Allele frequencies across the field area. Frequencies of musculus (red) and domesticus (blue) alleles are shown for the X chromosome marker (a), Y chromosome (b), and mtDNA (c). The pies are proportional to sample sizes. The bold lines indicate orientation of the X and Y fronts, respectively, estimated using the PAVA algorithm; the X marker orientation is almost identical to that of the consensus over all loci (see Table 1). M and D denote musculus and domesticus regions delimited by the green line.