Summary

Aphids produce males and females by a parthenogenetic process that gives the mother proximate control of sex allocation. Sex is an infrequent, usually annual, punctuation in a sequence of asexual generations. The aphid genome, initiated in the fertilized egg, is replicated in a sequence of bodies that make up the aphid clone and which can be thought of as a disaggregated hermaphrodite: selection acts at the clonal level to produce an optimal allocation in sperm and ova. Two crucial factors influencing sex allocation are the degree of within-clone mating and the timing by the clone of investment in males and mating females. Extreme sex ratios are very common in aphids, to an extent that is probably unique amongst diploid organisms. The aphids, unshackled from the constraints imposed by meiosis on sex determination, therefore provide an excellent opportunity for those interested in the evolutionary biology of sex allocation.

Introduction

Aphids are of special importance to evolutionary biologists because they are a diplodiploid group in which the mother clearly has proximate control of the sex of her offspring. They thus provide a genetic system other than haplodiploidy in which the default allocation ratio is not 0.5 (proportion investment in males, i.e. males/(males+females)). My aim in this chapter is to review current knowledge of aphid sex ratios, to provide a brief, accessible account of the relevant biology of these animals, and to highlight aphid groups and specific ideas that would be especially fruitful to study.