Biochemical and ultrastructural alterations in prothoracic glands of Heliothis virescens (F.) (Lepidoptera: Noctuidae) last instar larvae parasitized by Cardiochiles nigriceps Viereck (Hymenoptera: Braconidae).

The present study describes both the ultrastructural and the biochemical alterations induced by the parasitoid Cardiochiles nigriceps Viereck on the prothoracic glands of the last (fifth) instar larvae of its host Heliothis virescens (F.). Gross morphological degeneration or signs of necrosis of the prothoracic gland cells were not observed. A rough endoplasmic reticulum more developed and electrondense than in non-parasitized controls was the only ultrastructural change occurring in the prothoracic glands of parasitized host last instar larvae. However, although structurally undegraded, 5 days after parasitoid oviposition into newly moulted host last (fifth) instar larvae, the prothoracic gland cells appeared to be functionally dead and became permeable to trypan blue. In contrast, cells of the prothoracic glands explanted on day 4 of the fifth instar, 3 days after parasitism, performed as above, were still alive and did not absorb the trypan blue, as observed in synchronous non-parasitized controls. These glands were used to study in vitro the major biochemical alterations at cellular level induced by the parasitoid. Ecdysteroid production, RNA synthesis, protein synthesis and phosphorylation were studied in response to crude prothoracicotropic hormone (PTTH) extract, forskolin and dibutyryl cAMP (dbcAMP), these latter compounds mimicking the PTTH action at different levels of the signal transduction pathway. The prothoracic glands of H. virescens last instar larvae, parasitized by C. nigriceps, showed a severely depressed biosynthesis of ecdysteroids, which was not resumed with any of the stimulating treatments performed. This severely depressed biosynthetic activity was associated with a remarkable reduction in the rate of total protein synthesis and to a marked underphosphorylation of those proteins normally phosphorylated when healthy glands are challenged with dbcAMP. However, the early steps of the PTTH signal transduction pathway in the host prothoracic glands are apparently not altered by C. nigriceps parasitism. This is supported by the identical effect of PTTH and dbcAMP treatments on RNA synthesis in the prothoracic glands of both parasitized and non-parasitized larvae. In fact, in both cases a reduced level of [H-3]uridine incorporation into the total RNA synthesized in vitro was observed in stimulated glands. These experimental data would suggest the occurrence of a parasitoid-induced block of protein synthesis translation in host prothoracic glands, partly determined by the marked underphosphorylation of key target regulatory proteins.