Pathogenicity of mixed infections of Ascosphaera in solitary and social bees

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Dennis Welker


Recent research addressing honey bee (Apis mellifera) health issues have revealed multiple infections to be relatively common, but little is known about multiple-pathogen systems in any host, let alone bees. Understanding host-pathogen interactions is critical to understanding ecosystems and evolutionary biology. However, these interactions are complex, and more so when multiple pathogens infect the same host. Mathematical models help facilitate our understanding of these dynamics, but empirical data are needed to test the assumptions of these models. We tested the two most common mixed infection models (superinfection or co-infection) to determine which best fit a group of fungi closely associated with bees, the Ascosphaera. In addition, we evaluated the hypothesis that phylogenetic relatedness among the pathogens can predict superinfection vs. co-infection. We tested three fungal species, Ascosphaera apis, Ascosphaera aggregata and Ascosphaera larvis, in two bee hosts (A. mellifera and Megachile rotundata). Bee survival was not significantly different in mixed infections vs. infections with the most virulent pathogen only, for either host, but fungal growth within the host was significantly altered by mixed infections. In the host A. mellifera, only the most virulent pathogen was present in the host post infection (a superinfection). In M. rotundata, the most virulent pathogen was present along with the co-infecting species (co-infection). There was no evidence that phylogenetic similarity in this genus led to co-infections over superinfections. The results of mixed infections were host-specific, indicating a strong specificity of pathogens in this genus.

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