Characterization of a maize chromosome 4 centromeric sequence: Evidence for an evolutionary relationship with the B chromosome centromere

Previous work has identified sequences specific to the B chromosome chat ar e a major component of the B centromere. To address the issue of the origin of the B and the evolution of centromere-localized sequences, DNA prepared from plants without B chromosomes was probed to seek evidence for related sequences. Clones were isolated from maize line B73 without B chromosomes b y screening DNA at reduced stringency with a B centromeric probe. These clo nes were localized to maize centromere 4 using fluorescence in situ hybridi zation. They, showed homology to a maize centromere-mapped sequence, to mai ze B chromosome centromere sequences, and to a portion of the unit repeat o f knobs, which act as neocentromeres in maize. A representative copy was us ed to screen a BAC library to obtain these sequences in a larger context. E ach of the six positive BACs obtained was analyzed to determine the nature of centromere 4-specific sequences present. Fifteen subclones of one BAC we re sequenced and the organization of this chromosome 4-specific repeat was examined.