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. 1 From the Symposium Animal Behavior: Integra-ing sexual phenotypes (Breedlove, 1992; tion of Proximate and Ultimate Causation presented at GuStafson a n d D o n a h o e , 1 9 9 4). H o w e v e r , the Annual Meeting of the Society for Integrative and h j j. frequently not emphasized Comparative Biology, 26

Effects of Androgens on Social Behavior and Morphology of Alternative Reproductive Males of the Azorean Rock-Pool Blenny

“…While the original formulation of this hypothesis considered only one distinction between the ART, fixed vs plastic, in the upgraded version, further distinctions are made between reversible and irreversible plastic ART and between conditional and unconditional fixed ART. In the case of the plastic ART this further distinction generates another prediction at the endocrine level: that in reversible tactics the effects of hormones should be more activational while in irreversible tactics the effects should be organizational, since the transition to the "final" morphotype is permanent (like the original predictions, but now at a second hierarchical level) (Moore et al, 1998). In other words, the hormone differences that cause the differentiation of the alternative morphotypes are only needed, and thus should only be present, during the transition phase.…”

“…More recently, Moore et al (1998) developed a second generation of the relative plasticity hypothesis. While the original formulation of this hypothesis considered only one distinction between the ART, fixed vs plastic, in the upgraded version, further distinctions are made between reversible and irreversible plastic ART and between conditional and unconditional fixed ART.…”

“…A first prediction of this hypothesis is that hormone profiles would differ in plastic adult morphs but not in fixed ones (Moore, 1991;Moore, Hews, and Knapp, 1998). A second prediction is that in flexible ART, hormone manipulations should be effective in adults but not during early development (i.e., would have an activational effect), while in fixed ART hormone manipulations should be effective during early development but not in adults; that is, hormones would have an organizational effect (Moore, 1991;Moore et al, 1998). These predictions have been confirmed in several species from different phylogenetic groups.…”

Hormone levels of male African striped mice change as they switch between alternative reproductive tactics

“…The relative plasticity hypothesis predicts that plastic tactics are regulated by 69 activational hormonal effects, meaning that changes in hormone levels cause a switch 70 in tactic (Moore et al, 1998). In many species with male ARTs, individuals using 71 different tactics differ in their hormone levels.…”

“…The relative plasticity hypothesis states that changes of reproductive tactics 290 will be associated with changes in steroid hormone levels (Moore et al, 1998). 291…”