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

Natural infections often consist of multiple pathogens of the same or different species. When co-infections occur, pathogens compete for access to host resources and fitness is determined by how well a pathogen can reproduce compared to its competitors. Yet not all hosts provide the same resource pool. Males and females, in particular, commonly vary in both their acquisition of resources and investment in immunity, but their ability to modify any competition between different pathogens remains unknown. Using the Daphnia magna - Pasteuria ramosa model system, we exposed male and female hosts to either a single genotype infection or co-infections consisting of two pathogen genotypes of varying levels of virulence. We found that co-infections within females favoured the transmission of the more virulent pathogen genotype, while co-infections within male hosts resulted in the equal transmission of competing pathogen genotypes. This contrast became less pronounced when the least virulent pat...