Vertical transmission of Wolbachia bypassing the germ line 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 colonization of the host germline, subsequent transfer to developing oocytes, and utilization 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 novel Wolbachia transmission mode in the cedar bark aphid Cinara cedri that bypasses germline cells. After confirming stable Wolbachia infection in C. cedri, we visualized 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 specialized 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, utilizing 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.