RFLP MAPPING AND MARKER-ASSISTED SELECTION OF SOYBEAN CYST-NEMATODE RESISTANCE IN PI-209332

Soybean cyst nematode (Heterodera glycines Ichinohe; SCN) is one of th e most serious disease problems of soybean [Glycine mar (L.) Merrill], but the genetic basis of SCN resistance is complex and poorly underst ood. This study was conducted to identify genomic regions that confer partial resistance to SCN in soybean PI 209332 through the use of DNA genetic markers. By testing three different races of SCN, we also asse ssed race-specificities of putative resistance loci. Mapping was perfo rmed in two related populations, one an F-2:3 population and the other an F-5:6 recombinant inbred line population, both derived from a sing le cross between 'Evans' and PI 209322. To identify resistance loci, S CN disease responses for all lines in the mapping populations were con trasted with DNA marker genotypes at 98 restriction fragment length po lymorphism (RFLP) marker loci spread throughout the soybean genome. A major partial-resistance locus on linkage group 'G' (P < 0.0001) near RFLP marker C006V was effective against all three SCN race isolates te sted. This locus explained 35% of total phenotypic variation in the SC N disease response to a Race 1 SCN population, 50% of the response to a Race 3 SCN population, and 54% of the response to a Race 6 SCN popul ation. Additional putative SCN resistance loci (P < 0.002) were identi fied on soybean linkage groups 'D', 'J', 'L', and 'K'. Detecting some of these minor loci was complicated by strong masking phenotypic effec ts of the major resistance locus near C006V. Some of the putative resi stance loci behaved in a race-specific manner. Based on RFLP genotype at the F-2 generation, marker-assisted selection for the resistance lo cus on linkage group 'G' was evaluated and found to be 90% accurate in selecting F-5:6 lines resistant to SCN, comparable in accuracy to phe notypic selection for SCN resistance.