Ecological Significance of a Novel Nitrogen Fixation Mechanism in the Wax Scale Insect Ericerus pela

As a sessile wax scale insect, Ericerus pela heavily relies on its host plant for nutrition. While E. pela utilizes the nitrogen-poor plant sap as its primary nutrient source, the mechanisms by which this insect overcomes the nitrogen deficiency are poorly understood. In this study, we first confirm the nitrogen fixation capability of E. pela through isotopic tracer experiments and the acetylene reduction assay, which demonstrate that female adults exhibit an efficient nitrogen fixation rate. High-throughput sequencing further revealed 42 nitrogen-fixing bacterial species in the tissues of E. pela, most notably including Rhizobiales and Methylobacterium as the dominant species converting atmospheric nitrogen to ammonia. Several critical genes involved in nitrogen fixation, ammonia transporting, amino acid synthesis, and transportation were determined to be transcriptionally active across different developmental stages of E. pela. In addition, the symbiotic fungus Ophiocordyceps-located in the fat body of E. pela-was found to be capable of synthesizing all amino acids, including the essential amino acids required for the survival of E. pela. Taken together, this study demonstrates that E. pela has evolved a highly effective nitrogen acquisition system driven by symbiotic microorganisms, ensuring a sufficient nitrogen supply and enabling it to thrive on nitrogen-deficient food sources. Our findings reveal a unique evolutionary adaptation in which E. pela leveraged both bacterial nitrogen fixation and fungal amino acid synthesis to bolster its growth and development.