PHYSICAL ISOLATION AND SEQUENCE ANALYSIS OF THE REGION AROUND rhg1, A MAJOR CYST NEMATODE RESISTANCE LOCUS IN SOYBEAN.

¹ Department of Plant Pathology, University of Minnesota, St. Paul, MN 55108 USA

² Department of Agronomy and Plant Genetics, University of Minnesota, St. Paul, MN 55108 USA

³ Department of Biology, Northern Arizona University, Flagstaff, AZ 86011 USA

The rhg1 locus near the telomere of soybean molecular linkage group G (MLG-G) explains more than 50% of the phenotypic variation in disease response to the cyst nematode Heterodera glycines. As part of a soybean genomics project, we have used the techniques of bacterial artificial chromosome (BAC) library screening, chromosome walking, and BAC end/subclone sequencing to generate a broad picture of the rhg1 region of the MLG-G. We have constructed two BAC contigs, each flanking rhg1, that together encompass approximately 400 kb; the BAC-end closest to rhg1 is approximately 0.1 cM away. Soybean is an ancient polyploid containing, on average, 2.5 copies of each low-copy sequence (Shoemaker et al, 1996.Genetics 144:329-338), so it is not surprising that in the process of chromosome walking, we have identified 12 additional contigs containing over 1 Mb from homoeologous regions of the genome. We are now determining the map location of these contigs and exploring their relationship to the contigs on linkage group G. Sequence analysis of several BAC ends and subclones in a contig around marker AFLP170 (located less than 1 cM from rhg1) indicated a high density of coding sequences. GRAIL, BLAST and other computational tools uncovered a cluster of four receptor-like protein kinases, which appear to be a family of at least one functional gene and three pseudogenes. A neighboring cluster of phospholipases is located approximately 20 kb from the kinase cluster. The potential involvement of these genes in signal transduction awaits elucidation. Additional studies based on sequence analysis of physically isolated DNA fragments are also underway to determine gene density and organization in this interesting region of the soybean genome.