A Distal Heterochromatic Block Displays Centromeric Activity When Detached from a Natural Centromere

We repeatedly released a distal block of heterochromatin lacking a natural centromere in mitotic cells and assayed its segregation. At anaphase, control acentric fragments typically remained unoriented between daughter nuclei and were subsequently lost. Fragments containing the brown Dominant ( bw D ) heterochromatic element displayed regular anaphase movement upon release. These fragments were found to segregate and function based on both cytological and phenotypic criteria. We also found that intact bw D -containing chromosomes normally display occasional dicentric behavior, suggesting that bw D has centromeric activity on the intact chromosome as well. Our findings suggest that centromere competence is innate to satellite-containing blocks of heterochromatin, challenging models for centromere identity in which competence is an acquired characteristic.