Identification of a putative RNAse III (dicer homolog) gene in silkworm Bombyx mori

Like other invertebrates, silkworms also encounter a problem from microbial infection including from RNA viruses. In insects, RNA interference acts as a natural anitiviral response to RNA virus infection. Especially in Drosophila, it is proved that dicer mediated RNA interference directs innate immunity against RNA viruses. This information prompted us to identify similar RNAse III (dicer) gene in mulberry silkworm Bombyx mori. The Drosophila dicer gene was BLAST-searched with B. mori genome and single contig (GenBank accession n° AADK01001038) showed maximum homology with dicer gene, through which the RNAse III gene sequence was identified in the genome of silkworm B. mori. The RNAse III domain was present in the three regions with the length of 278 bp, 277 bp and 185 bp in the contig, possibly these three regions form exons. The primers were designed for three B. mori RNAse III regions and amplified through PCR. The region I was amplified only in pure Mysore silkworm strain whereas all three regions were amplified in Daizo strain. The PCR product sequences were translated and showed RNAse III domain with in the amplified product. The predicted B. mori RNAse III domain had phylogenetic relationship with other insect dicer genes. We presume that this RNAse III (dicer) would protect B. mori larvae from invading RNA viruses, which exists in the other insects.