Identification and comparison of the proliferation characteristics and whole genomes. (A) Serial passage of parent strain 85-7. Five plaques of 85-7 (originated with the same genome) were chosen for serial propagation over 40 passages, which generated five corresponding strains designated as A40, B40, C40, D40 and E40, respectively. (B) The virus growth curve of the 85-7 parent strain (P5) and five mutant viruses. Vero cells were infected at an MOI of 0.1 in the absence of trypsin. The supernatant was collected at the indicated times and plaque assays were carried out. (C) Comparison of the CPE of 85-7 parent strain (P5) and five mutant viruses at 24 and 48 hpi in the absence of trypsin. The C40 and E 40 strains show weaker cell–cell fusion capacity. (D) Comparison of the plaque phenotype of the 85-7 parent strain (P5) and five mutant viruses in the absence of trypsin. The C40 and E 40 strains had distinctly smaller plaques. (E) Identification of four major variable regions (V1–V4) in the genomes of the five mutant strains compared with the parent strain.