Maternal allocation shapes offspring phenotype by being tailored to parental phenotype and offspring sex. Because melanogenesis and immunity have partly common genetic control, maternal effects via egg immune factors and parental melanization may covary. Here, we tested whether barn swallows Hirundo rustica allocated antibodies against a novel antigen to their eggs according to embryo sex depending on maternal melanin-based coloration while controlling for paternal tail ornamentation. Egg antibody concentration increased with maternal plumage darkness in broods with relatively more females, whereas it decreased in broods with relatively more males. Thus, darker females allocated more antibodies to their eggs when offspring sex ratio was female biased, whereas paler females allocated more antibodies to the eggs when offspring sex ratio was male biased. These effects were independent of laying order. Our results are compatible with the hypotheses that maternal allocation to sons depends on their reproductive value as predicted by heritable melanin-based coloration and with the hypothesis of differential susceptibility of pheomelanic daughters to parasitism. These effects may depend on pleiotropy of the genes that control melanogenesis and immunity, which may differentially act on investment in self-maintenance and maternal effects. This study is the first study in vertebrates of the association between maternal effects on either sex via egg quality and parental melanization. The links between the genetic control of melanogenesis and of major fitness traits like immunity and the role of maternal effects in reproductive strategies prompt for more studies of the covariation between parental coloration and maternal sex-dependent allocation via egg quality.
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