CHROMOSOME-SPECIFIC MOLECULAR-ORGANIZATION OF MAIZE (ZEA-MAYS L.) CENTROMERIC REGIONS

A set of oat-maize chromosome addition lines with individual maize (Ze a mays L.) chromosomes present in plants with a complete oat (Avena sa tiva L.) chromosome complement provides a unique opportunity to analyz e the organization of centromeric regions of each maize chromosome. A DNA sequence, MCS1a, described previously as a maize centromere-associ ated sequence, was used as a probe to isolate cosmid clones from a gen omic library made of DNA purified from a maize chromosome 9 addition l ine. Analysis of six cosmid clones containing centromeric DNA segments revealed a complex organization. The MCS1a sequence was found to comp rise a portion of the long terminal repeats of a retrotransposon-like repeated element, termed CentA. Two of the six cosmid clones contained regions composed of a newly identified family of tandem repeats, term ed CentC, Copies of CentA and tandem arrays of CentC are interspersed with other repetitive elements, including the previously identified ma ize retroelements Huck and Prem2, Fluorescence in situ hybridization r evealed that CentC and CentA elements are limited to the centromeric r egion of each maize chromosome. The retroelements Huck and Prem2 are d ispersed along all maize chromosomes, although Huck elements are prese nt in an increased concentration around centromeric regions. Significa nt variation in the size of the blocks of CentC and in the copy number of CentA elements, as well as restriction fragment length variations, were detected within the centromeric region of each maize chromosome s tudied. The different proportions and arrangements of these elements a nd likely others provide each centromeric region with a unique overall structure.