Analysis of natural recombination and host-related evolutionary dynamics of avian avulavirus 1 isolates based on positive and negative selection from 1948 to 2017.

Adaptation and evolution of avian avulavirus 1, or Newcastle disease virus (NDV), has led to tremendous economic losses worldwide. The occurrence of natural recombination and selection pressure has been traced for NDV based on a few recent reports, but a dominant pattern based on genomic characteristics is lacking. Here, we used bioinformatics tools to search for evidence of recombination in all of the available complete genome sequence of NDV (462 sequences) using RDP4 software. Geographical linkage and host cell relationships of recombinant viruses were also investigated, and a study of the adaptive evolution of avian avulavirus 1 was performed. The results revealed that recombination events could occur in any gene fragment of the NDV genome. Moreover, class I NDV isolates from wild birds could associate to generate a putative recombinant virus with a class II genome backbone. In addition, not only avirulent-virulent hybrid genotypes but also virulent-virulent natural recombinant NDV viruses were generated. Investigation of geographic relationships of recombinant isolates indicated that the highest rate of recombination occurs in Asia and the Middle East, which can be influenced by vaccination failure, evasion of the immune response, live-bird markets, and the bird trade. The M and NP genes were found to have higher negative selection rates than the other genes, which might lead to the deletion of inadaptable sequences and result in more conserved sequences. Based on our analysis, the highest rate of positive selection was observed in the L, F and HN genes, which we suggest could lead to the occurrence of evolved viruses with high pathogenicity and a better chance of survival under extreme conditions.