Divergent selection for inherent fearfulness leads to divergent yolk steroid levels in quail

Avian yolk steroid levels may vary according to maternal condition or environmental factors, causing epigenetic modulation of offspring phenotype. Here we test whether artificial selection based on divergent duration of tonic immobility (TI) in Japanese quail (i.e., divergent inherent fearfulness) is correlated with divergent levels of yolk steroids. We analysed yolk steroid levels and egg quality in quail selected for either long or short duration of TI. Yolk steroids, i.e., immunoreactive progesterone and androstenedione, were found to be significantly less concentrated in eggs of the high inherent fearfulness line compared to eggs of the low inherent fearfulness line. A similar trend was found with testosterone levels. Larger eggs with lighter eggshell were also found in more fearful quail. Hence, the selection for divergent fearfulness has led to correlated changes in yolk steroid levels and egg quality. These data suggest that hormones of maternal origin, egg quality and genetic background may all contribute to line differences in phenotype. A modulation of progesterone concentrations by selection for behaviour in egg yolk is reported here for the first time. Although the effect of this hormone on avian embryos remains unknown, we argue it may have significant effects on phenotypic outcome.