NIJHOUT, H. FRED: Insect Polyphenisms and Adaptation

Subtle differences in the environment in which an organism develops can have profound consequences for the resulting phenotype. Temperature differences as small as ten degree Celsius, or photoperiod differences as small as 6 hours of light per day, can cause some species to develop such dramatically altered phenotypes that they were initially described as different species. Such phenotypic differences are not pathological, but are evolved adaptations to alternative environments. The ability to develop alternative phenotypes in response to specific environmental cues is called polyphenism. The developmental mechanisms underlying polyphenisms, and their ecological significance, are now beginning to be understood. The development of alternative phenotypes comes about through environmentally induced changes in the patterns of hormone secretion. In insects, juvenile hormone and ecdysone, the same hormones that control metamorphosis, also control the morphology of the type adult a larva will become upon metamorphosis. These hormones produce their phenotypic effects during well-defined critical periods in development, by altering the patterns of gene expression during that time. The control of alternative patterns of horn development in scarab beetles and color patterns in butterflies will be discussed in the context of the evolution of the developmental mechanisms that give rise to these traits.