Conservation of chromosomal location of nucleolus organizer in American marsupials (Didelphidae)

The distribution and expression of nucleolus organizer regions (NORs) were analyzed in seven species of marsupials representative of the three karyotypes (2n = 14, 18 and 22) found in the American family Didelphidae. Analyses comprised silver-staining of NORs and fluorescence in situ hybridization with an rDNA probe. In addition to confirming the variability in number and distribution of NORs in Didelphidae, we demonstrated the conserved location of NORs on one autosome pair in the three karyotypes. In Monodelphis domestica (2n = 18), the NOR on the X chromosome was not inactivated in females.