Locally adapted gut microbiomes mediate host stress tolerance.

While evidence for the role of the microbiome in shaping host stress tolerance is becoming well-established, to what extent this depends on the interaction between the host and its local microbiome is less clear. Therefore, we investigated whether locally adapted gut microbiomes affect host stress tolerance. In the water flea Daphnia magna, we studied if the host performs better when receiving a microbiome from their source region than from another region when facing a stressful condition, more in particular exposure to the toxic cyanobacteria Microcystis aeruginosa. Therefore, a reciprocal transplant experiment was performed in which recipient, germ-free D. magna, isolated from different ponds, received a donor microbiome from sympatric or allopatric D. magna that were pre-exposed to toxic cyanobacteria or not. We tested for effects on host life history traits and gut microbiome composition. Our data indicate that Daphnia interact with particular microbial strains mediating local adaptation in host stress tolerance. Most recipient D. magna individuals performed better when inoculated with sympatric than with allopatric microbiomes. This effect was most pronounced when the donors were pre-exposed to the toxic cyanobacteria, but this effect was also pond and genotype dependent. We discuss how this host fitness benefit is associated with microbiome diversity patterns.