Host immune status determines sexuality in a parasitic nematode

We examine the hypothesis that sexual reproduction by parasites is an adaptation to counter the somatic evolution of vertebrate immune responses. This is analogous to the idea that antagonistic coevolution between hosts and their parasites maintains sexual reproduction in host populations. Strongyloides ratti is a parasitic nematode of rats. It can have a direct life cycle, with clonal larvae of the wholly parthenogenetic parasites becoming infective, or an indirect life cycle, with clonal larvae developing into free-living dioecious adults. These free-living adults produce infective larvae by conventional meiosis and syngamy. The occurrence of the sexual cycle is determined by both environmental and genetic factors. By experimentally manipulating host immune status using hypothymic mutants, corticosteroids, whole-body γ-irradiation and previous exposure to S. ratti, we show that larvae from hosts that have acquired immune protection are more likely to develop into sexual adults. This effect is independent of the method of manipulation, larval density, and the number of days postinfection. This immune-determined sexuality is consistent with the idea that sexual reproduction by parasites is adaptive in the face of specific immunity, an idea which, if true, has clinical and epidemiological consequences.