Reproductive alliances and posthumous fitness enhancement in male ants

Ants provide excellent opportunities for studying the evolutionary aspects of reproductive conflict. Relatedness asymmetries owing to the haplodiploid sex determination of Hymenoptera create substantial fitness incentives for gaining control over sex allocation, often at the expense of the fitness i nterests of nest-mates. Under worker-controlled split sex ratios either the reproductive interests of the mother queen (when workers male bias the sex ratio) or the father (when workers female bias the sex ratio), but never t hat of both parents simultaneously, are fulfilled. When workers bias sex ra tios according to the frequency of queen mating, males which co-sire a colo ny have a joint interest in manipulating their daughter workers into rearin g a more female-biased sex ratio. Here we show that males of the ant Formic a truncorum achieve such manipulation by partial sperm clumping, so that th e cohort-specific relatedness asymmetry of the workers in colonies with mul tiple fathers is higher than the cumulative relatedness asymmetry across wo rker cohorts. This occurs because a single male fathers the majority of the offspring within a cohort. Colonies with higher average cohort-specific re latedness asymmetry produce more female-biased sex ratios. Posthumously exp ressed malt: genes are thus able to oppose the reproductive interests of th e genes expressed in queens and the latter apparently lack mechanisms for e nforcing full control over sperm mixing and sperm allocation.