Does the host matter? Testing the impact of host identity on the microbiome of a trematode parasite

Animal microbiomes have been recognized to have significant impact on animal behaviour, development, and health. For parasites, one of the sources of their associated microbes is likely their hosts, making host identity a potential factor explaining variation of microbial communities among conspecific parasites. Using the host-parasite system including the amphipod Paracalliope fluviatilis, and isopod Austridotea annectens, and their common trematode parasite Maritrema poulini, we hypothesized that host identity has an impact on the diversity and community composition of the microbiomes of M. poulini parasites. We focused on the bacterial component of the trematodes' microbiomes. We investigated the effect of host identity under both intraspecific (among individual isopods) and interspecific (between amphipods and isopods) host levels and predicted that the effect of host identity can be detected under both host hierarchical levels. We found an effect of host identity on both the diversity and community composition of parasite microbiomes between conspecific isopod hosts, while host identity only had an impact on microbiome community composition when comparing parasites infecting amphipod versus isopod hosts. Our study results show that host identity impacts both parasite microbiome composition and taxonomic diversity, providing indirect evidence that the assembly of parasite microbiomes is not merely random, and that horizontal transmission from the host to the parasite might play a key role in shaping parasite microbiomes.