PHYSICAL MAPPING OF 5S AND 18S-28S RDNA LOCI IN THE GENUS Glycine USING FISH.

Department of Plant and Soil Sciences, Alabama A&M University, Normal, AL 35762 USA

Wild germplasms are valuable sources of genes for crops, such as soybean, that possess limited genetic variability. Before these germplasms can be exploited, their genome organization and synteny must be unraveled. Soybean probably arose through an ancient allotetraploid event followed by the subsequent diploidization of the genome. However, wild Glycine species have not been intensively investigated for this ancient polyploidy. We determined the localization of 5S and 18S-28S rDNA sequences within Glycine using FISH. Two high stringency washes almost eliminated background hybridization and greatly enhanced the reliability of our results. We observed only a single major pair of 5S rDNA FISH signals in (2n=40) members of the subgenus Glycine and two major pairs in G. tomentella and G. tabacina (2n=80). One major pair of 18S-28S rDNA FISH signals was observed in G. canescens, G. clandestina, G. soja and G. max (2n=40). However, G. cytroloba (2n=40) exhibited two major and one minor pair of 18S-28s rDNA FISH signals. The polyploids G. tomentella and G. tabacina (2n=80) also exhibited two major and one minor pair of FISH signals. Our results clearly demonstrate the loss of a 5S rDNA locus from the `diploid' species and loss of two from the polyploids of Glycine. A similar diploidization of the 18S-28S rDNA array has occurred in (2n=40) G. canescens, G. clandestina, G. soja and G. max. However, the detection of four major sites in (2n=40) G. cytroloba suggests that the ancestral progenitor loci for 18S-28S rDNA may have been retained in this species. Both (2n=80) G. tomentella and G. tabacina , although of different genome types, showed two major loci per haploid genome, as opposed to four loci that would be expected in chromosomes that have undergone two doubling events in its evolutionary history. It is evident that the evolution of the subgenus Glycine is much more complex than represented in a simple diploid-to-tetraploid model.