C-type lectins in arthropods: Evolutionary diversification and prospects for pathogen defense

C-type lectins (CTLs) were initially characterized as calcium-dependent carbohydrate-binding proteins containing conserved carbohydrate-recognition domains (CRDs). However, accumulating evidence has revealed extensive structural and functional diversification within the C-type lectin superfamily. In arthropods, which depend exclusively on innate immunity, CTLs have evolved well beyond simple pathogen recognition receptors and function as integrative hubs that couple non-self detection with coordinated activation of immune effectors. This review systematically summarizes recent advances in arthropod CTL research, emphasizing their central roles in pathogen recognition, melanization, phagocytosis, antimicrobial peptide induction, antiviral defense, and microbiota homeostasis. Functional diversification is driven by the modulation of calcium dependence, variation in CRD motifs, domain remodeling, lineage-specific gene expansion, and RNA-level plasticity, thereby enabling flexible immune responses under diverse ecological pressures. Unlike vertebrate lectins, which amplify complement cascades, arthropod CTLs operate entirely within innate immune circuits and directly link immune recognition to prophenoloxidase activation and signaling pathways such as IMD and JAK/STAT. From an evolutionary perspective, CTL diversification demonstrates how innate immune systems acquire adaptability and functional complexity through domain recombination and gene-family expansion. These insights provide a unified framework for understanding arthropod immune evolution and offer theoretical guidance for vector control and disease-resistant aquaculture.