Host-parasite coevolution induces selection for condition-dependent sex

Sex and recombination remain one of the biggest riddles of evolutionary biology. One of the most prominent hypotheses, the Red Queen Hypothesis, claims that sex has evolved as a means to efficiently create genotypes that are resistant against coevolving parasites. However, previous models of the Red Queen have assumed that all individuals are equally likely to engage in sexual reproduction, regardless of their infection status, an assumption that may not be true in reality. Here, we consider a population genetic model of a host population coevolving with a parasite population, where the parasites are haploid and the hosts either haploid or diploid. We assume that the probability to engage in sex may be different in infected and uninfected hosts and ascertain the success of different reproductive strategies with a modifier-gene approach. Our model shows that in the large majority of the parameter space, infection-dependent sex is more successful than infection-independent sex. We identify at least two reasons for this: (i) an immediate short-term advantage of breaking-down gene combinations of unfit individuals and (ii) a selfish spread of the condition-dependent modifiers, in analogy to the 'abandon-ship' effect in single species. In diploids, these effects are often powerful enough to overcome the detrimental effects of segregation. These results raise the intriguing question of why infection-induced sex is not more commonly observed in nature.