Genomic conflict occurs when mothers, sons, and daughters bear asymmetrical costs and benefits associated with sib-mating. This conflict was quantified by using Hamilton's rule of altruism as a framework. We show that mothers are selected to prevent inbreeding in their offspring when CSis/BBro > 1, where CSis and BBro represent the sister's cost and the brother's benefit from a sib-mating. This relationship sometimes leads to conflict between a mother and her son, because sons are selected to mate their sisters when Csis/BBro < 2. Drosophila melanogaster provided a useful model system with which to test one application of this theory because i) brothers feasibly benefit from mating their sisters while mothers are harmed from such sib-mating, and ii) despite the smaller size of males relative to females, males have counter-intuitively longer egg-to-adult development time than their sisters.

We hypothesized that mothers use maternal effects to delay the development of their sons relative to their daughters, allowing the females of a brood to disperse before males eclose and become sexually competent -thus avoiding sib-mating. We further hypothesized that epigenetic modification of the Y chromosome could be the mechanism by which mothers manipulate the development of their sons. To test our hypothesis, we created females with paternally inherited Y chromosomes and males that lacked Y chromosomes. With these constructs, we predicted that sex-specific maternal effects would decrease the sexual dimorphism of development time when both sexes of offspring carry a Y chromosome, and would reverse the trend when sisters (but not brothers) carry a Y chromosome. No change in the development time of sons and daughters was observed in response to our experimental manipulation of the presence/absence of Y chromosomes in sons and daughters.

While our experimental results indicate that maternal epigenetic modification of a son's Y chromosome is not responsible for delayed male development in D. melanogaster, our modeling analysis clearly demonstrates the opportunity for mother-son conflict over a wide region of parameter space.