Figure 4. A recent selective sweep at exon 1 of lysin.

(A) A profile of the likelihood (y-axis) that a selective sweep originated at each site across the lysin gene region (x-axis). Values near exon 1 exceed the significance threshold (α = 0.01, dashed line). Black bars at the top of the plot denote exons. The thin, grey bar spans an unsequenced portion (∼6 kb) of intron 3. (B) Sliding window of polymorphism (Segregating sites) and Fay&Wu's H statistic plotted along the lysin gene region (window size = 1,000 bp, step size = 100 bp). Polymorphism near exon 1 is close to zero and recovers as the sliding window moves in the 3′ direction, consistent with a recent sweep originating in exon 1. The H statistic indicates an excess of high frequency derived alleles downstream of the putative sweep target, as predicted for genetic hitchhiking with a selective sweep. An asterisk denotes a significant rejection of the neutral model for the H value in that window (α = 0.05).