The activation of a neocentromere in Drosophila requires proximity to an endogenous centromere

The centromere is essential for proper segregation and inheritance of genet ic information. Centromeres are generally regulated to occur exactly once p er chromosome; failure to do so leads to chromosome loss or damage and loss of linked genetic material. The mechanism for faithful regulation of centr omere activity and number is unknown. The presence of ectopic centromeres ( neocentromeres) has allowed us to probe the requirements and characteristic s of centromere activation, maintenance, and structure. We utilized chromos ome derivatives that placed a 290-kilobase "test segment" in three differen t contexts within the Drosophila melanogaster genome-immediately adjacent t o (1) centromeric chromatin, (2) centric heterochromatin, or (3) euchromati n. Using irradiation mutagenesis, we freed this test segment from the sourc e chromosome and genetically assayed whether the liberated "test fragment" exhibited centromere activity. We observed that this test fragment behaved differently with respect to centromere activity when liberated from differe nt chromosomal contexts, despite an apparent sequence identity. Test segmen ts juxtaposed to an active centromeric produced fragments with neocentromer e activity, whereas test segments far from centromeres did not. Once establ ished, neocentromere activity was stable. The imposition of neocentromere a ctivity on juxtaposed DNA supports the hypothesis that centromere activity and identity is capable of spreading and is regulated epigenetically.