Evolution of sex-biased maternal effects in birds. IV. Intra-ovarian growth dynamics can link sex determination and sex-specific acquisition of resources

The evolutionary importance of maternal effects is determined by the interplay of maternal adaptations and strategies, offspring susceptibility to these strategies, and the similarity of selection pressures between the two generations. Interaction among these components, especially in species where males and females differ in the costs and requirements of growth, limits inference about the evolution of maternal strategies from their expression in the offspring phenotype alone. As an alternative approach, we examine divergence in the proximate mechanisms underlying maternal effects across three house finch populations with contrasting patterns of sex allocation: an ancestral population that shows no sex-biased ovulation, and two recently established populations at the northern and southern boundaries of the species range that have opposite sequences of ovulation of male and female eggs. For each population, we examined how oocyte acquisition of hormones, carotenoids and vitamins was affected by oocyte growth and overlap with the same and opposite sexes. Our results suggest that sex-specific acquisition of maternal resources and sex determination of oocytes are linked in this system. We report that acquisition of testosterone by oocytes that become males was not related to growth duration, but instead covaried with temporal exposure to steroids and overlap with other male oocytes. In female oocytes, testosterone acquisition increased with the duration of growth and overlap with male oocytes, but decreased with overlap with female oocytes. By contrast, acquisition of carotenoids and vitamins was mostly determined by organism-wide partitioning among oocytes and oocyte-specific patterns of testosterone accumulation, and these effects did not differ between the sexes. These results provide important insights into three unresolved phenomena in the evolution of maternal effects - (i) the evolution of sex-specific maternal allocation in species with simultaneously developing neonates of both sexes; (ii) the link between sex determination and sex-specific acquisition of maternal products; and (iii) the evolution of context-dependent modulation of maternal effects.