Plant Nematode Genetics Group, Department of Plant Pathology, North Carolina State University, Raleigh, NC 27695-7616, USA

The soybean cyst nematode (SCN), Heterodera glycines, is a sedentary endoparasite that causes substantial economic damage to soybean on a worldwide scale. Although management via the use of resistant soybean cultivars may be achieved, there are numerous genotypes (races) of SCN that may evade host resistance responses. Because SCN maintains an intimate parasitic relationship with its host, it is suspected that numerous nematode genes are involved in parasitic abilities. In order to identify how many nematode genes are involved and to isolate these genes, we have taken a genetic approach. To attain this goal, we have constructed a linkage map of the H. glycines genome. Controlled crosses have been performed between several SCN strains strains. Segregation analyses have revealed that inheritance of parasitic ability is Mendelian in nature. The genes controlling parasitic ability on a specific host differential appear to be unlinked loci. The SCN gene controlling ability to parasitize PI 88788 appears to be inherited as a dominant allele, whereas the genes conferring parasitic ability on PI 90763 and Pickett are recessive. We have screened approximately 1,000 RAPD 10mer primers against three SCN inbred parental strains, and 1,600 progeny lines. We have mapped parasitism loci using a RAPD-PCR approach with a modified bulk segregant analysis. Specifically, we have identified 2 DNA markers tightly linked to the PI 88788 parasitism locus. We have tentatively mapped the parasitism genes for PI 88788, PI 90763, and Peking to different linkage groups. Using a SCN transformation protocol developed in our labs, we will clone and characterize the PI 88788 parasitism gene.