ABSTRACT
Bombyx mori is a lepidopteran insect with important economic value. Bombyx mori nucleopolyhedrovirus (BmNPV) causes huge economic loss to silkworm industry in China every year. The objective of this study is to determine the anti-BmNPV mechanism of Bombyx mori strain NC99R, and to provide a basis for understanding the molecular mechanism of the silkworm resistance strain. The normal control Dazao (DZ) strain and the NC99R resistant strain were fed with occlusion bodies (OB). The median lethal dose (LD50) analysis of the DZ and NC99R showed that the LD50 of DZ was 1.2×10⁵ OBs/larva, while NC99R was 1.8×10⁶ OBs/larva. The LD50 of the NC99R was about 15 times higher than the DZ. The mortality of DZ and NC99R were analyzed, which were fed with 1×10⁶ OBs/larva and injection with 1×10⁶ BVs/larva. The results showed that the death peak of DZ was concentrated in the 4th to 6th day. And the death peak of NC99R was concentrated in the 6th to 8th day, with a delay of 1-2 days compared with the control. The BmNPV DNA copy number showed that the BmNPV genome in DZ proliferated rapidly. The copy number of BmNPV DNA in NC99R were increased slowly after oral infection and body injection. HE staining showed that midgut tissue has no significant difference between DZ and NC99R in the early stage of oral infection. At 96 h p.i., the nucleus of DZ midgut became larger and shedding. The NC99R had enlarged nuclei, but the cells were still arranged neatly. Finally, the expression of virus genes in different periods were analyzed by RT-PCR. The results indicated that the immediate early gene ie-1 expression levels began to down-regulate after 24 h p.i.. The early, late, and extremely late genes were also down-regulated, and finally maintained at a lower expression level.