Functional marker assisted improvement of productive mulberry silkworm breeds conferring resistance to Bombyx mori Bidensovirus (BmBDV)

Abstract

Flacherie is a widely prevalent disease in silkworms that causes huge economic losses to the sericulture industry. The Bombyx mori bidensovirus (BmBDV) is one of the causative agents of this disease. A major gene, nsd-2 (non-susceptibility to densovirus-2), a putative BmBDV receptor conferring resistance under recessive mutation condition has previously been identified. The natural deletion occurring in the nsd-2 gene has contributed to the evolution of BmBDV resistant silkworm breeds. However, nsd-2 has not yet been utilized as a marker or as a transgene for the development of BmBDV resistant breeds. In this study, we have demonstrated the use of the functional marker, nsd-2 for the development of absolute BmBDV resistant productive mulberry silkworm breeds. We screened 49 productive bivoltine breeds and identified 28 breeds carrying the nsd-2 resistant allele. Sixteen breeds having the resistant allele were identified in heterozygous condition at the nsd-2 locus, so these breeds could not be identified as resistant carrier when challenged with virus inoculation. Further, we have shown that selecting nsd-2 resistant allele in homozygous condition, at the nsd-2 locus is sufficient for the development of BmBDV resistant productive breeds like CSR6-R and CSR26-R. We have also validated the developed breeds for resistance to BmBDV through bioassays and found absence of viral genome at the molecular level thereby demonstrating complete resistance upon BmBDV infection, in the developed breeds.