Sex Investment Ratios in Social Hymenoptera

Abstract

Computer simulations of populations of social wasps were used to determine the direction of selection on dominant and recessive alleles affecting the sex investment ratio. Selection on queens always favors a colony (and personal) investment ratio of 1:1, while selection on workers favors biasing the ratio toward females. If all worker phenotypes in a colony have an equal effect on the investment ratio, selection favors a 3:1 ratio of females to males. If workers which prefer the more female-biased ratio determine investment irrespective of what other phenotypes are present in the colony, then the equilibrium sex investment ratio is 2:1 with dominant biasing alleles, and 3:1 with recessive biasing alleles. In the latter case, unstable gene frequency equilibria occur with alleles coding for a proportion of females greater than 3:1, so that stochastic processes can result in selection for a more female-biased investment ratio; the equilibrium frequency increases with increasing female bias. The presence of worker-queen and worker-worker conflict of interests over sex investment ratios is examined. The outcome of the conflict is discussed.