A dicentric chromosome of Drosophila melanogaster showing alternate centromere inactivation

Dicentric chromosomes are rarely found, because they interfere with normal cell division causing chromosome instability. By in situ hybridization of r egion-specific heterochromatic yeast artificial chromosomes we have found t hat the artificially generated C(I)A chromosome of Drosophila melanogaster has two potential centromeres: one carries all the sequences of the centrom ere of the Y chromosome and the other carries only a part of the Y centrome ric region that is rich in telomere-related sequences. Immunostaining with anti-Bub1 (a kinetochore-specific marker) shows that, in spite of the diffe rences in sequence, both centromeres can be active although as a rule only one at a time. In a small fraction of the chromosomes centromere inactive i on is incomplete, giving rise to true dicentric chromosomes. The centromere inactivation is clonally inherited, providing a new example of epigenetic chromosome imprinting and the possibility of genetically dissecting this pr ocess. The involvement of telomere-related sequences in centromere function is discussed.