Evolutionary origin of insect-Wolbachia nutritional mutualism

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.