Abstract
Host-associated microbiota have been claimed to play a role in hosts’ responses to parasitic infections, often protecting the hosts from infection. We tested for such a role in the crustacean Daphnia and the parasitic bacterium Pasteuria ramosa, a widely used model system for host-parasite interactions. We first determined the infection phenotype (i.e., resistotype) of eight clonal D. magna genotypes against four strains of P. ramosa by attachment test, followed by 16 S rDNA amplicon sequencing to determine if their genotype or their parasite resistotype influences the composition of their microbiome. We then reciprocally transplanted the microbiota of two host genotypes with opposite resistotypes to four P. ramosa isolates, followed by a reassessment of their resistotype after transplantation. We found significant differences in microbiome composition and structure between Daphnia genotypes and between Daphnia resistotypes to specific P. ramosa strains. Reciprocal microbiota exchange or making the Daphnia hosts bacteria-free, however, did not influence the resistotypes of the hosts. Thus, in contrary to what has been observed in some taxa, our results suggest that D. magna susceptibility to P. ramosa is strongly dictated by the genetic differences of the hosts and is still dependent on Daphnia’s first line of immune defense against the esophageal attachment of P. ramosa, which appears to be uninfluenced by the host’s microbiota.
Highlights
Until a few years ago, conventional wisdom stated that differences among host populations in resistance to pathogens are largely explained by host genetic differences[1,2,3] with environmental factors playing a subordinate role
It is not known whether it is the differences in overall host genetic background or the differences in specific immune traits that influences the microbiome composition in a given host taxa
These characteristics make Daphnia a good model to disentangle the contribution of host genetic background and microbiome composition, and to determine if the microbiome plays a role in Daphnia-P. ramosa interactions
Summary
Until a few years ago, conventional wisdom stated that differences among host populations in resistance to pathogens are largely explained by host genetic differences[1,2,3] with environmental factors playing a subordinate role. It is not known whether it is the differences in overall host genetic background or the differences in specific immune traits that influences the microbiome composition in a given host taxa Teasing apart these factors would allow to understand the role that the microbiome plays on immune traits and pathogen resistance. The main goal of this study is to determine if the microbiome plays a role in the resistance of D. magna to P. ramosa We evaluated this by determining the infection phenotype of 8 Daphnia genotypes (=clones) to four P. ramosa strains, followed by comparison of their microbiome structure and composition based on genotype and resistance phenotype (termed resistotype in this study) to four Pasteuria strains. Replacing the microbiome of the hosts or making the hosts bacteria-free have not altered their resistance or susceptibility to a P. ramosa pathogen
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