example, Rao-Arelli et al. (1992b) showed that several isolates of H. glycines were initially classified as one Soybean cyst nematode (SCN) (Heterodera glycines Ichinohe), race by this scheme. However, when later tested with the most destructive pest of soybean [Glycine max (L.) Merrill], is estimated to be responsible for almost nine million megagrams in resistant genotypes other than the standard differentials, annual yield loss worldwide. Host plant resistance is the most costthe isolates behaved as different races. To describe dieffective and environmentally friendly method of controlling SCN. P.R. Arelli, USDA-ARS, 605 Airways Blvd., Jackson, TN 38301.
Field resistance to cyst nematode (SCN) race 3 (Heterodera glycines I.) in soybean [Glycine max (L.) Merr.] cv 'Forrest' is conditioned by two QTLs: the underlying genes are presumed to include Rhg1 on linkage group G and Rhg4 on linkage group A2. A population of recombinant inbred lines (RILs) and two populations of near-isogenic lines (NILs) derived from a cross of Forrest × Essex were used to map the loci affecting resistance to SCN. Bulked segregant analysis, with 512 AFLP primer combinations and microsatellite markers, produced a high-density genetic map for the intervals carrying Rhg1 and Rhg4. The two QTLs involved in resistance to SCN were strongly associated with the AFLP marker EATGMCGA87 (P = 0.0001, R 2 = 24.5%) on linkage group G, and the AFLP marker ECCGMAAC405 (P = 0.0001, R 2 = 26.2%) on linkage group A2. Two-way analysis of variance showed epistasic interaction (P = 0.0001, R 2 =16%) between the two loci controlling SCN resistance in Essex × Forrest recombinant inbred lines. Considering the two loci as qualitative genes and the resistance as female index FI < 5%, jointly the two loci explained over 98% of the resistance. The locations of the two QTLs were confirmed in the NILs populations. Therefore SCN resistance in Forrest × Essex is bigenic. High-efficiency marker-assisted selection can be performed using the markers to develop cultivars with stable resistance to SCN.
The rhg1 gene or genes lie at a recessive or co-dominant locus, necessary for resistance to all Hg types of the soybean (Glycine max (L.) Merr.) cyst nematode (Heterodera glycines I.). The aim here was to identify nucleotide changes within a candidate gene found at the rhg1 locus that were capable of altering resistance to Hg types 0 (race 3). A 1.5 +/- 0.25 cM region of chromosome 18 (linkage group G) was shown to encompass rhg1 using recombination events from four near isogenic line populations and nine DNA markers. The DNA markers anchored two bacterial artificial chromosome (BAC) clones 21d9 and 73p6. A single receptor like kinase (RLK; leucine rich repeat-transmembrane-protein kinase) candidate resistance gene was amplified from both BACs using redundant primers. The DNA sequence showed nine alleles of the RLK at Rhg1 in the soybean germplasm. Markers designed to detect alleles showed perfect association between allele 1 and resistance to soybean cyst nematode Hg types 0 in three segregating populations, fifteen additional selected recombination events and twenty-two Plant Introductions. A quantitative trait nucleotide (QTN) [corrected] in the RLK at rhg1 was inferred that alters A87 to V87 in the context of H274 rather than N274. [corrected] Contiguous DNA sequence of 315 kbp of chromosome 18 (about 2 cM) contained additional gene candidates that may modulate resistance to other Hg-types including a variant laccase, a hydrogen-sodium ion antiport and two proteins of unknown function. A molecular basis for recessive and co-dominant resistance that involves interactions among paralagous disease-resistance genes was inferred that would improve methods for developing new nematode-resistant soybean cultivars.
Soybean cyst nematode (SCN) (Heterodera glycines Ichinohe) is the most important pest of soybean [Glycine max (L.) Merr.] in the world. A total of 17 quantitative trait locus (QTL) mapping papers and 62 marker–QTL associations have been reported for resistance to soybean cyst nematode in soybean. Conflicting results often occurred. The objectives of this study were to: (i) evaluate evidence for reported marker–QTL associations for resistance to SCN in soybean and (ii) extract relatively reliable and useful information from the reported marker–QTL associations in soybean. A meta‐analysis was conducted for QTL locations by comparing the 95% confidence intervals of the reported QTLs. QTLs for different races or different studies were classified into one cluster if their confidence intervals had a region in common. The QTLs of the same cluster may have a shared locus. QTLs for different races or different studies were classified into different clusters if their confidence regions had no region in common and were ≥ 20 cM away from each other. Different clusters may represent different loci. Reported SCN resistant QTLs were classified into three categories: suggestive, significant, and confirmed. Confirmed QTLs are credible and can be candidates for fine mapping and gene cloning. QTLs on linkage groups (LGs) G, A2, B1, E, and J were classified as confirmed. Two clusters of QTLs were identified on LG G. One of them is rhg1 One cluster of QTLs was identified near the end of LG B1, but one QTL may exist around the middle of LG B1. One cluster of QTLs was identified on LGs A2, E, and J, respectively. QTLs on LGs B2, C1, C2, D1a, D2, L, M, and N were classified into suggestive or significant. Confirmation studies are needed to lend credibility for these QTLs. A relationship between soybean QTLs and SCN races is discussed.
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