The ecdysteroid UDP-glucosyltransferase gene of Autographa californica nucleopolyhedrovirus alters the moulting and metamorphosis of a non-target insect, the silkworm, Bombyx mori (Lepidoptera, Bombycidae)

Abstract

The Autographa californica nucleopolyhedrovirus (AcMNPV) does not infect the silkworm and molecular studies on silkworm insusceptibility have not been performed. In cultured cells of the silkworm, the expression of viral genes has been reported. The expression of AcMNPV genes and their effect in vivo and in vitro was studied. In this study, the early gene, the ecdysteroid UDP-glucosyltransferase (egt) gene of AcMNPV, which inactivates the insect moulting hormone by sugar conjugation, was examined to determine whether it would alter the growth of the silkworm. Using wild-type (wt) AcMNPV, the egt gene deletion virus (vEGTDEL), and the virus carrying the egt promoter-lacZ cassette in vEGTDEL (vEGTZ), the egt promoter-driven expression in cultured cells and in nonproductive infection of the silkworm was characterized. Infection of cultured cells with vEGTZ at three different doses occurred in a single cell manner. When budded wt AcMNPV was injected into the fourth and fifth instar larvae, an increase in the amount of virus occurred and caused abnormal larval growth, which resulted in the prolongation or skipping of the larval instar, premature pupation, or death during the pupal stage. For infection of the fourth instar larvae, precocious metamorphosis was observed. When the same amount of vEGTDEL was injected, the alteration of growth did not occur. These results suggest that the egt gene was expressed in the primary infected cells of the silkworm, and that the EGT was secreted into the haemocoel, which significantly altered larval growth.