Two simple sequence repeat markers to select for soybean cyst nematode resistance coditioned by the rhg1 locus

The soybean cyst nematode (SCN) (Heterodera glycines Inchinoe) is the most economically significant soybean pest. The principal strategy to reduce or eliminate damage from this pest is the use of resistant cultivars. Identify ing resistant segregants in a breeding program is a difficult and expensive process which is complicated by the oligogenic nature of the resistance an d genetic variability in the pathogen. Fortunately, resistance at one SCN-r esistance locus, rhg1, is generally accepted as a necessity for the develop ment of resistant genotypes using any source of resistance and when challen ged by any SCN race. Thus, the development of SCN resistant cultivars would be expedited if an effective and rapid system were available to identify b reeding lines carrying a resistance allele at the rhg1 locus. In this study we report two simple sequence repeat (SSR) or microsatellite loci that cos egregate and map 0.4 cM from rhg1. Allelic variation at the first of these loci, BARC-Satt309, distinguished most, if not all, SCN-susceptible genotyp es from those carrying resistance at rhg1 derived from the important SCN-re sistance sources 'Peking', PI 437654, and PI 90763. BARC-Satt309 was also e ffective in distinguishing SCN resistance sources PI 88788 and PI 209332 fr om many, but not all, susceptible genotypes. BARC-Satt309 cannot be used in marker-assisted selection in populations developed from typical southern U S cultivars crossed with the important resistance sources PI 88788 or PI 20 9332 because these genotypes all carry the identical allele at the BARC-Sat t309 locus. A second SSR locus, BARC-Sat_168, was developed from a bacteria l artificial chromosome (BAC) clone that was identified using the primers t o BARC-Satt309. BARC-Sat_168 distinguished PI 88788 and PI 209332 from sout hern US cultivars such as 'Lee','Bragg' and 'Essex'. Both BARC-Satt309 and BARC-Sat_168 were used to assay Lines from SCN-susceptiblexSCN-resistant cr osses and proved to be highly effective in identifying lines carrying rhg1 resistance from those carrying the allele for SCN susceptibility at the rhg 1 locus.