Physiology underlying phenotypic plasticity and polyphenisms: introduction to the symposium.

Among the most important events in the development of an individual animal are the commitments to major life-history transitions (e.g., metamorphosis, sex determination, reproduction). In many cases, these commitments are accompanied by a switch from plastic (5environmentally sensitive) to canalized (5environmentally insensitive) development (West-Eberhard, 2003) for a given trait (e.g., the next developmental stage, gender). At this moment, individuals can move toward growing horns or lacking horns (beetles, Emlen and Nijhout, 1997), undergoing metamorphosis or growing to a larger larvae (toads, Denver, 1997; moths, Nijhout and Williams, 1974), developing wings with large or small eyespots (butterflies, Brakefield et al., 1998), becoming a queen or a worker (ants, Wheeler, 1991), or laying eggs or continuing to accumulate reserves (grasshoppers, Hatle et al., 2000; Moehrlin and Juliano, 1998). Before this transition, individuals are plastic and can adjust a given phenotype developmentally in response to environmental conditions. After the transition, individuals are canalized, with limited potential for adjusting the phenotype in response to environmental conditions. Because the phenotypes that result from these developmental trajectories can be drastically different, these transitions are critical points in animal life-histories (Crews, 2003; Nijhout, 1999). This symposium focuses on the physiological and endocrinological mechanisms that regulate such life-history transitions. These mechanisms are crucial links between the molecular biology of gene expression and life-history. The symposium includes papers on both continuous plasticity (the production of multiple phenotypes from one genotype) and discrete plasticity (a.k.a., polyphenisms; the production of two or more discrete phenotypes from one genotype). A major goal of the symposium is to reveal common patterns among the mechanisms that produce phenotypic canalization in a broad range of animals and for a variety of life-history transitions. How are drastically different phenotypes produced from genetically similar individuals? The answer may always involve hormones. Hormones control many important developmental events and probably control most environmentally sensitive life-history transitions (Ketterson and Nolan, 1992; Nijhout, 2003; Stearns,