Micro-scale signature of purifying selection in Marburg virus genomes

In the seven protein-coding genes in the Marburg virus (MARV) genome, the synonymous nucleotide diversity substantially exceeded the nonsynonymous nucleotide diversity, indicating strong purifying selection. Likewise, there was evidence of purifying selection on 5'UTR and 3'UTR, where nucleotide diversity (pi) was significantly less than piS in the coding regions. Nonsynonymous polymorphic sites showed significantly reduced mean gene diversity in comparison to other polymorphic sites, indicating that purifying selection at certain slightly deleterious nonsynonymous polymorphisms is ongoing. Moreover, nonsynonymous polymorphic sites showed significantly reduced gene diversity in comparison to adjacent synonymous sites, even though the vast majority of such adjacent synonymous sites were in the same codon or an adjacent codon. Thus purifying selection, in conjunction with recombination and/or backward mutation, can act to break up linkage relationships at a micro-scale in the MARV genome. The ability of purifying selection to break up linkage between synonymous and nonsynonymous polymorphisms on such a fine scale has not been reported in any other genome.