The variation of heterochromatin amount and localization using the genus Ctenomys as a model is analyzed in this work. This genus exhibits a significant variation in heterochromatin localization that range from pericentric areas in few chromosomes to whole heterochromatic arms. Its karyotypic variation includes the occurrence of a large number of chromosomal rearrangements (2n=10 to 70; NF=16 to 80) during its evolution. The possible mechanism which generated heterochromatin variation was analyzed in the meiotic nuclei, mainly in the pachytene of Ctenomys species from Uruguay. In the pachytene stage, several bivalents merge in densely stained chromocenters that are positive for C-banding and DAPI staining. Additionally, “in situ” hybridization was used with the repetitive PvuII Ctenomys sequence (RPCS) as a probe which was also localized in the pachytene chromocenters. In order to understand the heterochromatin behaviour in the chromocenter, we applied immunolocalization for two proteins: HP1 a chromatin gene silencer which transforms euchromatin into heterochromatin and BRCA1 a double stranded break (DSB) DNA repair factor. Positive immune signals for both proteins in the pachytene chromocenters suggests that the mechanics of meiosis could break DNA in the chromocenters due to physical forces leading to metaphase I. Based on these data we propose that heterochromatin exchanges could imply double strand breaks produced by these forces that in the repairing process may conduce to the spreading of heterochromatin. This also can explain the colocalization of BRCA1 and HP1 signals.