Newcastle disease virus evolution. II. Lack of gene recombination in generating virulent and avirulent strains.

Sequence analysis and comparison of the fusion glycoprotein genes of 11 Newcastle disease virus (NDV) isolates indicated a high degree of functional and structural constraint exerted on the change of the glycoprotein. However, synonymous nucleotide substitutions occurred frequently throughout the coding region. Facilitated by an analysis of synonymous difference (Ks) in pairwise strain comparison, we defined the branching orders of the strains and identified three distinct evolutionary lineages correlating with the virulence as expressed by mean death time (MDT) for chick embryo. The typically virulent strains with MDT of about 50 hr were associated with one lineage, while the typically nonvirulent strains with MDT of infinity were of another lineage. The third lineage consisted of both virulent and avirulent strains whose MDTs lay on a continuum from 50 to 120 hr. Synonymous substitutions were found to occur with almost the same rates in the adjacent hemagglutinin-neuraminidase and membrane protein genes as in the fusion protein gene, and the branching orders based upon the Ks for these genes were essentially identical to those derived from the fusion protein gene. Therefore, no gene exchange by recombination seems to have occurred to generate the strains of distinct lineages. Rather, the different strains appear to have evolved through various degrees of accumulation of point mutations. Besides these evolutionary features, the present study strongly supports the importance of the previously identified signals for gene expression and for the proteolytic activation of the gene product.