Bacterial endosymbionts initiate morphogenesis of symbiotic organs at specific locations in auchenorrhynchan insects of Hemiptera.

Plant sap-feeding insects (Hemiptera: Auchenorrhyncha) with rich species diversity generally harbor obligate endosymbionts within their specialized symbiotic organs (i.e., bacteriomes) to supplement them with essential amino acids (EAAs) and B vitamins that are unavailable in their nutritionally unbalanced diet, representing a typical model of insect-microbe symbioses. However, the processes of symbiont translocation and morphogenesis of bacteriomes during embryonic development remain unclear in Auchenorrhyncha. Here, we assessed symbiont replacement events in representative species across the five auchenorrhynchan superfamilies, and investigated the symbiont communities of eggs and morphogenesis of symbiotic organs during embryonic development as well as nutritional roles that related obligate symbionts play using multiple approaches. We revealed differences in the provision of EAAs by the same obligate symbiont across various auchenorrhynchan lineages, and demonstrated that the morphogenesis of symbiotic organs is closely associated with the symbiont community: (i) hosts acquiring only obligate symbiotic bacteria form bacteriomes to harbor them; (ii) hosts having only a yeast-like fungal symbiont (YLS) form fat bodies to harbor the YLS cells, but no bacteriomes evolved; and (iii) hosts harboring both obligate symbiotic bacteria and YLS form bacteriomes to harbor bacteria, and YLS gradually migrate to the fat bodies with the development of the host insects, although they initially co-colonized the bacteriomes. The results indicate that only the obligate bacterial symbiont(s) initiate the morphogenesis and formation of the bacteriomes. It highlights adaptive mechanisms underlying the origin and evolution of symbiotic organs in plant sap-feeding insects and provides new insights into their co-evolution with microbial partners.