Evolution of the heterochromatic regions on maize B long arm based on the sequence structure of CL-repeat variants

Evolution of heterochromatic regions in the B long arm was studied in two directions: construction of a phylogenetic tree from mutational variants of CL-repeat and analysis of the XbaI fragments carrying CL-repeat and the repeat structural variants. Using tertiary trisomes and hypoploids of a set of B-10L translocations, the fragments associated with CL-repeat and the variants in each of the three distal heterochromatic (DH) regions were identified. Twenty fragments comprising the CL-repeat were observed in the B-chromosome, and each was assigned to an individual DH region. Four deletions, one insertion, and a large number of mutational variants from each of the three DH regions were isolated and sequenced. The sequences of 27 mutational variants were used to establish a phylogenetic tree which divided the 27 variants into three groups, each of which was associated with a distinct DH region and elucidated an evolution order of the three DH regions. According to the tree, the DH2 was the earliest DH region, which gave rise to the DH3 to be followed by the DH1. The distributions of the fragments including CL-repeat and structural variants in the three DH regions were consistent with such evolution order.