COMPARTMENTALIZATION OF DISTANT CHROMOSOMAL SUBREGIONS IN INTERPHASE NUCLEI OF Arabidopsis thaliana

¹ Laboratory of Molecular Biology, Wageningen Agricultural University, Dreijenlaan 3, 6703 AH, Wageningen, The Netherlands

² Laboratory of Genetics, Wageningen Agricultural University, Dreijenlaan 2, 6703 AH, Wageningen, The Netherlands

The interphase nucleus of eukaryotes appears as a complex organization of heterochromatic and euchromatic domains. The former is highly condensed and genetically inert, whereas the latter is less condensed representing sites of (potential) transcriptional activity. Both domains can easily be distinguished in DAPI stained pachytene chromosomes. During interphase the chromosomes occupy separate territories with different shapes, while the heterochromatic domains are no longer distinguishable as discrete chromosomal segments, especially in eukaryotes with large genomes. However, in interphase nuclei of A. thaliana a small number (5 - 10) of heterochromatic domains appear as distinct, DAPI-bright regions, so-called chromocenters. All centromeric regions are confined to these chromocentres. Using FISH with different probe combinations, including repeats ((peri)centromeric, telomeric) and several unique DNA sequences (YACs, cosmids etc.) we have identified individual chromocentres and established the presence of other non-centromeric chromosomal segments in these heterochromatic domains. Moreover, we are able to trace entire chromosome arms relative to the chromocentres, and establish possible associations between otherwise euchromatic regions and heterochromatic domains. These studies provide a solid basis for further investigating the relationship between chromatin organization, compartmentalization and gene silencing at the nuclear level.