Hormonal Control and Evolution of Alternative Male Phenotypes: Generalizations of Models for Sexual Differentiation

Abstract

SYNOPSIS. An organism's phenotype results from an interaction of environment and genotype. Sex steroids play a role in translating sexual genotype into phenotype. The focus of this research has been to extend the model of sex steroid hormone action in sexual differentiation to individual variation in reproductive phenotype. The hypothesis generated, called the relative plasticity hypothesis, has been tested in a species with alternative phenotypes, tree lizards (Urosaurus ornatus). Such species are useful models for tests of these ideas because variation is extreme and easily studied. These tests have shown that permanent differentiation of the territorial and nonterritorial phenotypes is accomplished by hormonal mechanisms operating during early development. These are similar to organizational actions of hormones classically described for sexual differentiation. A unique finding of this work is that the adrenal hormone progesterone regulates differentiation of the two male types. Furthermore, the nonterritorial male tree lizard appears to conditionally switch tactics between sedentary satellite and nomadic behavior. Nomadic behavior is triggered under stressful environmental conditions. Nonterritorial tree lizards show greater inhibition of reproductive hormones following stress than do territorial tree lizards, suggesting that a differential sensitivity of reproductive hormones to stress is the endocrine basis of conditional tactic switching in this morph. This mechanism is similar to the classical activational effects of hormones. Together, these findings indicate that individual variation in sexual phenotype is produced by mechanisms similar to sexual differentiation. Further refinement of the models and integration with some evolutionary ideas is explored.