A free‐living Serratia symbiotica strain enhances aphid development, potentially through alteration of host nutritional composition

BACKGROUND Aphids harbor diverse microbial communities that influence their development, reproduction, and stress tolerance. In our previous work, we identified a free‐living strain of Serratia symbiotica , SsMj, which is highly abundant in the gut of Megoura crassicauda . However, the biological effects of SsMj on its aphid host remain largely unknown. RESULTS In this study, SsMj– M. crassicauda were generated by rearing newly hatched nymphs separately from their parthenogenetic SsMj+ parents. Absolute quantification of the microbiome revealed that, apart from the loss of SsMj, the overall bacterial abundance and diversity did not differ significantly between SsMj– and SsMj+ aphids. Nevertheless, SsMj– individuals exhibited slower development, smaller body size, reduced survival, and produced more offspring compared to their SsMj+ counterparts. Metabolomic analyses further showed that SsMj– aphids accumulated higher levels of several sugars but lower concentrations of multiple amino acids. Consistently, the insulin‐like peptide ( ILP ) gene showed elevated expression across developmental stages in SsMj– aphids, which is likely to reflect a response to nutrient imbalance. RNA interference targeting ILP significantly delayed development, confirming its regulatory role in aphid growth. Comparative genomics showed that the SsMj genome contains a high number of genes involved in amino acid synthesis pathways than both obligate and facultative S. symbiotica strains, a pattern consistent with other free‐living strains. CONCLUSION Our findings indicate that S. symbiotica plays an essential role in aphid nutrient metabolism, and is likely to be facilitating the conversion of dietary sugars into amino acids to support host development. The fitness benefits conferred by this free‐living S. symbiotica strain suggest a close, mutualistic‐like association with its aphid host, highlighting its ecological and physiological significance in insect–microbe interactions.