Proximate Mechanisms of Phenotypic Plasticity in Amphibian Metamorphosis

Abstract

SYNOPSIS. Amphibians exhibit extreme plasticity in the duration of the larval period. Probably the most important environmental variable for a tadpole is water availability, and some amphibian species have been shown to be capable of accelerating metamorphosis in a desiccating larval habitat. Current data support the view that the response to pond drying is not simply a consequence of unavoidable environmental effects but rather an adaptive response to the risk of mortality in the drying pond. The acceleration of development by pond drying results in the precocious activation of the endocrine systems that control metamorphosis, the thyroid and the interrenal. Furthermore, a single neurohormonal stimulus, corticotropin-releasing hormone (CRH) activates both of these endocrine systems. CRH is the primary vertebrate stress neurohormone; thus, tadpoles mount a classical endocrinological stress response to accelerate development and avoid mortality in the desiccating habitat. Comparison of recent data in mammals with amphibians suggests that the neurohormonal stress response is a phylogenetically ancient developmental trigger that allows the developing organism to assess habitat quality and respond accordingly.