Larval-Transcriptome Dynamics of Ectropis grisescens Reveals Differences in Virulence Mechanism between Two EcobNPV Strains

Simple Summary Baculoviruses are virulent pathogens of a number of important insect-pest species. Ectropis grisescens is a serious leaf-eating pest in tea plantations, which causes incalculable losses to tea production. Ectropis obliqua nucleopolyhedrovirus (EcobNPV), as a biological insecticide, has been applied to control E. grisescens. Most prior detailed studies of EcobNPV-strain infections have focused on virulence bioassay. In the previous period, our group screened a highly efficient strain, EcobNPV-QF4, which is more virulent for E. grisescens than EcobNPV-QV. In this study, we compared the virulence of the strains using the leaf-disc method and generated larval-transcriptomes of infected E. grisescens at different time-points after infection. Understanding the transcriptional responses of the host to viral infection is critically important for understanding the mechanism of the differences in virulence between the two strains. The biological insecticide, Ectropis obliqua nucleopolyhedrovirus (EcobNPV), has been applied to control the major tea-pest Ectropis grisescens. Previously, the virus strain EcobNPV-QF4 showed higher a mortality rate (58.2% vs. 88.2%) and shorter median lethal-time (13.9 d vs. 15.4 d) on E. grisescens than the strain EcobNPV-QV. However, the mechanism of the difference in virulence between the two strains remains unclear. Using the leaf-disc method, we detected the virulence of the two strains on 3(rd)-instar larvae, and found that median lethal-dose (LD50) of EcobNPV-QF4 is 55-fold higher than that of EcobNPV-QV (4.35 x 10(8) vs. 7.89 x 10(6)). Furthermore, fourteen larva transcriptomes of E. grisescens were subsequently sequenced at seven time-points after ingestion of the two virus strains, yielding 410.72 Gb of raw reads. Differential gene-expression analysis shows that 595, 87, 27, 108, 0, 12, and 290 genes were up-regulated in EcobNPV-QF4 at 0, 2, 6, 12, 24, 36 h and 48 h post ingestion (hpi), while 744, 68, 152, 8, 1, 0, 225 were down-regulated. KEGG enrichment showed that when the virus first invades (eats the leaf-discs), EcobNPV-QF4 mainly affects pathways such as ribosome (p-value = 2.47 x 10(-29)), and at 48 hpi EcobNPV-QF4, causes dramatic changes in the amino-acid-synthesis pathway and ribosome pathway (p-value = 6.94 x 10(-13)) in E. grisescens. Among these, thirteen key genes related to immunity were screened. The present study provides the first ever comprehensive analysis of transcriptional changes in E. grisescens after ingestion of the two strains of EcobNPV.