Prophenoloxidases 1 and 2 from the spruce budworm, Choristoneura fumiferana:: Molecular cloning and assessment of transcriptional regulation by a polydnavirus

Immune challenge in arthropods is frequently accompanied by melanization of the hemolymph, a reaction triggered by the activation of prophenoloxidase (PPO). Because their immature stages are spent inside the hemocoel of insect larvae, endoparasitoids have evolved strategies to escape or counter melanin formation. Very little molecular information is available on these endoparasitoid counterstrategies. We have sought to shed light on the inhibition of melanization in the spruce budworm, Choristioneura fumiferana, by the parasitic wasp Tranosema rostrale, by cloning two host PPO homologs and studying their transcriptional regulation after parasitization. The two polypeptides are encoded by transcripts of similar to 3.3 kb (for CfPP01) and 3.0 kb (for CfPP02) and possess structural features typical of other insect PPOs. While there appears to be a single CfPP02 gene in the C. fumiferana genome, we detected three CfPP01 mRNA variants displaying insertions/cleletions in the 3' untranslated region, suggesting that there may be more than one CfPP01 gene copy. Both CfPP01 and CfPP02 were expressed at high levels in C. fumiferana 6th instars, and parasitization by T. rostrale had no apparent impact on the level of their transcripts. Injection of a large dose (0.5 female-equivalent) of polydnavirus-laden calyx fluid extracted from T. rostrale, which is known to inhibit melanization in C. fumiferana, only caused a transient decrease in CfPP01 and CfPP02 transcript accumulation at 2-3 d post injection. It thus appears that transcriptional downregulation of C. fumiferana PPO by T. rostrale plays a minor role in the inhibition of hemolymph melanization in this host-parasitoid system.