Origins of Heterochromatic Repatterning in White-Footed Mice, Peromyscus leucopus

There appear to be at least three potential mechanisms to explain karyotypic orthoselection resulting in heterochromatic repatterning in the white-footed mouse, Peromyscus leucopus. These include pericentric inversions where the chromosomal break occurs in the hetero-chromatic regions flanking the centromere, insertions of blocks of ribosomal genes into the short-arm heterochromatic material, and either rolling circle amplification-insertion or the translocation of heterochromatic material. We provide data that are not easily compatible with the hypothesis that a single mechanism appears adequate to explain all heterochromatic repatterning in the karyotype of P. leucopus. Our conclusions are based on results obtained through in situ hybridization using three different fluorescently labeled probes: one to identify heterochromatin, one to identify the ribosomal genes, and one to identify the telomere. The direction of change in chromosomal evolution was established by cladistical analysis of 30 species of Peromyscus.