Sodalis transmission via the venom and sperm of the parasitoid wasp Spalangia cameroni

Microbial symbionts of multicellular hosts originate from free-living ancestors, but their mechanisms of establishment remain largely enigmatic. Here we studied the acquisition and transmission routes in a nascent symbiosis between a cultivable Sodalis sp. (termed here Sodalis SC) and its host, the parasitoid wasp Spalangia cameroni (Hymenoptera: Spalangiidae). Aposymbiotic wasp larvae and adults acquired Sodalis SC by feeding, but the bacterium was transmitted vertically only by adults that acquired it during their larval stages. Using fluorescence in situ hybridization, we observed a successive invasion of the bacterium from the larva's digestive tract through the gut epithelium into the hemolymph and multiple host organs. However, the oocytes were completely devoid of Sodalis SC, whereas the females' venom reservoir became densely colonized. We demonstrate experimentally that Sodalis SC is injected with the females' venom into the fly hosts and later acquired by feeding larvae, which subsequently develop into infected adults. Additionally, we found that Sodalis SC is transmitted paternally at a low frequency via the seminal fluids, and females that inherit Sodalis SC from their fathers continue to transmit it via their venom. To test the specificity of the symbiosis, we replicated these experiments using Sodalis praecaptivus, a close free-living relative of Sodalis SC that can initiate symbiosis with insect hosts. By artificial infection, Spalangia cameroni readily acquired S. praecaptivus, maintained this bacterium in the venom reservoir and injected it to flies during parasitism. However, Sodalis praecaptivus vertical transmission was not achieved due to insufficient proliferation in the fly host.