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Metamorphosis

Hormones

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.

Resources

Books

Carde, Ring, and Vincent H. Resh, eds. Encyclopedia of Insects. San Diego, CA: Academic Press, 2003.

Chu, H.F., and L.K. Cutkomp. How to Know the Immature Insects. 2nd ed. Burr Ridge, IL: McGraw-Hill Higher Education, 1992.

Fox, H. Amphibian Morphogenesis. Totowa, NJ: Humana Press, 1984.

Quiri, P.R. Metamorphosis. Danbury, CT: Franklin Watts, Inc., 1991.

Peter A. Ensminger

KEY TERMS

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Alternation of generations: —General feature of the life cycle of many plants, characterized by the occurrence of multiple reproductive forms which often have very different overall body patterns.
Endocrine system: —A system of glands and other structures that secrete hormones to regulate certain body functions such as growth and development of sex characteristics.
Evolution: —In biology, inheritable changes occurring over a time span greater than one generation.
Gene: —A discrete unit of inheritance, represented by a portion of DNA located on a chromosome. The gene is a code for the production of a specific kind of protein or RNA molecule, and therefore for a specific inherited characteristic.
Gene expression: —Molecular process in which a gene is transcribed into a specific RNA (ribonucleic acid), which is then translated into a specific protein.
Hormone: —Chemical regulator of physiology, growth, or development which is typically synthesized in one region of the body and active in another and is typically active in low concentrations.
Imago: —Adult form of an insect which develops from a larva and often has wings.
Larva: —Immature form (wormlike in insects; fishlike in amphibians) of a metamorphic animal which develops from the embryo and differs radically from the adult.
Molting: —Shedding of the outer layer of an animal, such as the cuticle during growth of insect larvae.
Natural selection: —The differential survival and reproduction of organisms, producing evolutionary change in populations.
Neoteny: —Retention of larval or juvenile characteristics in a sexually mature adult.

Additional topics

Science EncyclopediaScience & Philosophy: Mathematics to Methanal trimerMetamorphosis - General Features, Insects, Hormones, Amphibians