Biomedical subjects

E M Callery

Publications and source records attributed to E M Callery.

4 recordsLinked to original sources

PubMed2001

Frogs without polliwogs: evolution of anuran direct development.

Direct development is the assumption of the adult morphology without progression through an intervening, morphologically distinct, free-living larval phase. We discuss the ecological factors contributing to the evolution of this derived life-history strategy in frogs, and the developmental modifications that facilitate such an unusual mode of embryogenesis. Studies on the Puerto Rican tree frog, Eleutherodactylus coqui, have identified several such modifications, including developmental adaptations for dealing with increased egg size, and loss of tadpole structures. Surprisingly, this direct developer still undergoes a thyroid hormone-dependent metamorphosis, which occurs before hatching. We suggest how the ancestral biphasic developmental pattern may have been rearranged during the evolution of direct development.

Animals↗

PubMed2000

Thyroid hormone-dependent metamorphosis in a direct developing frog.

The direct developing anuran, Eleutherodactylus coqui, lacks a tadpole, hatching as a tiny frog. We investigated the role of the metamorphic trigger, thyroid hormone (TH), in this unusual ontogeny. Expression patterns of the thyroid hormone receptors, TRalpha and TRbeta, were similar to those of indirect developers. TRbeta mRNA levels increased dramatically around the time of thyroid maturation, when remodeling events reminiscent of metamorphosis occur. Treatment with the goitrogen methimazole inhibited this remodeling, which was reinitiated on cotreatment with TH. Despite their radically altered ontogeny, direct developers still undergo a TH-dependent metamorphosis, which occurs before hatching. We propose a new model for the evolution of anuran direct development.

Animals↗

PubMed2000

Opercular development and ontogenetic re-organization in a direct-developing frog.

The direct-developing frog, Eleutherodactylus coqui, has eliminated the tadpole stage from its ontogeny, and lacks many larval characters. We demonstrate that the dermal folds of E. coqui are homologous with the opercular folds of metamorphosing frogs. In both E. coqui and its metamorphic counterparts the opercular folds grow over the developing forelimb before perforating to free the entrapped limb. Opercular perforation in E. coqui occurs even in the absence of the forelimb but shows no signs of thyroid hormone dependence. The condensation of E. coqui development appears due to the excision of the extended larval period of developmental stasis. Analysis of opercular development, when viewed in conjunction with other developmental characters, suggests the ontogenetic period in the ancestral Eleutherodactylus life-history from which the tadpole was likely eliminated.

Animals↗

PubMed1996

Developmental regulation of the urea-cycle enzyme arginase in the direct developing frog Eleutherodactylus coqui.

Direct developing organisms obviate the larval intermediary from their ontogeny, hatching as miniature adults. To investigate this phenomenon, we have examined the developmental expression of arginase in the direct developing frog Eleutherodactylus coqui. An enzyme in the ornithine-urea cycle, the activation of liver arginase is necessary for the switch from ammonotelism to ureotelism which occurs when many frogs metamorphose and assume a terrestrial existence. Arginase enzyme activity is detectable at low levels in late prehatching stages of E. coqui, and increases at hatching, at which point the protein becomes detectable on Western blots. The activity increases gradually during posthatching development, reaching maximal levels at approximately the same time as yolk resorption is completed. Thyroid hormone is responsible for upregulating arginase activity during metamorphosis in Rana, but the role of thyroid hormone in direct developing frogs is unknown. A high dose (250 nM) of the thyroid hormone analogue 3,3'5-triiodo-L-thyronine (T3) caused precocious induction of arginase protein and activity, showing that even in a direct developing frog, some level of responsiveness to the metamorphic trigger, thyroid hormone, has been retained.

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