Molecular evolution of the hypervariable region of the attachment glycoprotein gene in human respiratory syncytial virus subgroup B genotypes BA9 and BA10.

We studied the molecular evolution of the C-terminal 3rd hypervariable region in the attachment glycoprotein gene of human respiratory syncytial virus subgroup B (HRSV-B) genotypes BA9 and BA10. We performed time-scaled phylogenetic analyses using Bayesian Markov chain Monte Carlo methods. We also performed a genetic distance analysis (p-distance analysis), positive and negative selection analyses, and a Bayesian skyline plot (BSP) analysis. We found that genotype BA9 diverged from the common ancestor of genotypes BA7, BA8, and BA10, while genotype BA10 diverged from the ancestor of genotypes BA7 and BA8. Strains of both genotypes were distributed worldwide. BA9 and BA10 diverged between 1999 and 2001. Both BA9 and BA10 evolved rapidly (about 4.8×10(-3)substitutions/site/year) and formed three distinct lineages in a 10-year period. BA10 strains belonging to lineage 3 had large genetic distances (p-distance>0.07). Thus, it may be possible to classify these strains as a new genotype, BA11. No positive selection site was detected in either genotype. Phylodynamic analyses showed that the effective population size of BA10 decreased gradually since 2010 and BA9 slightly decreased since 2009. The results suggested that the recently prevalent HRSV-B genotypes BA9 and BA10 evolved uniquely, leading to epidemics of HRSV-B worldwide over a 15-year period.