Dependence of ecdysteroid metabolism and development in host larvae on the time of baculovirus infection and the activity of the UDP-glucosyl transferase gene

Abstract

Infection of fourth-instar gypsy moth ( Lymantria dispar, Lepidoptera: Lymantriidae) larvae with the wild-type (Wt) gypsy moth baculovirus, LdNPV on the first day post-molt, or infection of fifth instars on the fifth day post-molt, results in elevated ecdysteroid levels in both hemolymph and the media of cultured prothoracic glands, when examined by radioimmunoassay and compared to mock-infected controls. In contrast, insects infected with an ecdysteroid UDP-glucosyl transferase minus [ egt(-)] mutant virus show ecdysteroid levels and gland activity similar to that found in controls. UDP-glucosyl transferase causes sugar-conjugation of ecdysteroids containing a hydroxyl group at the C-22 position, and our ecdysteroid antiserum recognizes these conjugated ecdysteroids. Since we found that 20-hydroxyecdysone inhibits gland activity in culture and glucose-conjugated ecdysone, the major form of ecdysteroid in the hemolymph of Wt baculovirus-infected animals does not, the elevated level of hemolymph ecdysteroids and heightened gland activity in Wt virus-infected animals are most likely due to low levels of 20-hydroxyecdysone resulting from sugar conjugation of ecdysone by EGT. Interestingly, hemolymph ecdysteroid titer and gland activity of fifth-instar larvae infected with either the Wt or egt(-) virus on the first day post-molt remain at basal levels throughout the instar, with death occurring prior to pupation, suggesting that LdNPV may have a second, egt-independent mechanism for regulation of host ecdysteroid levels.