Joachim L. Dagg 

A hitherto unrecognized genetic predisposition for sperm competition exists in aphids. The females of a clone are diploid for the autosomes and sex chromosomes: AA/XX (where A stands for a haploid set of autosomes). Production of males is parthenogenetic, except for the random loss of one X chromosome. Male offspring is diploid for the autosomes but haploid for the sex chromosomes (AA/X0). Hence, each clone produces two brotherhoods of males. The relatedness of males sharing the same X chromosome will be 1. Their relatedness to males of the same clone that received the other X chromosome will be less than 1. Sperm results from meiosis and is haploid, but the sperm cells that do not receive an X chromosome die. The surviving sperm cells share the male’s X chromosome, which makes up the largest part of the genome. Thus, the two brotherhoods of males of a clone produce two kinships of sperm. Sperm cells carrying the same X chromosome will share a part of the genome that is larger than 0.5. Those carrying different X chromosomes will share a part less than 0.5. The above can lead to sperm competition between males from the same clone. Competition from males of different clones can increase sperm competition even further. Ecological factors mixing different clones will have this effect. Sperm cooperation within ejaculates is a possible adaptation to sperm competition. Cystic spermatogenesis, in turn, is a physiological preadaptation for sperm cooperation. Sperm cooperation should thus be as frequent in aphids as in related taxa, like scale insects. Yet, the evidence is inconclusive. This discrepancy between theory and evidence demands an explanation.