Immune stress and facultative sex in a parasitic nematode

It has been suggested that sexual reproduction in parasites may be advantageous because it helps evade genotype-specific host immune responses. Indirect support for this hypothesis has recently come from work on Strongyloides ratti, a parasitic nematode of rats that develops and reproduces sexually or asexually. In this species, host immune responses against S. ratti lead to a higher proportion of individuals reproducing sexually. However, an alternative explanation for these results is that sex is favoured by general environmental stress, including host responses against antigen sources other than S. ratti. Here we test this hypothesis, by determining how host immunity against two other parasitic nematode species (Nippostrongylus brasiliensis & Strongyloides venezuelensis) and commonly used mammalian antigens (sheep red blood cells) affects the likelihood of S. ratti larvae developing sexually. Our results show that increased levels of sex occur in response to immune responses generated against these other species, and not just host immunity elicited by S. ratti. This is consistent with the idea that sex is favoured under stressful conditions, and does not support the immune evasion hypothesis.