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.