What is the molecular mechanism behind metamorphosis?
Both amphibian metamorphosis and insect metamorphosis are triggered by the release of hormones, but the particular hormones that are used differ in the two kinds of animals.
In the case of amphibians (frogs, salamanders, and similar animals), metamorphosis is triggered by the release of thyroid hormone, which begins a process during which most of the tissues in the body are remodeled. In the case of frogs, the tail is reabsorbed, the gills degenerate, the tongue grows, and the small intestine is shortened. For salamanders, the change is less dramatic, but tail structure changes and the gills are reabsorbed. The thyroid hormone used for amphibian metamorphosis is the same hormone that the human body uses to regulate body weight. Hyperthyroidism and hypothyroidism are conditions that result from irregularities in thyroid hormone regulation in the human body.
For insects, two hormones are involved. One hormone, called ecdysone, is required for an insect to molt or change its skin. This happens several times during insect development. (Most caterpillars, for example, molt about five times before they become a pupa or chrysalis, each time becoming a larger caterpillar). The second important hormone is juvenile hormone. When juvenile hormone is present in combination with ecdysone, an insect will molt into a form similar to its current form. When juvenile hormone is absent and ecdysone is present, the molt will be into a new form. Ecdysone on its own will cause a caterpillar to turn into a chrysalis, for instance, and a chrysalis will turn into a butterfly. If we were to add juvenile hormone to the largest molt of a caterpillar, we could cause it to molt into an even bigger caterpillar, rather than a pupa.
In conclusion, in insects, it is the absence of juvenile hormone that allows for the remodeling of tissues to take on a new form, while in amphibians, it is the presence of thyroid hormone that causes the remodeling of tissues. These hormonal changes initiate a cascade of changes in gene expression, which scientists are still trying to figure out in both systems—it is a very active field of research.
For Web sites that describe these mechanisms in further detail, see: