Transcriptional activity of constitutive heterochromatin in the mammal Microtus agrestis (Rodentia, Cricetidae).

Two cytological approaches were applied to the analysis of genetic activity in euchromatin and heterochromatin in the European field vole Microtus agrestis. The first is based on the transcriptional activity of prematurely condensed chromosomes, the second on in situ hybridization of labeled cellular RNA to metaphase chromosomes. The results show that in cultivated fibroblasts, the transcriptional activity of constitutive heterochromatin is in the same order of magnitude as that of euchromatin; its chromosomal pattern is nonrandom. It is concluded that--as has already been shown for insects and amphibians--also in mammals, constitutive heterochromatin is transcribed in its decondensed state but is genetically inactive when forming chromocenters at interphase.