Queens versus workers: sex-ratio conflict in eusocial Hymenoptera

Studies of sex-ratio conflict in the eusocial Hymenoptera (ants, bees and wasps) have provided the most rigorous tests of kin selection theory. The hymenopteran haplodiploid system of sex determination generally renders workers more closely related to their sisters than to their brothers, whereas queens are equally related to their sons and daughters. Kin selection theory therefore predicts that resource allocation into male or female reproductives is a source of queen–worker (i.e. parent–offspring) conflict. Under the traditional assumption of worker control, sex ratios should evolve towards female bias, shifting away from the optimum of the queen, an even sex ratio. Three decades of research on sex-ratio conflict largely supported worker control, but recent studies have revealed queen-controlled sex ratios even in societies previously thought to operate under worker control. Recent studies have further documented that queen–worker sex-ratio conflict is modulated by other within-colony conflicts, such as those over colony growth or worker reproduction. Shared-control, multiconflict models are now needed to encompass the dynamic balance between queen and worker power over the colony sex ratio.