Molecular organization of large fragments in the maize B chromosome: indication of a novel repeat.

The supernumerary B chromosome has no apparent effects on plant growth, and its molecular makeup is difficult to unravel, due to its high homology to the normal complement, which prevents conventional cloning. This difficulty was overcome previously by microdissecting the B chromosome under the microscope to result in 19 B clones, one of which is B specific and highly repetitive, dispersing over one-third of the B long arm and most regions of the centromeric knob. To gain insights into the molecular structure of the B chromosome, this sequence was used to screen a genomic library constructed from W22 carrying 16 B's. Five clones (>10 kb each) were isolated, and all were repetitive, showing homology with A chromosomes in Southern and FISH analyses. Two of them were further characterized and sequenced. Each is composed of several restriction fragments with variable degrees of repetitiveness. Some of these are B specific and others have variable degrees of homology with the A chromosomes. The order of each characteristic group is not contiguous; they intersperse within those of other groups. Sequence analysis reveals that their sequences ( approximately 26 kb) have no homology with any published gene other than sequences of transposable elements (retrotransposons and MITEs) and the B as well as the A centromeres. We uncovered a 1.6-kb CL-repeat sequence, seven units of which were present in the two clones in defective forms. Those repeats mostly arrange in tandem array in the B chromosome. Moreover, we detected transposition of a retrotransposon and a MITE element involved in the genesis of these two sequences.