Microbial protection favors parasite tolerance and alters host-parasite coevolutionary dynamics

Coevolution between hosts and parasites is a major driver of rapid evolutionary change(1) and diversification.(2)(,)(3) However, direct antagonistic interactions between hosts and parasites could be disrupted(4) when host microbiota form a line of defense, a phenomenon widespread across animal and plant species.(5)(,)(6) By suppressing parasite infection, protective microbiota could reduce the need for host-based defenses and favor host support for microbiota colonization,(6) raising the possibility that the microbiota can alter host-parasite coevolutionary patterns and processes.(7) Here, using an experimental evolution approach, we co-passaged populations of nematode host (Caenorhabditis elegans) and parasites (Staphylococcus aureus) when hosts were colonized (or not) by protective bacteria (Enterococcus faecalis). We found that microbial protection during coevolution resulted in the evolution of host mortality tolerance—higher survival following parasite infection—and in parasites adapting to microbial defenses. Compared to unprotected host-parasite coevolution, the protected treatment was associated with reduced dominance of fluctuating selection dynamics in host populations. No differences in host recombination rate or genetic diversity were detected. Genomic divergence was observed between parasite populations coevolved in protected and unprotected hosts. These findings indicate that protective host microbiota can determine the evolution of host defense strategies and shape host-parasite coevolutionary dynamics.