Host sexual dimorphism affects the outcome of within-host pathogen competition

Abstract

Natural infections often consist multiple pathogens of the same or different species. In multiple infections, pathogens compete for access to host resources and fitness is determined by how well a pathogen can reproduce compared to its competitors. Given the propensity for males and females to exhibit variation in pathogen-induced reduction in lifespan or fecundity, we explore how host sex may modulate the competitive ability of pathogens, potentially favouring the transmission of different pathogen genotypes. Using the Daphnia magna - Pasteuria ramosa model system, we exposed male and female hosts to either a single genotype infection or coinfections consisting of two pathogen genotypes of varying levels of virulence, measured as pathogen-induced reduction in host lifespan. We found that co-infections within females generally favoured the transmission of the more virulent pathogen genotype. Conversely, co-infections within male hosts resulted in equal transmission of competing genotypes, or favoured the transmission of the less virulent pathogen genotype in treatments where it established prior to the more virulent competitor. These results suggest that sex is a form of host heterogeneity which may influence the evolution of virulence within co-infection contexts and that one sex may be a reservoir for pathogen genetic diversity in nature.