Comparison of novel MLB-clade, VA-clade and classic human astroviruses highlights constrained evolution of the classic human astrovirus nonstructural genes.

Eight serotypes of human astroviruses (the classic human astroviruses) are causative agents of diarrhea. Recently, five additional astroviruses belonging to two distinct clades have been described in human stool, including astroviruses MLB1, MLB2, VA1, VA2 and VA3. We report the discovery in human stool of two novel astroviruses, astroviruses MLB3 and VA4. The complete genomes of these two viruses and the previously described astroviruses VA2 and VA3 were sequenced, affording seven complete genomes from the MLB and VA clades for comparative analysis to the classic human astroviruses. Comparison of the genetic distance, number of synonymous mutations per synonymous site (dS), number of non-synonymous mutations per non-synonymous site (dN) and the dN/dS ratio in the protease, polymerase and capsid of the classic human, MLB and VA clades suggests that the protease and polymerase of the classic human astroviruses are under distinct selective pressure.