Phenotypic and molecular insights into a cypovirus isolated from Antheraea assamensis Helfer (Lepidoptera: Saturniidae) and modelling of its polyhedrin protein structure.

Abstract

Antheraea assamensis Helfer (A. assamensis) or Muga silkworm is popularly known for producing golden silk and endemic to the region of Northeast India. The present work characterizes a cypovirus variant infecting A. assamensis larvae, exhibiting characteristic symptoms of flacherie disease. Scanning electron microscope and transmission electron microscope imaging revealed the presence of polyhedral occlusion bodies (OBs) and virion particles measuring 40-50 nm in size. The cypovirus strain comprised of 10 dsRNA genome segments, which were sequenced, assembled and annotated. The encoded viral proteins from different genomic fragments were studied. The phylogenetic analysis of the RNA-dependent RNA polymerase and polyhedrin revealed a close relationship with the previously classified Antheraea mylitta cypovirus 4. The strain was characterized as Antheraea assamensis cypovirus 4 (AaCPV4) with substantial genomic and proteomic evidence that was previously unexplored. The peptide fingerprints of the polyhedrin protein were analysed in the diseased and healthy silkworm lysate by using LC-MS/MS. The polyhedrin protein of AaCPV4 was modelled by different in silico methods and compared with the previously reported cypovirus strains. The multimeric models of polyhedrin were studied and demonstrated the mechanism of formation of OB geometry. Our study provides new insights into the complete genome of AaCPV4 and its viral proteins, which were previously unknown. The present work will help in understanding the differentiation of CPV4 variants infecting Antheraea species and different host adaptations.