Recent evolution of NOR-bearing and sex chromosomes of the North African rodent Lemniscomys barbarus.

The karyotype and meiotic phases of Lemniscomys barbarus from Morocco were extensively studied with G- and C-banding, Ag-NOR and fluorochrome staining, in situ hybridization with an rDNA probe, and synaptonemal complex analysis. Comparison of the data with those previously published for an Algerian specimen revealed in the Moroccan specimens the presence of large heterochromatic segments in the sex chromosomes, a new nucleolar organizer at chromosome pair 1, and silent NORs on both sex chromosomes - features that are not present in the Algerian specimen. These findings demonstrate that during the very recent evolution of the karyotype of this species a new NOR was acquired by pair 1, possibly by amplification of ribosomal genes after a translocation event. This new NOR changed the preference of activation of the NORs in these individuals and became the preferentially activated NOR. Another autosome-sex chromosome translocation led to the presence of NORs on the sex chromosomes, which were then inactivated by the invasion of repetitive sequences. These silent NORs may be involved in the pairing of the two sex chromosomes.