GENOMIC ORGANIZATION AND EVOLUTION OF THE SOYBEAN SB92 SATELLITE SEQUENCE

Repetitive DNA sequences comprise a large percentage of plant genomes, and their characterization provides information about both species an d genome evolution. We have isolated a recombinant clone containing a highly repeated DNA element (SB92) that is homologous to ca. 0.9% of t he soybean genome or about 10(5) copies. This repeated sequence is tan demly arranged and is found in four or five major genomic locations. F ISH analysis of metaphase chromosomes suggests that two of these locat ions are centromeric. We have determined the sequence of two cloned re peats and performed genomic sequencing to obtain a consensus sequence. The consensus repeat size was 92 bp and exhibited an average of 10% n ucleotide substitution relative to the two cloned repeats. This high l evel. of sequence diversity suggests an ancient origin but is inconsis tent with the limited phylogenetic distribution of SB92, which is foun d at high copy number only in the annual soybeans. It therefore seems likely that this sequence is undergoing very rapid evolution.