Insects in the Order Odonata are highly subject to infection by gregarine parasites. However, despite the important ecological roles that insects play in every ecosystem in which they exist, little research has been devoted to the description of insect immunity. Insects rely heavily on the rapid actions of innate immune mechanisms to prevent infection. We characterized the melanization response in the hemolymph of green darner dragonfly (Anax junius) nymphs. Incubation of chymotrypsin-activated hemolymph with L-DOPA resulted in volume- and time-dependent production of dopaquinone via the phenoloxidase (PO) enzyme, with biphasic accumulation of product. The PO activity was temperature-dependent, with a stepwise increase from 20℃ - 35℃ and maximum activity measured at 35℃ - 40℃. The formation of product was also inhibited in a concentration-dependent manner by diethylcarbonate, a specific inhibitor of PO activity, which indicated that the observed activity was due to the presence of PO enzyme. The rate of formation and quantity of melanin was dependent on exposure to different titers of bacteria. This is the first characterization of both PO activity and melanization response in green darner dragonflies.
The pea aphid Acyrthosiphon pisum hosts different facultative symbionts(FS)which provide it with various benefits,such as tolerance to heat or protection against natural enemies(e.g.,fungi,parasitoid wasps).Here,we investigated whether and how the presence of certain FS could affect phenoloxidase(PO)activity,a key component of insect innate immunity,under normal and stressed conditions.For this,we used clones of A.pisum of difTerent genetic backgrounds(LLOl,YR2 and T3-8V1)lacking FS or harboring one or two(Regiella insecticola,Hamiltonella defensa,Serratia symbiotica Rickettsiella viridis).Gene expression and proteomics analyses of the aphid hemolymph indicated that the two A.pisum POs,PPOl and PP02,are expressed and translated into proteins.The level of PPO genes expression as well as the amount of PPO proteins and phenoloxidase activity in the hemolymph depended on both the aphid genotype and FS species.In particular,H.defensa and R.insecticola,but not S.symbiotica-h R.viridis,caused a sharp decrease in PO activity by interfering with both transcription and translation.The microinjection of different types of stressors(yeast,Escherichia coli,latex beads)in the YR2 lines hosting different symbionts affected the survival rate of aphids and,in most cases,also decreased the expression of PPO genes after 24 h.The amount and activity of PPO proteins varied according to the type of FS and stressor,without clear corresponding changes in gene expression.These data demonstrate that the presence of certain FS influences an important component of pea aphid immunity.