Unstable chromosome sites and evolutionary rearrangements in akodont rodents (Cricetidae).

Evolutionary rearrangements producing changes in chromosome 1 of Akodon molinae were traced by comparing the G banding patterns of the karyotypes from six species of akodont rodents. It was possible to subdivide chromosome 1 of A. molinae into unstable and stable regions. Most of the spontaneous rearrangements of chromosome 1 appearing in passages 116-128 of a continuous line of A. molinae cells (AKm line) occurred in the unstable regions which comprise repetitive DNA sequences favouring the setting up of heteroduplexes leading to rearrangements. When AKm cells were irradiated with UV light it was observed that unstable regions of chromosome 1 showed higher rates of unscheduled DNA synthesis (UD) than stable areas. A differential degree of condensation making certain regions of the chromatin fibril more accessible to repair enzymes or a better target for damage, is probably the cause of the variable response to UV light, and perhaps to most clastogenic agents (including those responsible for spontaneous rearrangements). Thus, the distribution of repetitive DNA sequences, the structure of the chromatin fibril and the efficiency of the DNA repair machinery may be important factors in the origin of spontaneous chromosomal rearrangements.