MECHANISM OF PARASITISM-INDUCED ELEVATION OF HEMOLYMPH GROWTH-BLOCKING PEPTIDE LEVELS IN HOST INSECT LARVAE (PSEUDALETIA-SEPARATA)

Growth-blocking peptide (GBP) has been purified for the first time fro m the haemolymph of the host armyworm Pseudaletia separata whose growt h is inhibited and shows developmental arrest in the last larval insta r stage when parasitized by the parasitoid wasp Cotesia kariyai. GBP n aturally occurs in the haemolymph of lepidopteran larvae but its conce ntration is very low during the last larval instar in comparison with that in the penultimate larval instar. However, by 24 h after parasiti zation or polydnavirus (PdV)-infection on day 0 of the last larval ins tar, a four-fold increase in GBP level, compared with synchronous non- parasitized control larvae, is observed. Although Northern blot analys is indicates that GBP mRNA is transcribed in brain-nerve cord and fat body, plasma GBP is likely to be secreted mainly from fat body because the GBP mRNA level is approximately 100-fold higher in fat body than that in brain-nerve cord. RT-PCR analysis demonstrates the constant ex pression of GBP mRNA in both parasitized (or PdV-infected) and non-par asitized larval fat body, which suggests that parasitism does not infl uence transcriptional level, but might influence post-transcriptional level to elevate plasma GBP concentration. This interpretation was sup ported by estimating GBP precursor levels in fat body of PdV-infected and non-infected larvae. Virus infection appears to elevate the GBP pr ecursor levels in fat body to about six times greater than that in non -infected last instar larvae by 6h after PdV-injection. The GBP proces sing enzyme activity that occurs in Golgi body-rich extract of the fat body is increased by about 90% after parasitization or PdV-injection. (C) 1998 Elsevier Science Ltd. All rights reserved.