The IMD pathway in Hemipteran: A comparative analysis and discussion.

The evolutionary patterns of the genes in the IMD pathway in hemipterans were characterized and compared. The hemipteran insects were clustered into two groups. One group that encompasses whitefly, plant lice, and scale insect partially lacks the IMD pathway and all antimicrobial peptide (AMP) genes, with the vast majority of IMD pathway and all AMP genes being absent in aphids. The reasons for the absence of the IMD pathway and AMP genes in hemipterans were analyzed based on aphids, in terms of fitness costs. In case of limited resources, aphids have to make a trade-off between the necessary costs such as clean food sources, the essential amino acids supplied by primary bacterial symbionts for survival, nutrients and/or protections against stress provided by secondary symbionts, and the high reproductive capacity, and the costs that do not increase the fitness. Obviously, aphids have to abandon the strong immune system, especially the AMPs and IMD pathway which is mainly against Gram-negative bacteria. The common ground shared with aphids may be the reason for the absence of the IMD pathway and AMP genes in other hemipteran insects.