5S rRNA genes in tribe Phaseoleae: array size, number, and dynamics

The organization of 5S rRNA genes in plants belonging to tribe Phaseoleae was investigated by clamped homogeneous electric field gel electrophoresis and Southern blot hybridization. Representatives of subtribe Glycininae included the diploid species Neonotonia wightii and Teramnus labialis, as well as three soybean accessions: an elite Glycine max (L.) Merr. cultivar (BSR101), an unadapted G. max introduction (PI 437.654), and a wild Glycine soja (PI 468.916). A cultivar of Phaseolus vulgaris (kidney bean), a member of subtribe Phaseolinae, was also examined. We determined the number of 5S rDNA arrays and estimated the size 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 approximately 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 origin, our data strongly suggest that one of the two progenitor 5S loci has been lost during diploidization of soybean. Neonotonia wightii, the 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 exhibited a more complex arrangement of 5S rRNA genes, having at least three arrays, each comprising a few hundred copies of a distinct repeat unit. Although each array in P. vulgaris exhibits a high degree of homogeneity with regard to the sequence of the repeat unit, heterogeneity in array size (copy number) was evident when individual plants were compared. A cis-dependent molecular drive process, such as unequal crossing-over, could account for both the homogenization of repeat units within individual arrays and the observed variation in copy number among individuals.