Co-evolution of marine worms and their chemoautotrophic bacterial symbionts: unexpected host switches explained by...

Mutualistic associations of bacteria and invertebrates are widespread and encompass an enormous diversity on the side of both partners. The advantages gained from the symbiosis favour reciprocal adaptations that increase the stability of the association and can lead to codiversification of symbiont and host. While numerous examples of a strictly vertical transfer of the symbionts from parent to offspring among intracellular associations abound, little is known about the fidelity of the partners in extracellular associations, where symbionts colonize the surface or body cavity of their host. In this issue of Molecular Ecology, Zimmermann et al. () investigated the evolutionary history of the symbiotic association between a monophyletic clade of sulphur-oxidizing Gammaproteobacteria and two distantly related lineages of marine worms (nematodes and annelids). The study supports the surprising conclusion that partner fidelity does not necessarily increase with the intimacy of the association. Ectosymbionts on the cuticle of the nematodes seem to be cospeciating with their hosts, whereas endosymbionts housed in the body cavity of the annelids must have originated multiple times, probably by host switching, from ectosymbionts of sympatric nematodes. This excellent case study on the evolutionary history of invertebrate-microbe interactions supports the emerging concept that the co-evolutionary processes shaping such mutualistic symbioses include both codiversification and ecological fitting.