5S RIBOSOMAL-RNA GENES IN TRIBE PHASEOLEAE - ARRAY SIZE, NUMBER, AND DYNAMICS

The organization of 5S rRNA genes in plants belonging to tribe Phaseol eae was investigated by clamped homogeneous electric field gel electro phoresis and Southern blot hybridization. Representatives of subtribe Glycininae included the diploid species Neonotonia wightii and Teramnu s labialis, as well as three soybean accessions: an elite Glycine mar (L.) Merr. cultivar (BSR101), an unadapted G. mar introduction (PI 437 .654), and a wild Glycine soja (PI 468.916). A cultivar of Phaseolus v ulgaris (kidney bean), a member of subtribe Phaseolinae, was also exam ined. We determined the number of 5S rDNA arrays and estimated the siz e and copy number of the repeat unit for each array. The three soybean accessions all have a single 5S locus, with a repeat unit size of sim ilar to 345 bp and a copy number ranging from about 600 in 'BSR101' to about 4600 in the unadapted soybean introduction. The size of the 5S gene cluster in 'BSR101' is the same in roots, shoots, and trifoliate leaves. Given that the genus Glycine probably has an allotetraploid or igin, our data strongly suggest that one of the two progenitor 5S loci has been lost during diploidization of soybean. Neonotonia wightii, t he diploid species most closely related to soybean, also has a single locus but has a repeat unit of 520 bp and a copy number of about 1300. The more distantly related species T. labialis and P. vulgaris exhibi ted a more complex arrangement of 5S rRNA genes, having at least three arrays, each comprising a few hundred copies of a distinct repeat uni t. Although each array in P. vulgaris exhibits a high degree of homoge neity with regard to the sequence of the repeat unit, heterogeneity in array size (copy number) was evident when individual plants were comp ared. A cis-dependent molecular drive process, such as unequal crossin g-over, could account for both the homogenization of repeat units with in individual arrays and the observed variation in copy number among i ndividuals.