Meeting Abstract
S5-1.3 Friday, Jan. 4 Hormone-mediated maternal effects in birds � evolution of mechanisms SCHWABL, Hubert*; MARTIN, Thomas E.; Washington State University, Pullman; University of Montana, Missoula huschwabl@wsu.edu
Birds are widely used models to study hormone-mediated maternal effects. This research focused on functions of yolk testosterone showing that 1) traits as diverse as growth, immune defense, coloration, and behavior can be modified by maternal androgens; 2) individuals, populations, and species vary in yolk concentrations of androgens. Yet, the proximate mechanisms causing variation in yolk androgen concentrations remain little understood and intra-specific studies produced mixed results. We apply a comparative approach using data on the androgens androstenedione (A), testosterone (T), and 5a-dihydrotestosterone (DHT) to understand evolution of these mechanisms. A, T, and DHT are successive metabolites in the ovarian androgen synthesis pathway and DHT is the endpoint and most potent androgen in this chain. These conversions are catalyzed by the enzymes 17b-hydroxysteroid-dehydrogenase (A to T) and 5a-reductase (T to DHT). Across species DHT correlates positively with its precursor T, while T does not correlate with its precursor A. Compared to non-passerines, passerines have more T for a given concentration of its precursor A suggesting that substrate is not limiting. Moreover, in passerines, T and particularly DHT are related to development rate and therefore they may be more important to increase androgen exposure of the embryo than A. We propose that selection acted on expression and/or activity of these enzymes to achieve optimal exposure of the embryo to the more potent androgens. The synthesis of T from A and of DHT from T are single-step enzymatic reactions that likely reflect single gene control and, therefore, may respond easily to natural selection. Offspring mortality rate and physiological tradeoff costs from androgen exposure of mother and embryo seem to be ultimate factors driving this process.
