Replication of Bombyx mori Densonucleosis Virus (Zhenjiang isolate) in Different Silkworm Strains

Abstract:

The invertebrate parvovirus Bombyx mori Densonucleosis Virus type 3（Zhenjiang isolate）,named BmDNV-3，is a kind of bidensovirus. The most obvious characteristic in the genome of BmDNV-3 is that it has 2 sets of DNA molecular (VD1, VD2),and each of them is encapsidated respectively in the form of single-stranded liner DNA (+VD1,-VD1,+VD2,-VD2) in equal percentage. So the BmDNV-3 has 4 kinds of virions. Furthermore the sequence of BmDNV-3 is able to encode DNA polymerase itself. Some strains of silkworm revealed complete resistance to BmDNV-3, so they didn't fall sick. To investigate the difference in the process of infection and replication between the 2 virions(VD1,VD2) of this bidensovirus, and the difference of the increment in the resistant or susceptible host, the 5th instar larvae of the susceptible silkworm strain (HUABA 35 )and the resistant silkworm strain(QIUFENG d) were inoculated determinate dose of BmDNV-3 by oral ingestion. Then the midgut were collected at 9 timepoints. The silkworm cytoplasm actin A3 was used to be normalized gene, so the number of cells in collected tissue could be determined. The result shows that whatever in the susceptible silkworm strain or in the resistant one, the copies of VD1 and VD2 in the genome of BmDNV-3 collected at the different timepoint were almost at the equal level respectively, so that the VD1 and VD2 were replicated with synchronization. The process of infection in the susceptible silkworm strain was devided into 3 partitions, latent period(2～12 hours post inoculation), exponential phase (12～36 hours post inoculation)and stationary phase (36～96 hours post inoculation and there are about 2×10⁵ copies per cell). In the resistant silkworm strain, the virus were replicated at a very low level, that was from 6～10 copies 2 hours post inoculation to 150～200 copies 96 hours post inoculation (about 20 times). So we predict that the resistance in some of the silkworm strains from BmDNV-3 was a kind of chronic representation that the host carried virus without being caused flacherie.