Major histocompatibility complex heterozygote advantage and widespread bacterial infections in populations of Chinook salmon (Oncorhynchus tshawytscha).

Despite growing evidence for parasite-mediated selection on the vertebrate major histocompatibility complex (MHC), little is known about variation in the bacterial parasite community within and among host populations or its influence on MHC evolution. In this study, we characterize variation in the parasitic bacterial community associated with Chinook salmon (Oncorhynchus tshawytscha) fry in five populations in British Columbia (BC), Canada across 2 years, and examine whether bacterial infections are a potential source of selection on the MHC. We found an unprecedented diversity of bacteria infecting fry with a total of 55 unique bacteria identified. Bacterial infection rates varied from 9% to 29% among populations and there was a significant isolation by distance relationship in bacterial community phylogenetic similarity across the populations. Spatial variation in the frequency of infections and in the phylogenetic similarity of bacterial communities may result in differential parasite-mediated selection at the MHC across populations. Across all populations, we found evidence of a heterozygote advantage at the MHC class II, which may be a source of balancing selection on this locus. Interestingly, a co-inertia analysis indicated only susceptibility associations between a few of the MHC class I and II alleles and specific bacterial parasites; there was no evidence that any of the alleles provided resistance to the bacteria. Our results reveal a complex bacterial community infecting populations of a fish and underscore the importance of considering the role of multiple pathogens in the evolution of host adaptations.