Molecular characteristics and evolutionary analysis of a very virulent infectious bursal disease virus. 
Infectious bursal disease virus (IBDV) poses a significant threat to the poultry industry.  Viral protein 2 (VP2), the major structural protein of IBDV, has been subjected to frequent mutations that have imparted tremendous genetic diversity to the virus.  To determine how amino acid mutations may affect the virulence of IBDV, we built a structural model of VP2 of a very virulent strain of IBDV identified in China, vvIBDV Gx, and performed a molecular dynamics simulation of the interaction between virulence sites.  The study showed that the amino acid substitutions that distinguish vvIBDV from attenuated IBDV (H253Q and T284A) favor a hydrophobic and flexible conformation of β-barrel loops in VP2, which could promote interactions between the virus and potential IBDV-specific receptors.  Population sequence analysis revealed that the IBDV strains prevalent in East Asia show a significant signal of positive selection at virulence sites 253 and 284.  In addition, a signal of co-evolution between sites 253 and 284 was identified.  These results suggest that changes in the virulence of IBDV may result from both the interaction and the co-evolution of multiple amino acid substitutions at virulence sites.