Analysis of the molecular evolutionary characteristics of neuraminidases in influenza B virus strains isolated in Jiangsu province from 2010 to 2012
Objective 
To analyze the substitutions at amino acid residues of neuraminidases (NAs) in influenza B virus strains isolated in Jiangsu province from 2010 to 2012 and to further understand the genetic evolution of NAs in those influenza B virus strains. 
 
 
Methods 
Forty strains of influenza B virus isolated in Jiangsu province from 2010 to 2012 were screened out for this study. A two-step reverse transcription PCR (RT-PCR) was performed to amply the gene fragments encoding the neuraminidases of influenza B virus strains. The PCR products were purified and then sequenced in an ABI 3730XL Genetic Analyzer. The evolutionary characteristics of NA gene were analyzed by using DNAStar, Bioedit, MEGA 5.0 and BEAST 1.8.0 softwares. 
 
 
Results 
The phylogenetic tree analysis of the NA genes showed that the NAs of 28 Victoria strains were derived from the Yamagata lineage. There were reassortments between the Victoria lineage-HA and theYamagata lineage-NA. Some of the strains added a glycosylation site at position 462. No substitution was found in important enzyme active sites and neuraminidase inhibitor resistant sites. The Bayesian MCMC analysis showed that the estimated mean evolutionary rate for NA gene was 1.74×10-3 (95%HPD: 1.46×10-3-2.06×10-3) substitutions/site/year. The dN/dS ratio (ω), an indicator of selective pressure, was 0.24. 
 
 
Conclusion 
The important amino acid sites of NA were relatively conservative and the evolutionary rate for NA gene was low. The dN/dS ratio was less than one, indicating that the NA gene was under purifying selection. 
 
 
Key words: 
Influenza B virus; Neuraminidases; Evolution