ORIGINS OF HETEROCHROMATIC REPATTERNING IN WHITE-FOOTED MICE, PEROMYSCUS-LEUCOPUS

There appear to be at least three potential mechanisms to explain kary otypic orthoselection resulting in heterochromatic repatterning in the white-footed mouse, Peromyscus leucopus. These include pericentric in versions where the chromosomal break occurs in the heterochromatic reg ions 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 mater ial. We provide data that are not easily compatible with the hypothesi s that a single mechanism appears adequate to explain all heterochroma tic repatterning in the karyotype of P. leucopus. Our conclusions are based on results obtained through in situ hybridization using three di fferent 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.