HAYES, Tyrone B.; Univ. of California, Berkeley: Evolutionary Developmental Endocrinology: Evolution of Mechanisms and Mechanisms of Evolution

As in all vertebrates, hormones are critical in amphibian growth and development. Thyroid hormones regulate larval development and metamorphosis and have a permissive role in sexual development, glucocorticoids modulate thyroid hormone activity and effect immune function and somatic growth, and estrogens and androgens regulate sex differentiation. These developmental effects of hormones play important roles in the evolution of amphibians. As examples, estrogen receptor expression is regulated by thyroid hormones in frogs and this mechanism may have served as a developmental constraint that prevented the evolution of neoteny in anurans, early thyroid hormone synthesis and secretion and increased tissue sensitivity are responsible for the evolution of accelerated metamorphosis in desert-adapted Pelobatid frogs, interactions between thyroid hormones and glucocorticoids are responsible for adaptive responses to habitat desiccation, and variation in aromatase expression is responsible for the evolution of sexual dichromatism in African reedfrogs Hyperolius. As a result of the importance of hormones in development and evolution, hormone mimics (endocrine-disrupting contaminants) in the environment may have serious consequences for exposed amphibian populations. Compounds that inhibit thyroid hormone synthesis and/or corticoid synthesis can disrupt metamorphic responses to pond desiccation, compounds that increase corticoid synthesis can cause immuno-suppression and compounds that interfere with androgen and estrogen activity can disrupt sexual development and reproduction. Mixtures of contaminants (such as pesticides in agricultural runoff) can have even more drastic interactive effects. As a result, some populations may have evolved mechanisms of “resistance” to the endocrine-disrupting effects of pesticides ultimately altering the roles of endogenous hormones in development.