Organization and evolution of a higher plant alphoid-like satellite DNA sequence

Two recombinant plasmids containing, respectively, three and eight tandem repeats of a 177 base-pair (bp) element from radish nuclear DNA have been isolated. These plasmids were used as probes to investigate the organization and the copy number of this element within the genome. This sequence is present in congruent to 0.6 million copies. Restriction analysis provides evidence for sequence heterogeneity and reveals the occurrence of non-overlapping subfamilies. Nine units were sequenced and found to be remarkably conserved. However, sequences in the two clones clearly belong to two distinct subgroups. Our data suggest that these sequences evolved in a concerted manner and that homogenization mechanisms such as gene conversions certainly took place. The 177 bp sequence is made from three 60 bp blocks that are derived from a common ancestor. Exchanges between the three blocks probably occurred before they became fixed as a patchwork of short sequences, the 177 bp element. This unit of 177 bp was then amplified in several steps. The presence of such a repeated sequence can be detected in other Cruciferae when hybridizations are carried out under low stringency conditions. Direct comparison with a previously published mustard satellite DNA sequence indicates a similar organization and a 75% homology. Homology was also found with shorter regions (congruent to 60 bp) of broad bean and corn satellite DNA. Finally, homology was also found with several animal alphoid sequences, suggesting that this family also occurs in the plant genomes.