Males are under different selective pressures than females, which results in differences in the physiology of the two sexes to maximize their fitness. In terms of immunity, males are typically considered as the ‘sicker sex’, where immunocompetence is reduced to favour increased reproductive output. However, male social Hymenoptera are also haploid and therefore lack allelic variation at the individual level, which can also lead to reduced immunocompetence. Over the last decade, several studies have provided evidence for a higher susceptibility to disease in males of social Hymenoptera, without clarifying whether this susceptibility was a direct consequence of their haploid condition or the result of a ‘live hard, die young’ overall evolutionary strategy. In the present study, we used an experimental approach of bacterial challenge to test the immune response of males and females in two species of social Hymenoptera (honey bees, Apis mellifera; paper wasps, Polistes dominula), where males show very different life-history traits. Drones benefit from colony protection for most of their life, whereas P. dominula males leave their colonies and have to survive for weeks at leks. If the haploid condition is responsible for a higher susceptibility in males, we should expect a lower immune response in males of both species compared to females. Conversely, if the immunocompetence depends on the life-history traits of males, an opposite trend is expected in males of the two species. Our results do not support the ‘haploid susceptibility hypothesis’ but are in accordance with the different life history of males from the two species.
The authors thank the two anonymous reviewers for their helpful comments. We also thank Alex McMenamin and the members of the Grozinger Laboratory for their assistance during the present study. The research was co-funded by the project PRIN 2008 (prot. 2008KZ82RE) and the University of Florence.
- Apis mellifera
- haploid susceptibility
- immune challenge