Bidirectional Selection for Yolk Testosterone Content in Japanese Quail

A broad variability of yolk androgen levels is considered as an important source of adaptive phenotypic plasticity. This variability of maternal androgen deposition is explained by environmental variation and also by genetic differences among avian mothers. We experimentally investigated such heritable variation of testosterone (T) levels in the egg through the bidirectional selection of Japanese quail. The response to selection for yolk T concentrations was evaluated in downward (low egg T, LET) and upward (high egg T, HET) directions over five generations. Moreover, the correlative response of circulating sex steroids to selection was examined by a comparison of plasma T and estradiol levels between the HET and LET females. We observed a distinct pattern of the response to selection in the HET and LET lines with the stronger response occurring in the upward direction and a limited potential for selection in the downward direction. In the HET line, the population mean increased more than double as compared to the initial population but it seems to be still below its plateau. The LET line reached the selection limit in the second generation suggesting that yolk T concentrations were close to their physiological minimum. Estradiol levels were higher in the circulation of the HET than LET females without changes in plasma T concentrations underlying specific control mechanisms for yolk and plasma hormone levels.