Parasitic castration by Xenos vesparum depends on host gender.

Host castration represents a mechanism used by parasites to exploit energy resources from their hosts by interfering with their reproductive development or to extend host lifespan by removing risks associated with reproductive activity. One of the most intriguing groups of parasitic castrators is represented by the insects belonging to the order Strepsiptera. The macroparasite Xenos vesparum can produce dramatic phenotypic alterations in its host, the paper wasp Polistes dominula. Parasitized female wasps have undeveloped ovaries and desert the colony without performing any social task. However, very little attention has been given to the parasitic impact of X. vesparum on the male phenotype. Here, we investigated the effects of this parasite on the sexual behaviour and the morpho-physiology of P. dominula males. We found that, differently from female wasps, parasitized males are not heavily affected by Xenos: they maintain their sexual behaviour and ability to discriminate between female castes. Furthermore, the structure of their reproductive apparatus is not compromised by the parasite. We think that our results, demonstrating that the definition of X. vesparum as a parasitoid does not apply to infected males of P. dominula, provide a new perspective to discuss and maybe reconsider the traditional view of strepsipteran parasites.