Non-Canonical, Somatic-Dependent Vertical Transmission of Wolbachia in an Aphid.

Wolbachia, a widespread endosymbiotic bacterium that infects a broad range of arthropods and nematodes, relies on vertical transmission from mother to offspring. This process often involves colonisation of the host germline, subsequent transfer to developing oocytes, and utilisation of host yolk protein transport mechanisms such as vitellogenin uptake. However, the transmission strategies employed by Wolbachia in viviparous insects such as aphids are poorly understood. Here, we demonstrate a non-canonical Wolbachia transmission mode in the cedar bark aphid Cinara cedri: the somatic-dependent vertical transmission. After confirming consistent Wolbachia infection in C. cedri, we visualised the localization of Wolbachia, along with the obligate symbionts Buchnera aphidicola and Serratia symbiotica. Consistent with previous reports, Wolbachia in C. cedri were predominantly observed within maternal and embryonic bacteriocytes, the specialised cells housing obligate symbionts. Notably, Wolbachia cells were rarely detected in germline cells or early-stage embryos and were directly transmitted from maternal bacteriocytes to developing embryos, coinciding with obligate symbiont transfer. These results suggest that Wolbachia in C. cedri has evolved a unique "piggybacking" strategy, utilising the obligate symbiont transmission system. Our study highlights the diversity of endosymbiont maternal transmission strategies and provides new insights into the underlying molecular mechanisms of action.