Host Genetic Constraints on the Horizontal Transmission of Daphnia-associated Microbiota.

The taxonomic composition of Daphnia microbiota is affected not only by external environmental conditions, but also by the host's internal physiological state, which is partly governed by genetic factors. However, the extent to which host genetics constrain the composition of associated bacterial communities remains unclear. In the present study, we conducted mixed-culture experiments using obligately parthenogenetic Daphnia cf. pulex individuals from genetically distinct lineages. The results obtained showed that the taxonomic composition of host-associated microbiota significantly differed between genotypes, both within and across lineages, with certain bacterial taxa being exclusive to specific genotypes. When genetically distinct hosts were co-cultured, some bacterial taxa initially exclusive to one genotype appeared in the microbiota of another, indicating the horizontal transmission of microbiota between hosts. Nevertheless, the overall taxonomic composition of microbiota was largely unaffected by the presence of genetically different hosts. These results suggest that although the horizontal transfer of microbiota occurs between different Daphnia genotypes, it is not extensive enough to override genotype-specific microbiota compositions. Therefore, in D. cf. pulex, host genetics play a major role in shaping the composition of the associated microbiota.