The Control of Polyphenic Development
In polyphenic development an insect is able to develop two or more discretely different adult phenotypes, depending on external signals received during larval life. The differences between these alternative phenotypes are often as great as the differences between species (indeed, alternative phenotypic forms have occasionally been described as different species). There are, however, no genetic differences between the alternative forms and every individual is, at the beginning of its life, capable of developing into either form.
Here are some examples
Polyphenic development is regulated by means of discrete developmental switches. Hormones serve as the physiological mediators of these switches. In most cases, the same hormones that regulate metamorphosis (ecdysone and juvenile hormone) also regulate polyphenic development. In addition, some polyphenic switches are regulated by neurosecretory hormones.
At one or more points in the life of an insect larva there are critical periods of hormone sensitivity. Superimposed on these, there is a complex pattern of hormone secretion and degradation. The hormone-sensitive period acts as a binary developmental switch. If the hormone is above a particular threshold during a hormone-sensitive period, the tissue follows one developmental pathway, if not, an alternative developmental pathway is chosen. The hormone sensitive periods of different tissues do not need to coincide in time.
The different variants of the mechanisms by which this works are illustrated below. During the life of a larva there is a period of environmental sensitivity when the summation of specific signals (such as temperature, photoperiod, nutrition, or pheromones) somehow reprogram the central nervous system. This reprogramming results in al altered pattern of hormone secretion later in life, typically just before metamorphosis (the period indicated by ABC). Depending on the species, one of four switching mechanisms comes into play: (1) a hormone may rise above or fall below a threshold; (2) a threshold may be raised or lowered; (3) hormone secretion may occur at a different time and miss a hormone-sensitive period; (4) a hormone sensitive period may shift. Depending on whether the hormone is above or below the threshold during the hormone-sensitive period, one of two alternative developmental pathways is taken, resulting in insects with different morphologies after metamorphosis.
Through the integration if relevant environmental signals by the central nervous system, development can be made responsive to a varying environment. This mechanism enables the evolution of specific adult adaptations to different environments. BOOK SECTION