Heterochromatin patterns were analyzed in the genus Ctenomys from Uruguay which 
exhibits high karyotype variability. Different amounts and localizations of 
heterochromatin were observed in species and populations analyzed. While species 
as C. rionegrensis presented heterochromatic arms in all the chromosomes of the 
karyotype, other species like C. torquatus showed only few chromosomes with 
pericentric heterochromatin. At the pachytene stage, bivalents merge in densely 
stained chromocenters. We detected in these chromocenters the typical highly 
repeated DNA of this genus after in situ hybridization, the M31 chromodomain 
through immunofluorescence as well as dense Giemsa staining after C-banding. In 
species that present low amounts of heterochromatin, only 1 or 2 chromocenters 
were observed in which bivalents merge as observed in C. rionegrensis. After 
BRCA1 immunodetection we observed in early pachytene cells positive spots 
located over heterochromatic chromocenters that strongly suggest heterochromatic 
DNA repair. Mechanical stress mainly due to increasing chromatin compactness 
before metaphase I might be a mechanism to spread heterochromatin between 
different chromosomes within a karyotype.