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.