Figure 2: Proportion of shared and privately segregating polymorphisms and coalescence analysis in the South American and Holarctic (HOL) populations.

(a) Proportion of private alleles, fixed differences and shared polymorphisms among SNPs found in all possible pairwise comparisons involving the HOL and the two South American populations, SA-A and SA-B (South American mosaic strains were excluded from the analysis). (b) Genome-wide estimation of relative time to coalescence for the HOL and the two South American populations. The tree was built from an alignment of 51,159 high-quality sites (Phred quality score >Q40) partitioned over the 16 chromosomes. Each partition represents regions without evidence for intra-locus recombination. The scale bar depicts estimated substitutions per site. Node ages are printed in bold near the nodes of the population whose coalescence they estimate. Branch lengths are printed in italics above the branches. One representative strain of the divergent Australasian population was used to root the tree. The insert shows the marginal posterior densities of the time to the most recent common ancestor (tmrca) in each population.