Objective: To study the evolutionary characteristics and rules of two lineages on influenza B virus.
Methods: A total of 126 HA1 sequences of strains isolated during 1940 to 2012 were downloaded from the GenBank. Time of the most recent common ancestor (TMRCA) and divergence of the two lineages were calculated based on the data from phylogenetic analysis of HA1 gene, using Bayesian Markov Chain Monte Carlo (Bayesian-MCMC) and molecular clock method.
Results: The average amino acid variant ratios were ranged from 5.4% to 10.2% within the strains of influenza B virus isolated during 1978 to 2010. Compared with the Victoria-like strains, all Yamagata-like strains showed an amino acid deletion at 163(th) site, while some of them showing a deletion at position 166. HA1 gene of influenza B virus seemed not have been affected by positive selection except a few sites. The evolutionary average rate on HA1 gene was 2.138×10(-3) substitutions/site/year (95% HPD: 1.833×10(-3) - 2.437×10(-3) substitutions/site/year). The estimated dates for TMRCA of the two lineages of influenza B virus could be dated back to 1971 (95% HPD: 1969 - 1972), while the divergence times of the two lineages were 1973 (95% HPD: 1971 - 1974) and 1977 (95% HPD: 1975 - 1978) respectively.
Conclusion: Significant differences were found on HA1 gene between earlier and recent identified strains of Victoria and Yamagata lineage. Differences between the two lineages increased and showing the potential of dividing themselves into different subtypes in the future. More attention should be paid to these trends and the related epidemiological significance.