Horizontally transferred NADAR genes contribute to immune defense of ladybird beetles against bacterial infection

Abstract

Horizontal gene transfer (HGT) is now widely recognized as an important mechanism contributing to host immunity and adaptation. Ladybird beetles, with their diverse diets and habitats, encounter a broad spectrum of microbial threats, making effective immune responses critical for their survival. However, the immune roles of HGT-acquired genes in ladybirds remain largely unexplored. To address this gap, we investigated HGT of a NADAR (NAD- and ADP-ribose-associated) domain-containing gene from microorganisms to insects. Phylogenetic analyses revealed that NADAR genes in ladybird beetles form a well-supported clade nested within a larger group composed primarily of bacterial sequences, providing strong evidence for an HGT origin. Sampling across 69 ladybird species suggests that NADAR genes originated in the Coccinellidae family and were subsequently retained or duplicated across ladybird genomes, indicating their functional importance. Using the ladybird Cryptolaemus montrouzieri as a model, we observed that the expression levels of CmNADAR1 and CmNADAR2 were significantly upregulated in response to bacterial infection. Immune challenges combined with RNA interference targeting NADAR genes led to reduced survival rates and marked necrosis in intestinal tissues, compared to controls exposed to either bacterial infection or dsRNA alone. Together, our results demonstrate that NADAR genes in ladybird beetles were acquired through horizontal gene transfer and contribute to immune defense against bacterial infection.