Experimental evolution makes microbes more cooperative with their local host genotype

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Science  23 Oct 2020:
Vol. 370, Issue 6515, pp. 476-478
DOI: 10.1126/science.abb7222

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Microbial selection drives adaptation

Many legumes have a host-symbiote relationship with nitrogen-fixing bacteria, or rhizobia, that provides a benefit to both the plant and the microbe. Batstone et al. experimentally evolved the association between five legume accessions and different bacterial isolates. Rather than observe selection by the host for bacterial associations (host choice), mutations accumulated within a bacterial plasmid and increased the strength of the mutualism. Thus, local and recent associations between bacterial strains and plant genotypes are due to selection for bacterial adaptation.

Science, this issue p. 476


Advances in microbiome science require a better understanding of how beneficial microbes adapt to hosts. We tested whether hosts select for more-cooperative microbial strains with a year-long evolution experiment and a cross-inoculation experiment designed to explore how nitrogen-fixing bacteria (rhizobia) adapt to legumes. We paired the bacterium Ensifer meliloti with one of five Medicago truncatula genotypes that vary in how strongly they “choose” bacterial symbionts. Independent of host choice, E. meliloti rapidly adapted to its local host genotype, and derived microbes were more beneficial when they shared evolutionary history with their host. This local adaptation was mostly limited to the symbiosis plasmids, with mutations in putative signaling genes. Thus, cooperation depends on the match between partner genotypes and increases as bacteria adapt to their local host.

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