Michael C. Moore

The discovery by Schwabl that maternal steroid hormones are transferred to the egg yolk and have effects on the phenotype of offspring revealed a new pathway for non-genetic maternal effects. The initial model relied on passive transfer. The thinking was that steroids passively entered the lipophillic yolk during yolk deposition and then were deposited in the yolk until they were passively delivered to the embryo as the yolk was used. Subsequent studies revealed that the system is much more dynamic than that. Here, we explore questions about how dynamic the system really is and look at questions like: Is transfer of maternal steroids to the yolk passive or is it actively regulated? At what stages of the maternal reproductive cycle are steroids transferred? During reproduction, how dynamic are the levels of yolk steroids? Especially in the case of potentially deleterious steroids (e.g., androgens in female offspring; glucocorticoids), once deposited can they come out of the yolk over time? Can they be metabolized by the yolk or by the embryo? During incubation, how much do steroid levels in the yolk change? Can steroids diffuse from the yolk to the embryo prior to yolk utilization? Does the embryo contribute to yolk steroid levels as it develops? We believe that comprehensive answers to questions like these will eventually allow us to generate a much more accurate and complete model of the transfer and utilization of yolk steroids and that this model will be much more dynamic and active than the one initially proposed.

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.