Non-binomial sex allocation and brood sex ratio variances in the parasitoid Hymenoptera [review]

Much theoretical and empirical work suggests that in many situations brood sex ratio can have important fitness consequences. The development of sex ratio theory and its empirical testing have, however, largely focussed on the mean sex ratio. Here I focus on the fitness consequences of sex ratio variance. Many species of parasitoid Hymenoptera lay broods of eggs and I use these as examples to show that variances either lesser or greater than binomial can be selected for and are observed. If broods of offspring are confined to each other for mating opportunities, low brood sex ratio variances (precise sex ratios) can be highly advantageous over binomial variances. Greater than binomial sex ratio variances may be advantageous if these increase offspring survival or avoid disadvantages associated with inbreeding. Non-binomial sex allocation can be detected by the examination of brood sex ratio variances or by the sequence in which male and female eggs are laid. Problems associated with these approaches are discussed. Constraints on control of sex allocation and offspring mortality complicate the production of optimal brood sex ratios by mothers. In highly inbreeding species, the primary sex ratio should be adjusted in response to the probability of developmental mortality among offspring. It seems that primary sex ratio should also be adjusted according to the distribution of mortality within broods and to sexually differential mortality. The selective advantage of control of brood sex ratio variance is affected by outbreeding opportunities. Control of sex allocation in some other taxa is briefly discussed. It is concluded that although the importance of non-binomial sex ratio variances has been recognized under a limited set of conditions, new aspects of the importance of greater than binomial variances, and the value of precise sex ratios under a wider set of conditions, are only beginning to be explored.