Microparasite manipulation of an insect:: the influence of the egt gene on the interaction between a baculovirus and its lepidopteran host

1. Parasites and pathogens manipulate their hosts in a variety of ways that are thought to enhance their fitness. However, it is rare to be able to link such phenotypic changes to specific genes. 2. Here the effect of a single pathogen gene is examined. The ecdysteroid UDP-glucosyltransferase (egt) gene of insect baculoviruses produces an enzyme that interferes with host moulting. 3. The effect of the egt gene was examined by comparing two baculoviruses that differed only in the expression of this gene. All three fitness traits examined - pathogenicity, infection duration and pathogen productivity - were affected by deletion of the gene. 4. Trichoplusia ni larvae in all five instars died earlier when infected with the egt-minus virus compared with those infected by the wild-type Autographa californica nucleopolyhedrovirus. 5. Unexpectedly, the egt-minus virus was more pathogenic to final instar larvae than the wild-type virus. Virus genotype and dose both influenced insect development. 6. Wild-type infected insects had a significantly higher yield of virus at death, cadaver weight and yield of virus per unit weight than those infected with the egt-minus virus. 7. The size of the virus challenge had a major influence on the outcome of the interaction. The consequences of these data for pathogen fitness are discussed.