TARGETED COMPARATIVE GENOME ANALYSIS AND QUALITATIVE MAPPING OF A MAJOR PARTIAL-RESISTANCE GENE TO THE SOYBEAN CYST-NEMATODE

A major partial-resistance locus to the soybean cyst nematode (Heterod era glycines Ichinohe; SCN) was Identified on linkage group 'G' of soy bean [Glycine max (L.) Merr.] using restriction fragment length polymo rphisms (RFLPs). This locus explained 51.4%, (LOD=10.35) of the total phenotypic variation in disease response in soybean Plant Introduction (PI) 209332, 52.7% (LOD=15.58) in PI 90763, 40.0% (LOD=10.50) in PI 8 8788, and 28.1% (LOD=6.94) in 'Peking'. Initially, the region around t his major resistance locus was poorly populated with DNA markers. To i ncrease marker density in this genomic region, first random, and later targeted, comparative mapping with RFLPs from mungbean [Vigna radiata (L.) R. Wilcz.] and common bean (Phaseolus vylgaris L.) was performed , eventually leading to one RFLP marker every 2.6 centimorgans (cM). E ven with this marker density, the inability to resolve SCN disease res ponse into discrete Mendelian categories posed a major limitation to m apping. Thus, qualitative scoring of SCN disease response was carried out in an F-5:6 recombinant inbred population derived from 'Evans' x P I 209332 using a 30% disease index cut-off for resistance. Using the c omputer program JoinMAP, an integrated map of the region of interest w as created, placing the SCN resistance locus 4.6 cM from RFLP marker B 53 and 2.8 cM from Bng30. This study demonstrates how a combination of molecular-mapping strategies, including comparative genome analysts, Join mapping, and qualitative scoring of a quantitative trait, potenti ally provide the necessary tools for high-resolution mapping around a quantitative-trail locus.