Three different fluorochrome and specific counterstain combination (DAPI/AMD, DA/DAPI and CMA/DA) treatments were applied to the chromosomes of four Microtidae (Rodentia) species. The results complete the data obtained in our previous paper (Burgos, M., Jiménez, R., & Dìaz de la Guardia, R., Genome 30:540-546, 1988) and prove that the changes in the constitutive heterochromatin in the evolution of the karyotypes of these species are not only due to gain or loss of heterochromatin, but are qualitative with respect to their nucleotide composition, repeated base pair organization or DNA-protein complex modification. These variations lead to the differential response to the fluorescence dye combinations used.

The sex chromosomes of Microtus cabrerae are characterized by large segments of constitutive heterochromatin in both the X and Y. Alkali treatment, enzymatic digestion (trypsin), and fluorochrome staining reveal considerable heterogeneity within these heterochromatic blocks and allow us to distinguish several regions. In addition, variation in the length of these segments is also present, probably as a result of deletions. A possible explanation for the origin of these heterochromatic blocks is discussed.