Significance How sophisticated mutualism has arisen from less-intimate associations is of general interest. Here we address this evolutionary issue by looking into the bedbug. Wolbachia endosymbionts are generally regarded as facultative/parasitic bacterial associates for their insect hosts, but in the bedbug, exceptionally, Wolbachia supports the host’s growth and survival via provisioning of vitamins. In the bedbug’s Wolbachia genome, we identified a gene cluster encoding the complete synthetic pathway for biotin (vitamin B7), which is not present in other Wolbachia genomes and is presumably acquired via lateral transfer from a coinfecting endosymbiont. The Wolbachia -provisioned biotin contributes to the bedbug’s fitness significantly, uncovering an evolutionary transition from facultative symbiosis to obligate mutualism facilitated by lateral gene transfer in the endosymbiont lineage.

Evolutionary origin of insect– Wolbachia nutritional mutualism