Interference competition influences virulence in Metarhizium coinfections

Coinfections are common in a nature and often involve competition among coinfecting parasites to extract resources from the host. As a consequence of this competition, parasites can face trade-offs between investing in competition with other parasites and exploiting the host for their own growth and transmission. Such within-host interactions among parasites can either increase or decrease virulence depending on how they alter parasite density, resource use, and host damage. We tested the inhibitory effect of different species of the entomopathogenic fungus Metarhizium on one another in vitro and the impact of coinfection on virulence towards Tribolium castaneum. We found that Metarhizium pemphigi is able to inhibit all other isolates of Metarhizium tested, whereas Metarhizium pingshaense was relatively resistant to inhibition by other Metarhizium strains. Virulence towards the host, Tribolium castaneum, differed very little among isolates tested. We did, however, observe differences between virulence in single and coinfections in two cases. Where there were virulence differences, these were both consistent with in vitro inhibition patterns and interestingly resulted in reduced virulence in coinfection. This is inconsistent with broader patterns of virulence in coinfection, where virulence seems to be driven by the more virulent parasite. It is, however, consistent with data from other systems where there is evidence of interference competition. Our data thus support the idea that, in contrast to other forms of between parasite interaction, interference competition results in reduced virulence in coinfection.