Intra-individual conflicts between autosomal and X-linked altruistic genes: evolutionary perspectives of sex-specific grandmothering.

Alloparental care by females toward their grandoffspring can evolve by kin selection. Previous theoretical studies predicted that selection favors autosomal and X-chromosomal genes, causing altruism toward maternal grandoffspring and paternal granddaughters, respectively, and two corresponding types of biased grandparental investment are suggested by empirical studies on human populations. Using discrete-time two-locus-two-allele models, I examined a possible conflict between the autosomal and the X-chromosomal altruistic genes over the carrier female's time and resources. This conflict is expected to occur when each grandmother has access to only maternal or paternal grandchildren as a result of her residence situation. The conditions under which each or both kinds of altruistic genes evolve (against non-altruistic genes) mainly represent the conflicting relationship between the autosomal and X-chromosomal altruistic genes. In addition, depending on the settings, the models exhibit bistable or periodic behaviors, and one type of gene can be considered parasitic in the latter behavior. On the whole, the results suggest that the X-chromosomal altruistic genes rather than the autosomal ones exhibit more difficulty increasing or fixing with this kind of conflict.