5-Azadeoxycytidine induced heteromorphic undercondensation in the C-blocks of X chromosomes of three mammalian species, Pipistrellus abramus, Millardia meltada, and Apodemus argenteus

5-azadeoxycytidine (5-aza-dC)-induced undercondensation (UC) was described with cultured lung fibroblasts from three mammalian species, Pipistrellus abramus , Millardia meltada , and Apodemus argenteus , all of which carry a large amount of C-heterochromatin (C-block) on their X chromosomes. The C-blocks were GC-rich in P. abramus , AT/GC-balanced in M. meltada , and AT-rich in A. argenteus , as revealed by fluorescent staining with quinacrine mustard (QM) and chromomycin A 3 (CMA). P. abramus conspicuously undercondensed, irrespective of autosomes or gonosomes, in all C-band regions including C-blocks. The extent of UC was most prominent in P. abramus , rather weak in M. meltada , and weakest in A. argenteus , suggesting a close relation to the amount of GC base pairs contained in C-heterochromatin. Thus 5-aza-dC treatment may be an easy method for detect-ing GC-rich heterochromatin. The appropriate conditions for inducing UC in the C-blocks were 10 -5 –10 -6 M and 2–3 hours. The decondensing effect of 5-aza-dC on the C-blocks was time- and concentration-dependent in all species examined, asynchronously acting with certain time lag between the early-replicating X and late-repli-cating X. The heteromorphic UC between the C-blocks of the XX homologue was observed with a frequency of 8% at maximum, reflecting the allocyclic behavior of the C-blocks.