Arrangements of macro- and microchromosomes in chicken cells

Arrangements of chromosome territories in nuclei of chicken fibroblasts and neurons were analysed employing multicolour chromosome painting, laser con focal scanning microscopy and three-dimensional (3D) reconstruction. The ch icken karyotype consists of 9 pairs of macrochromosomes and 30 pairs of mic rochromosomes. Although the latter represent only 23% of the chicken genome they contain almost 50% of its genes. We show that territories of microchr omosomes in fibroblasts and neurons were clustered within the centre of the nucleus, while territories of the macrochromosomes were preferentially loc ated towards the nuclear periphery. In contrast to these highly consistent radial arrangements, the relative arrangements of macrochromosome territori es with respect to each other (side-by-side arrangements) were variable. A stringent radial arrangement of macro- and microchromosomes was found in mi totic cells. Replication labelling studies revealed a pattern of DNA replic ation similar to mammalian cell nuclei: gene dense, early replicating chrom atin mostly represented by microchromosomes, was located within the nuclear interior, surrounded by a rim of late replicating chromatin. These results support the evolutionary conservation of several features of higher-order chromatin organization between mammals and birds despite the differences in their karyotypes.