Genome identification of C-type lectins in Bactrocera dorsalis and functional characterization of BdCTL-S12 with a broad-spectrum pathogen recognition

BACKGROUND Bactrocera dorsalis (Diptera: Tephritidae) is a highly adaptable invasive pest that causes significant economic losses. Long-term chemical control strategies have led to high levels of insecticide resistance and severe environmental risks. C-type lectins (CTLs), key pattern recognition receptors in insect innate immunity, are named for their calcium-dependent carbohydrate recognition domains (CRDs) and they play a critical role in detecting and binding to foreign pathogens. This study aims to identify B. dorsalis CTLs (BdCTLs) and investigate their roles in pathogen defense. RESULTS A total of 42 BdCTL genes were identified. Based on the number of CRDs, 36 genes were classified into the S subfamily, while the remaining six were assigned to the X subfamily. Spatio-temporal expression analysis by qPCR revealed that BdCTL genes were expressed throughout all developmental stages with a higher expression observed in the midgut, Malpighian tubules, and fat body. The expression levels of BdCTL-S4 , BdCTL-S12 , and BdCTL-S31 were significantly upregulated under the infection of the entomopathogenic fungus Beauveria bassiana . Notably, BdCTL-S12 also exhibited upregulated expression patterns in response to four different pathogens bacteria with sustained expression over time. Using RNA interference (RNAi), we successfully knocked down BdCTL-S12 expression, achieving a silencing efficiency of 68%. Survival assays demonstrated a 10–30% reduction in the survival rate of B. dorsalis after pathogen infection following dsRNA injection compared to the control group. Further in vitro experiments confirmed that BdCTL-S12 can agglutinate and bind to pathogens in a calcium-dependent manner. This binding was competitively inhibited by polysaccharides, underscoring a key functional mechanism of BdCTL-S12 in pathogen recognition. CONCLUSION In summary, we identified 42 BdCTLs and demonstrated that BdCTL-S12 is critical for pathogen recognition and immune defense in B. dorsalis . These results advance our understanding of the molecular mechanisms underlying B. dorsalis immunity. Specifically, elucidating the role of CTLs may facilitate the development of microbial-based biological control strategies (e.g., entomopathogenic fungi or bacteria) against this invasive pest. Additionally, RNAi-mediated knockdown of CTL-encoding genes could weaken the immune system of Bactrocera flies, increasing their susceptibility to microbial infections and thereby enhancing the effectiveness of biological control. © 2025 Society of Chemical Industry.