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 A₃ (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 detecting GC-rich heterochromatin. The appropriate condi-tions 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-replicating 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.

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The Iwagaki oyster Crassostrea nippona is an important marine species in Japan. While some studies have reported the karyotype of oysters, inconsistencies among these results have been found, probably due to the small chromosome sizes and similarities among the species. Indeed, the karyotype of C. nippona has not been clearly established. In this study, we analyzed the karyotype of C. nippona and found that C. nippona contained seven pairs of metacentric, two pairs of metacentric/submetacentric, and one pair of submetacentric chromosomes. These results differ from those of previous studies.

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