Abstract
The Bombyx mori nucleopolyhedrovirus (BmNPV) La strain is a variant isolated in Laos (Iwashita, 1993). Although DNA fingerprinting analysis revealed nucleotide sequence variations in the genomes of BmNPV La and T3 (the type strain of BmNPV), other features of La, including virulence and growth properties, are unknown. Here we compared the pathogenicity and replication of La and T3 in B. mori larvae and cultured cells. Larval bioassays revealed that the median lethal dose of La BV was approximately 2-fold lower compared with that of T3. We found that La induced earlier host liquefaction compared with T3, and the median lethality time of La was approximately 6h shorter compared with that of T3. Further, La-infected B. mori larvae released more occlusion bodies (OBs) into the hemolymph compared with those infected with T3. These results show that the virulence of La is higher compared with that of T3. Moreover, La may be suitable for use in a BmNPV-based expression vector system, because La produced approximately 4-fold and 1.75-fold higher levels of polyhedrin mRNA and protein, respectively, compared with T3. Moreover, we found that multicapsid occlusion-derived viruses (ODVs) were more numerous in La-infected cells compared with those in T3-infected cells. However, there was no significant difference in the median lethal oral dose. Together, these results show that La exhibits unique features that mediate pathogenesis, production of the Polyhedrin protein, and the formation of multicapsid ODVs.
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