A temperature-sensitive (ts) mutant of bombyx mori nucleopolyhedrovirus (BmNPV), ts-S1, contains a mutation in a putative RNA polymerase gene, which is involved in late viral gene expression. When ...4th-instar silkworm larvae were infected with 1.0 x 10(5) TCID50 of ts-S1 per larva and reared at 33 degrees C, the titre of budded virus (BV) and number of occlusion bodies (OBs) in the haemolymph of the infected larvae were very low in the early stage but markedly increased in the late stage of infection. In contrast, a rapid increase in both BV titre and OB number was detected in the haemolymph of infected larvae reared at 25 degrees C. LD50 values of ts-S1 and wild type BmNPV (wtBmNPV) for 4th-instar larvae were 2.41 and 0.96 TCID50 per larva at 25 degrees C, and > 1.0 x 10(6) and 1.70 TCID50 per larva at 33 degrees C, respectively. These results indicate that the virulence of ts-S1 for the larvae reared at 33 degrees C was markedly reduced. To examine further the reduction of ts-S1 virulence at the non-permissive temperature of 33 degrees C, silkworm larvae were infected with ts-S1 at the multiplicity of 1.0 x 10(2) - 1.0 x 10(6) TCID50 per larva, reared for various time at 33 degrees C and then shifted to 25 degrees C. Longer rearing periods at 33 degrees C resulted in better survival rates indicating that the reduction of virulence of ts-S1 was proportional to cumulative rearing time at 33 degrees C. When large virus inocula were used, a growth alteration of larvae was preferentially induced. However, when small virus inocula were used, the appearance of abortive infection due to the non-permissive temperature became more evident.
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