Sex ratios of an aphid subject to local mate competition with variable maternal condition

Abstract

Fisher¹ was the first to argue that natural selection would adjust the sex ratio so as to equalize parental investment (PI) in the two sexes, where mating is at random. Hamilton² then showed that female-biased sex ratios would be favoured where male siblings compete for matings, a situation referred to as ’local mate competition‘ (LMC). In Hamilton's original model and in most subsequent LMC models, the females founding a local breeding population have equal amounts of PI to be allocated between sons and daughters. But in nature, females may differ in total PI and, as a consequence, in fecundity. Here I describe a model for the sex ratios of n co-foundresses, all of which differ in total PI. The model shows that each female with more than a specified minimum amount of PI is selected to make the same absolute investment in sons. All previously published cases of dramatic sex-ratio control occur in haplodiploid species. Here I test my theoretical model against sex-ratio data for an aphid, Prociphilus oriens, which has a normal diploid genetic system.