ABSTRACT
Here we investigated the bacterial endosymbionts of weevils of the genus
Curculio
. From all four species of
Curculio
weevils examined, a novel group of bacterial gene sequences were consistently identified. Molecular phylogenetic analyses demonstrated that the sequences formed a distinct clade in the
Gammaproteobacteria
, which was not related to previously known groups of weevil endosymbionts such as
Nardonella
spp. and
Sodalis
-allied symbionts. In situ hybridization revealed that the bacterium was intracellularly harbored in a bacteriome associated with larval midgut. In adult females, the bacterium was localized in the germalia at the tip of each overiole, suggesting vertical transmission via ovarial passage. Diagnostic PCR surveys detected high prevalence of the bacterial infection in natural host populations. Electron microscopy identified the reduced cell wall of the bacterial cells, and the bacterial genes exhibited AT-biased nucleotide composition and accelerated molecular evolution, which are suggestive of a long-lasting endosymbiotic association. On the basis of these results, we conclude that the novel endocellular bacteria represent the primary symbiont of
Curculio
weevils and proposed the designation “
Candidatus
Curculioniphilus buchneri.” In addition to “
Ca.
Curculioniphilus,” we identified
Sodalis
-allied gammaproteobacterial endosymbionts from the chestnut weevil,
Curculio sikkimensis
, which exhibited partial infection frequencies in host insect populations and neither AT-biased nucleotide composition nor accelerated molecular evolution. We suggest that such
Sodalis
-allied secondary symbionts in weevils might provide a potential source for symbiont replacements, as has occurred in an ancestor of
Sitophilus
grain weevils.