Melanization is an effective defence reaction used by mosquito hosts to kill malarial and filarial worm parasites. Although phenoloxidase (PO) has long been considered to be the key enzyme in the biosynthesis of melanotic material in insects, there is no direct evidence verifying its role in parasite melanization. To elucidate the role of PO in the melanization of microfilariae (mf) by mosquitoes, a double subgenomic Sindbis (dsSIN) recombinant virus was used to transduce Armigeres subalbatus mosquitoes with a 600 base antisense RNA targeted to the highly conserved copper-binding region of an Ar. subalbatus PO gene. Compared with controls, haemolymph PO activity in mosquitoes transduced with antisense RNA was significantly reduced. When these mosquitoes were challenged with Dirofilaria immitis mf, the melanization of mf was almost completely inhibited. These data verify that PO is an essential component of the biochemical pathway required for the melanization of parasites, and that the dsSIN expression system represents a useful tool in the functional analysis of endogenous gene expression in mosquitoes.
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