein concentration in thyroxine treated gills as compared to control.

Thyroxine-induced gill resorption in the axolotl (Ambystoma mexicanum).

AbstractNormal gill structure and thyroxine induced resorptive changes were studied in Ambystoma mexicanum. The gill is normally composed of a mesenchymal core covered with a multilayered epithelium. The general architecture is simpler than that of the teleost and elasmobranch, but the vascular arrangement is analogous. There are three basic cell types in the epithelium: a characteristic epithelial cell containing tonofibrils and mucus, a ciliated cell with an ultrastructure similar to that of the chloride cell, and the mucin‐filled Leydig cell. The basal lamella and mesenchymal tissue appear typical of amphibians.Cytologic changes during thyroxine induced gill resorption varied with cell type. Some epithelial cells demonstrated a cytoplasmic response with swelling of mitochondria and rough endoplasmic reticulum and late, lytic nuclear changes, while others remained viable and went on to cornify. Ciliated cells showed early changes in nuclear chromatin pattern followed by rapid, progressive dilatation of endoplasmic reticulum. Leydig cells sustained variable changes leading to collapse of the perinuclear mucus, and cells of this type were absent in mature epidermis. Early basement membrane changes included widening and reduplication of the adepidermal membrane followed by morphologic fraying of collagen plies. There is no cytologic evidence to suggest that autolysis plays a major role in gill tissue dissolution.Resorption involved the maintenance of structural integrity in the face of diminishing physical dimensions. The epithelium became cornified, the basement lamellae dissolved, and the mesenchymal tissue was resorbed through action of macrophages in an orderly distal to proximal direction.

T3‐ and T4‐induced rates of metamorphosis in immature and sexually mature larvae of Ambystoma tigrinum (amphibia: Caudata)

AbstractRates of T3‐ and T4‐induced metamorphosis in sexually mature and immature tiger salamander larvae (Ambystoma tigrinum) from a single local population were proportional to daily injected doses (1.0, 10 1, 10 2, or 10 3 μg T3, or T4). Larvae were maintained at 16° C on 12L:12D photoperiod, and were fed beef liver daily. Metamorphic rates were compared as the mean number of days to achieve 50% resorption of the gills (GR50). Saline‐injected and untreated control larvae exhibited no metamorphic tendencies during the experiments. Immature larvae were induced to metamorphose with as little as 0.05 ng T3 or T4/g body weight/day. Small, sexually mature larvae were less sensitive to a given dose of T4, especially at the lowest doses. Large sexually mature larvae did not differ from small larvae. It is presumed that the observed differences in sensitivity would not account solely for the observations of neoteny in this species. Metamorphosis was not influenced by sex. T3 and T4 are equipotent inducers of metamorphosis in immature larvae under conditions of this experiment except that some animals receiving the lowest dose of T3 did not undergo metamorphosis. Larvae receiving 10−1 μg T4/day did not metamorphose at 5° C, and gill resorption was slower for larvae at 10° C as compared to 16° C. GR50 for larvae maintained at 21° C was not different from 16° C.