Horizontal gene transfer
The C. panzhihuaensis genome contains four copies of an insecticidal toxin gene family named fit D that was acquired via ancient horizontal transfer from fungi (Lui et al ., 2022). Thesefit D genes encode proteins similar to the fit toxin and MCF toxin of Photorhabdus bacteria, which are lethal to insects. Experimental injection of the synthesized C. panzhihuaensis FitD protein into moth larvae resulted in significantly higher mortality than controls, demonstrating that this protein provides resistance against at least some insect herbivores. Phylogenetic analyses indicate thefitD gene family originated from bacteria and the cycad sequences share a common ancestor with fungi fit D with subsequent gene family expansion in the ancestor of C. panzhihuaensis andCycas debaoensis . The acquisition of this toxin gene family represents an important innovation that likely enhanced the defense capacities of Cycas against insect pests and illustrates how horizontal transfer of foreign genes can provide adaptive benefits.