As in insects, a complex interaction of hormones in the amphibian larva precipitates metamorphosis. Ultimately, two major classes of hormones act together to control amphibian metamorphosis: the thyroid hormones (made by the thyroid gland) and prolactin (made by the pituitary gland). Thyroid hormones function somewhat like the molting hormones of insects, in that an increase of their concentration relative to prolactin leads to metamorphosis of the larva into the adult. Prolactin functions somewhat like the juvenile hormones of insects, in that it tempers the action of the thyroid hormones. In most species, thyroid hormones increase dramatically in concentration during metamorphosis and this stimulates resorption of certain larval organs and differentiation of new adult organs.
Developmental biologists often investigate amphibian metamorphosis by experimentally manipulating hormone levels. For example, injection of thyroxine into a young larva can induce metamorphosis, although the injection must be at an appropriate stage of larval development and injection of high levels can lead to developmental abnormalities. If the thyroid gland is removed from a larva, it will not metamorphose into the adult form; moreover, a larva without a thyroid will metamorphose into an adult if thyroid tissue is implanted.
The relative ease with which these and other experimental manipulations of hormone levels can alter metamorphosis indicates that hormones have a profound effect on development. It also indicates that the endocrine system is relatively malleable. These two features suggest that natural selection may dramatically affect the course of animal evolution by altering the endocrine system.
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Peter A. Ensminger