The biology of melanization reactions in mosquitoes

osquitoes are able to limit infection with filarial parasites through a resistance response known as melanotic encapsulation, wherein the parasite is recognized then surrounded by a capsule that blackens and ultimately kills the parasite. Melanization proceeds through a catecholamine metabolizing pathway that begins with the hydroxylation of tyrosine by the enzyme phenol oxidase, a chemical process that is also essential for tanning of the cuticle and egg chorion. Identification of specific steps in the regulation of this pathway could lead to a better understanding of what makes some mosquitoes inefficient vectors. We have identified a reproductive cost, in the form of delayed egg development and oviposition, to Armigeres subalbatus undergoing a defense response against the parasite, Brugia malayi. Because of reduced tyrosine levels in the developing ovaries of these mosquitoes, it is proposed that the chemistries of egg chorion tanning and parasite melanization are not independent. Hemolymph of Ar. subalbatus undergoing an active defense response contained higher dopa decarboxylase (Ddc) activity, an enzyme known to be involved in egg chorion tanning reactions. Consequently, we initiated studies to isolate mosquito Ddc cDNA. The gene encodes a tissue specific transcript that is up-regulated in the ovaries in response to bloodfeeding. Gene structure was evaluated by sequence analysis of a cosmid clone containing the entire gene. Four introns are contained within the 11kb gene. Two kilobases of the DNA sequence...