Could sex be maintained through harmful males?

All else being equal, parthenogenetic females should produce as many surviving daughters as sexual couples produce daughters plus sons. Hence the resources spent on producing sons are a cost of sex and parthenogenetic females economize on sons. It has recently been shown that a small competitive advantage of sexual individuals can recoup this large reproductive disadvantage, while the adaptation behind the competitive advantage might differ from case to case. One hypothesis that has not yet been considered as a potential competitive advantage is that males could differentially harm parthenogenetic females, for example, through harassment, toxic seminal fluids, or infanticide. Harmful male functions result from the selection for males that maximise their fitness at the expense of females in the context of sexual conflict. Unless parthenogenetic lineages can maintain their resistance against harmful male functions, a competitive advantage for sex should be a by-product benefit of sexual conflict. Mutations that make males harm parthenogenetic females worse than sexual ones, however, can be seen as evolutionary spite. The spiteful trait is not the production of costly sons, but the production of males that discriminate against parthenogenetic females. Spiteful behaviour can be positively selected, if it acts against negatively related victims. Sexual and parthenogenetic individuals within a population should usually be negatively related, because the genomes of the sexual individuals are bound together by recombination while those of parthenogenetic individuals will be identical except for divergence through mutation. Some unusual cases of parthenogenesis are discussed in the light of this new hypothesis and an experimental approach for testing it is suggested.