biostudies-literatureRafaluk-Mohr CLeverhulme TrustOxford University PressNatural Environment Research CouncilBiotechnology and Biological Sciences Research Council246-256https://www.ebi.ac.uk/biostudies/studies/S-EPMC6121859biostudies-literatureUnknown2(3)Species interactions can shift along the parasitism-mutualism continuum. However, the consequences of these transitions for coevolutionary interactions remain unclear. We experimentally coevolved a novel species interaction between <i>Caenorhabditis elegans</i> hosts and a mildly parasitic bacterium, <i>Enterococcus faecalis</i>, with host-protective properties against virulent <i>Staphylococcus aureus</i>. Coinfections drove the evolutionary transition of the <i>C. elegans-E. faecalis</i> relationship toward a reciprocally beneficial interaction. As <i>E. faecalis</i> evolved to protect nematodes against <i>S. aureus</i> infection, hosts adapted by accommodating greater numbers of protective bacteria. The mutualism was strongest in pairings of contemporary coevolved populations. To generaEvolution lettersMutual fitness benefits arise during coevolution in a nematode-defensive microbe model.PMC6121859RPG-2015-165BB/M011224/1NE/N014979/1Ashby BDahan DAKing KCRafaluk-Mohr CfalseMutual fitness benefits arise during coevolution in a nematode-defensive microbe model.Species interactions can shift along the parasitism-mutualism continuum. However, the consequences of these transitions for coevolutionary interactions remain unclear. We experimentally coevolved a novel species interaction between <i>Caenorhabditis elegans</i> hosts and a mildly parasitic bacterium, <i>Enterococcus faecalis</i>, with host-protective properties against virulent <i>Staphylococcus aureus</i>. Coinfections drove the evolutionary transition of the <i>C. elegans-E. faecalis</i> relationship toward a reciprocally beneficial interaction. As <i>E. faecalis</i> evolved to protect nematodes against <i>S. aureus</i> infection, hosts adapted by accommodating greater numbers of protective bacteria. The mutualism was strongest in pairings of contemporary coevolved populations. To genera2018-01-01T00:00:00Z2018 Jun2025-04-19T16:22:07.222Z2019-03-26T23:58:14ZS-EPMC61218593028368010.1002/evl3.58