Identifying novel sources of resistance to the soybean cyst nematode.

Abstract

Soybean cyst nematode (SCN, Heterodera glycines Ichinohe) is the most serious yieldlimiting pathogen on soybean [Glycine max (L.) Merr.]. Utilizing genetic resistance is an effective method to control SCN. Most commercial SCN-resistant cultivars in the North Central USA are developed from two sources of resistance, PI 88788 and Peking. However, frequent use of a limited number of resistance sources has shifted virulence phenotypes of SCN populations (HG Types) and the new types seem to overcome originally resistant cultivars. The main purpose of this study is to search for new sources of SCN resistance that are different from Peking or PI 88788 and to identify genetic regions that are associated with novel resistance loci. Since Peking is not resistant to HG Type 1- (race 14) and PI 88788 is not resistant to HG Type 2- (race 1), 17 soybean cultivars and accessions that were reported resistant to HG Type 1- or/and HG Type 2- were tested against 13 different nematode populations including race 1, race 2, race 3, race 4 and race 14. Most of the lines tested had high or moderate resistance to race 1, race 2 and race 3 populations and can serve as an alternative resistance sources to PI 88788. However, most of the lines were susceptible to race 4 and the two race 14 nematode populations. Only PI 633736 has a high level of resistance to all the nematode populations used. PI 417091, PI 404166, PI 567516C, PI 629013 have moderate or high resistance to race 4 and at least one of the two race 14 populations. The different resistance spectrums of those lines indicate that there should be novel genes in PI resistance spectrums of those lines indicate that there should be novel genes in PI 633736, PI 417091, PI 404166, PI 567516C and PI 629013 that are different from Peking and PI 88788. QTLs conferring resistance to an HG Type 2.5.7 population (race 1) were sought with 92 MN0095 × PI 567516C F2:3 families from greenhouse (Experiment 1) and 92 F2:3 families from field (Experiment 2) using 1536 SNP markers. Altogether, 5 QTLs were declared for Experiment 1 and Experiment 2, including 2 significant QTLs (genome-wide type I error =0.05) and 3 suggestive QTLs (LOD > 3). The two significant QTLs were detected on chromosome 10 and chromosome 19 and the three suggestive QTLs were detected on chromosome 8, chromosome 18, and chromosome 20. The QTL with the highest LOD score, located on chromosome 10 was detected in both Experiment 1 and Experiment 2 and was recently reported by another group. This QTL has not been identified in other sources of SCN resistance. This QTL has significant additive effect, and explained 22.2% and 22.4% total variance in Experiment 1 and Experiment 2, respectively. The QTLs on chromosome 19 was detected only in Experiment 1. It had significant dominance effect, and explained 12.7% of total variance. The suggestive QTL mapped on chromosome 18 in Experiment 2 was at or near the rhg1 locus. Haplotype analysis of rhg1 and Rhg4 genes for the 17 resistant soybean germplasm lines revealed that PI 567516C and Peking share the same rhg1 allele. Markers closest to rhg1 and the QTL on chromosome 10 might be considered for use in marker assisted selection.