Diversification of endosymbiosis: replacements, co-speciation and promiscuity of bacteriocyte symbionts in weevils

The processes and mechanisms underlying the diversification of host–microbe endosymbiotic associations are of evolutionary interest. Here we investigated the bacteriocyte-associated primary symbionts of weevils wherein the ancient symbiont Nardonella has experienced two independent replacement events: once by Curculioniphilus symbiont in the lineage of Curculio and allied weevils of the tribe Curculionini, and once by Sodalis -allied symbiont in the lineage of grain weevils of the genus Sitophilus . The Curculioniphilus symbiont was detected from 27 of 36 Curculionini species examined, the symbiont phylogeny was congruent with the host weevil phylogeny, and the symbiont gene sequences exhibited AT-biased nucleotide compositions and accelerated molecular evolution. These results suggest that the Curculioniphilus symbiont was acquired by an ancestor of the tribe Curculionini, replaced the original symbiont Nardonella , and has co-speciated with the host weevils over evolutionary time, but has been occasionally lost in several host lineages. By contrast, the Sodalis -allied symbiont of Sitophilus weevils exhibited no host–symbiont co-speciation, no AT-biased nucleotide compositions and only moderately accelerated molecular evolution. These results suggest that the Sodalis -allied symbiont was certainly acquired by an ancestor of the Sitophilus weevils and replaced the original Nardonella symbiont, but the symbiotic association must have experienced occasional re-associations such as new acquisitions, horizontal transfers, replacements and/or losses. We detected Sodalis -allied facultative symbionts in populations of the Curculionini weevils, which might represent potential evolutionary sources of the Sodalis -allied primary symbionts. Comparison of these newcomer bacteriocyte-associated symbiont lineages highlights potential evolutionary trajectories and consequences of novel symbionts after independent replacements of the same ancient symbiont.