A Decade of QTL Mapping for Cyst Nematode Resistance in Soybean

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. versity better within the nematode, the race system has Resistance is present among soybean plant introductions (PIs) and been replaced with a new scheme that includes addirelated wild species, such as Glycine soja Sieb. and Zucc. Molecular tional differentials or indicator lines and classifies nemamarker technology has ushered in a decade devoted to the identificatode populations as “H. glycines types” (HG types) (Nition and characterization of quantitative trait loci (QTL) underlying black et al., 2002). In this review, SCN populations used SCN. These genetic mapping efforts uncovered numerous locations in mapping studies will be referred to by race designaof SCN resistance QTL in many PIs. In more than a decade of mapping tions because most of this work was done before the SCN resistance QTL, there is some consistency in the results. In development of the new HG type system. almost all studies involving various sources of resistance, the QTL conferring the greatest level of resistance mapped to the region conThe primary methods for controlling SCN include taining rhg1 on linkage group (LG) G. In addition, a major resistance planting resistant cultivars and rotation with nonhost QTL was mapped in many sources to the region containing Rhg4 on crops. Breeders and nematologists have been successful LG A2. The mapping of QTL to these regions from many sources in developing SCN resistant cultivars. Host plant resissuggests that these sources may have resistance genes in common, tance has allowed soybean production to continue in which has caused concern over the possible dependence on a few many growing areas where production could no longer resistance genes. Recently, two independent research groups reported be profitable because of SCN infestations. It was esticloning candidate genes for rhg1 and Rhg4. Despite these advances, mated that the resistant cultivar Forrest alone prevented there is some degree of trepidation, especially in the public sector, crop losses worth $405 million from 1975 to 1980 (Bradon the use of rhg1 and Rhg4 genetic mapping and cloning information ley and Duffy, 1982). in SCN resistance breeding because of intellectual property issues. Plant introductions from the USDA Soybean Germplasm Collection have been screened for resistance to nematode populations and resistant lines have been S cyst nematode is estimated to cause the identified (Ross and Brim, 1957; Epps and Hartwig, greatest yield loss to soybean compared to any other 1972; Anand and Gallo, 1984; Anand et al., 1988; Young, pest worldwide. Most recent estimates include a loss of 1990, Rao-Arelli et al., 1997). Arelli et al. (2000) identi7.6 million megagrams in the USA and nearly nine milfied 118 exotic soybean PIs resistant to combinations of lion worldwide in 1998 (Wrather et al., 2001). Soybean SCN races 1, 2, 3, 5, or 14. Anand et al. (1988) reported cyst nematode causes yield reductions by feeding on that the only PI they found with resistance to nearly all plant nutrients, retarding root growth, and inhibiting SCN races tested was PI 437654, a claim that was later Bradyrhizobium japonicum (Kirchner) Buchanan novalidated by the work of Diers et al. (1997b). Diers et al. dulation (Riggs and Schmidtt, 1987). (1997b) evaluated a set of 38 SCN resistant PIs with The genetically diverse field populations of H. glyH. glycines races 1, 2, 3, 5, and 14 in the greenhouse. cines combined with the limited germplasm base of comAmong these 38 PIs, only PI 437654 and PI 438489B mercial soybean for resistance could potentially lead were classified as resistant to all five SCN races in their to population shifts over time (Colgrove et al., 2002). test. PI 437654 was the more resistant of the two, with Diversity in SCN has been described with a physiologia higher number of cysts produced on PI 438489B. In cal race system that utilizes four resistant soybean genoaddition, previous work showed that PI 438489B was types and a susceptible standard as differential lines susceptible to other isolates of races 2 and 14 (Rao(Golden et al., 1970). However, this race system does Arelli et al., 1992b). Diers et al. (1997b) also evaluated not account for all of the diversity present in SCN. For the genetic relationships among the PIs with 201 genomic clones used as restriction fragment length polymorV.C. Concibido, Monsanto Company, 800 N. Lindbergh Blvd., St. Louis, MO 63167; B.W. Diers, Department of Crop Sciences, University of phism (RFLP) markers. They found that the clustering Illinois at Urbana–Champaign, 1101 W. Peabody Dr., Urbana, IL 61801; of the PIs based on markers was consistent with resisP.R. Arelli, USDA-ARS, 605 Airways Blvd., Jackson, TN 38301. Received 14 July 2003. *Corresponding author (vergel.c.concibido@ Abbreviations: AFLP, amplified fragment length polymorphism; BAC, monsanto.com). bacterial artificial chromosome; LG, linkage group; LRR, leucinerich repeat; MAS, marker-assisted selection; QTL, quantitative trait Published in Crop Sci. 44:1121–1131 (2004).  Crop Science Society of America locus; R-gene, resistance gene; SCN, soybean cyst nematode; SNP, single nucleotide polymorphism; SSR, simple sequence repeat. 677 S. Segoe Rd., Madison, WI 53711 USA