Densovirus infection in silkworm Bombyx mori and genes associated with disease resistance

Abstract

The silkmoth Bombyx mori has been bred in captivity for around 5,000 years and it is now a
completely domesticated species of the silkmoth. The larva of B. mori feeds only on Mulberry leaves so it is a monophagous insect. Silk cocoons obtained from this species are the primary source of commercial silk and this makes B. mori an economically important insect. However, the silk industry suffers significant losses due to various viral infections during the larval stages. One of the frequently affecting silkworm viruses is the B. mori Densovirus (BmDV). The BmDV is further classified into two types: B. mori Densovirus-1 (BmDV-1) and B. mori Densovirus-2 (BmDV-2). However, BmDV-2 is excluded from the family of Parvoviridae and is referred to a new family Bidnaviridae. To date, three isolates of BmDVs have been reported. This virus has been found to be a causative agent of the commonly occurring fatal silkworm disease, ‘flacherie’. BmDVs have been found to be a highly diverse group of viruses. While most of the strains of B. mori are susceptible to BmDV, few races have been found to be completely resistant to the virus. Studies have shown both dominant and recessive alleles to be responsible for the resistance. So far four genes have been reported conferring resistance against BmDV-1 and BmDV-2. These are the Nid-1 and nsd-1 genes against BmDV-1 and nsd-2 and nsd-Z genes against BmDV-2. Details about Densovirus with special reference to B. mori and the resistant genes present against it and the studies undertaken towards screening of BmDV resistant silkworm races have been discussed in this review.