Mutations in each CDS were analyzed by the method of Nei and Gojobori [11] . Codon aligned sequences for each dataset were analyzed using the Perl-based SNAP program (http://www.hiv.lanl.gov/content/sequence/SNAP/SNAP.htm l) [12] in order to calculate the variability of each CDS. The selective pressure was measured by comparing the rate of non-synonymous nucleotide substitutions per nonsynonymous site (d N ) against that of synonymous substitutions per synonymous site (d S ). The ratio d N /d S was used as an index to assess positive selection. A ratio d N /d S >1 means positive (diversifying) selection, =1 means neutral selection, and <1 means negative (purifying) selection.