Endosymbiotic bacteria of the genus Wolbachia represent the most successful symbiotic bacteria in the terrestrial ecosystem. The success of Wolbachia has been ascribed to its remarkable phenotypic effects on host reproduction, such as cytoplasmic incompatibility, whereby maternally inherited bacteria can spread in their host populations at the expense of their host’s fitness. Meanwhile, recent theoretical as well as empirical studies have unveiled that weak and/or conditional positive fitness effects may significantly facilitate invasion and spread of Wolbachia infections in host populations. Here, we report a previously unrecognized nutritional aspect, the provision of riboflavin (vitamin B2), that potentially underpins the Wolbachia-mediated fitness benefit to insect hosts. A comparative genomic survey for synthetic capability of B vitamins revealed that only the synthesis pathway for riboflavin is highly conserved among diverse insect-associated Wolbachia strains, while the synthesis pathways for other B vitamins were either incomplete or absent. Molecular phylogenetic and genomic analyses of riboflavin synthesis genes from diverse Wolbachia strains revealed that, in general, their phylogenetic relationships are concordant with Wolbachia’s genomic phylogeny, suggesting that the riboflavin synthesis genes have been stably maintained in the course of Wolbachia evolution. In rearing experiments with bedbugs (Cimex lectularius) on blood meals in which B vitamin contents were manipulated, we demonstrated that Wolbachia’s riboflavin provisioning significantly contributes to growth, survival, and reproduction of the insect host. These results provide a physiological basis upon which Wolbachia-mediated positive fitness consequences are manifested and shed new light on the ecological and evolutionary relevance of Wolbachia infections. IMPORTANCE Conventionally, Wolbachia has been regarded as a parasitic bacterial endosymbiont that manipulates the host insect’s reproduction in a selfish manner, which tends to affect a host’s fitness negatively. Meanwhile, some theories predict that, at the same time, Wolbachia can directly affect the host’s fitness positively, which may potentially reconcile the negative effect and facilitate spread and stability of the symbiotic association. Here we demonstrate, by using comparative genomic and experimental approaches, that among synthetic pathways for B vitamins, the synthetic pathway for riboflavin (vitamin B2) is exceptionally conserved among diverse insect-associated Wolbachia strains, and Wolbachia’s riboflavin provisioning certainly contributes to growth, survival, and reproduction in an insect. These findings uncover a nutritional mechanism of a Wolbachia-mediated fitness benefit, which provides empirical evidence highlighting a “Jekyll and Hyde” aspect of Wolbachia infection.
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