Transcriptional Reshaping of Bacteriocytes in the Aphid-Serratia Symbiosis.

The facultative endosymbiont Serratia symbiotica significantly influences the ecological fitness of its aphid host. However, the molecular mechanisms by which Serratia affects the host's symbiotic organ, the bacteriocyte, remain poorly understood. Here, we conducted a comparative transcriptomic analysis of bacteriocytes from Serratia- and Serratia+ pea aphid (Acyrthosiphon pisum) strains. Our analysis revealed that Serratia colonization extensively modulates gene expression within bacteriocytes. Key metabolic pathways were significantly altered: genes involved in ribosomal biogenesis and oxidative phosphorylation were upregulated, while those in fatty acid biosynthesis were downregulated. Furthermore, we observed a complex reshaping of the immune profile, characterized by a broad downregulation of immune recognition and signaling components alongside an upregulation of specific effector genes and antioxidant enzymes. These findings suggest that Serratia induces a state of enhanced anabolic capacity and energy production in bacteriocytes, coupled with strategic reallocation of resources and a finely tuned immune response that balances symbiont tolerance with control. Our RT-qPCR validation confirmed the RNA-seq results, further supporting these results. This study provides the first bacteriocyte-specific transcriptomic resource for the aphid-Serratia interaction system, offering novel insights into the molecular integration of a facultative symbiont into host physiology.