Drivers and consequences of coevolution in protective symbiosis

All organisms in nature are targets for parasite attack. Over a century ago, it was first observed that symbiotic species living in hosts can provide a strong barrier against infection, beyond the host’s own defence responses. We now know that ‘protective’ microbial symbiont species are key components of plant, animal, and human microbiota, shaping host health in the face of parasite infection. I have shown that microbes can evolve within days to protect, providing the possibility that microbe-mediated defences can take-over from hosts in fighting with parasites over evolutionary time. This new discovery of an evolvable microbe-mediated defence challenges our fundamental understanding of the host-parasite relationship. Here, I will use a novel nematode-microbe interaction, an experimental evolution approach, and assays of phenotypic and genomic changes (the latter using state-of-the-art sequencing and CRISPR-Cas9 technologies) to generate new insights into the drivers and consequences of coevolving protective symbioses. Specifically, the objectives are to test: (i) the ability of microbe-mediated protection to evolve more rapidly than host-encoded resistance, (ii) the impacts of evolvable protective microbes on host-parasite coevolution, and the effect of community complexity, in the form of (iii) parasite and (iv) within-host microbial heterogeneity, in shaping host-protective microbe coevolution from scratch. Together, these objectives will generate a new, synthetic understanding of how protective symbioses evolve and influence host resistance and parasite infectivity, with far-reaching implications for tackling coevolution in communities.