Fine Mapping of a Major Insect Resistance QTL in Soybean and its Interaction with Minor Resistance QTLs

Utilization of native insect resistance genes can be an important component for managing insects in soybean [Glycine max (L.) Merr.]. A major quantitative trait locus (QTL-M) for insect resistance from PI 229358, controlling antibiosis and antixenosis, was previously identified on linkage group (LG) M and was found to increase the effectiveness of a Bacillus thuringiensis (Bt) transgene in soybean. The objectives of this study were to fine-map QTL-M using recombinant substitution lines (RSLs) identified from a ‘Benning’ backcross population, and to evaluate the main effects and the epistatic interactions between QTL-M and other resistance QTLs on LGs G and H using near-isogenic lines (NILs) in a Benning genetic background. The effect of QTL-M was still detectable in the Benning NILs when they were evaluated for resistance to corn earworm [CEW, Helicoverpa zea (Boddie)]. The two minor resistance QTLs only provided insect resistance when QTL-M was also present in the Benning NILs. The QTL-M was fine-mapped to an approximately 0.52-cM region after the first round of phenotyping the RSLs for resistance to CEW and soybean looper [SBL, Pseudoplusia includens (Walker)]. These results should increase the feasibility of cloning QTL-M and help guide the development of insect resistant soybean cultivars. CLASSICAL BREEDING for insect resistance in soybean has faced several obstacles. First, since soybean resistance to insect defoliation is a quantitatively inherited trait (Sisson et al., 1976; Rufener et al., 1989), simple backcrossing has not been successful in transferring the full complement of resistance genes from unadapted germplasm accessions. In several insect resistance breeding programs based on phenotypic selection, only a major QTL on LG M was consistently selected, while minor insect resistance QTLs were often lost during breeding (Narvel et al., 2001), resulting in a resistance level lower than that of the original donor. Second, the main sources of insect resistance in soybean, the three Japanese plant introductions PI 229358, PI 227687, and PI 171451, are of low agronomic value, increasing problems with linkage drag (Van Duyn et al., 1971; Lambert and Tyler, 1999). Furthermore, the insect resistance in these PIs exhibits two distinct mechanisms: antibiosis (adverse effects on the insect life history), and antixenosis (discouragement of insect colonization and/or feeding) (Painter, 1951; Kogan and Ortman, 1978; Lambert and Kilen, 1984). The genetic independence of antibiosis and antixenosis has been suggested, although their effects may overlap (Painter, 1951; Rector et al., 1999, 2000). Depending on the phenotypic screen used, breeders often select for one of the two resistance modes, potentially resulting in the loss of resistance alleles for the other. The difficulty of obtaining a high level of insect resistance inmany crops via conventional breeding prompted genetic engineering with crystal protein genes from Bt (Stewart et al., 1996). However, the widespread use of Bt genes has raised issues about the evolution of Btresistant insect populations (Gould, 1998; Walker et al., 2002b). Pyramiding Bt genes and native resistance genes in the same plant increases the efficacy of both (Walker et al., 2002b, 2004). Therefore, the identification, characterization, and utilization of native soybean resistance genes could help the management of insect resistance to insecticidal proteins and broaden the resistance of plants with Bt genes. Association of restriction fragment length polymorphism marker data with CEW defoliation and weight gain data from F2–derived lines of a Cobb 3 PI 229358 population allowed Rector et al. (1998, 2000) to identify antixenosis QTLs on LGs M (QTL-M), D1b (QTLD1b), and H (QTL-H). The QTLs for antibiosis were identified on LGs M and G (QTL-G), at which the resistance alleles were all contributed by PI 229358. In SoyBase (http://soybase.org; unverified future URL), QTL-M, QTL-H, and QTL-G were first listed as CEW 11, CEW 1-2, and CEW 6-1, respectively. Of these, QTL-M, the major insect resistance QTL, was initially identified in an approximately 30-cM interval, with its peak at marker A584_4 on LG M. QTL-M contributed approximately 35 and 20% of phenotypic variations for antixenosis and antibiosis to CEW, respectively. It also enhances the effectiveness of a Bt cry1Ac transgene in soybean (Walker et al., 2004). Narvel et al. (2001) later presented evidence that QTL-M was in the approximately 8-cM region flanked by simple sequence repeat (SSR) markers Satt220 and Satt175. These authors also mapped QTL-H in the 0.5-cM Sat_122–Satt541 interval on LG H, QTL-D1b in the 3.7-cM Satt141–Satt290 interval on LG D1b, and QTL-G in the 4.4-cM Satt472– Satt191 interval on LG G. Many early QTL mapping studies in plants reported QTLs from populations of limited size and from statistical tests that producedhigh rates ofType I errors (Fasoula et al., 2004). Putative QTLs detected at a less stringent comparison-wise significance level (P . 0.0001) need to be further analyzed before use in marker-assisted selection (MAS) programs (Bernardo, 2004). The effects S. Zhu, D.R. Walker, H.R. Boerma, and W.A. Parrott, Center for Applied Genetic Technologies and Dep. of Crop & Soil Sciences; J.N. All, Dep. of Entomology, Univ. of Georgia, Athens, GA 30602. Received 7 June 2005. *Corresponding author ([email protected]). Published in Crop Sci. 46:1094–1099 (2006). Genomics, Molecular Genetics & Biotechnology doi:10.2135/cropsci2005.06-0109 a Crop Science Society of America 677 S. Segoe Rd., Madison, WI 53711 USA Abbreviations: Bt, Bacillus thuringiensis; CEW, corn earworm; LG, linkage group; MAS, marker-assisted selection; NIL, near-isogenic line; QTL, quantitative trait locus; RSL, recombinant substitution line; SBL, soybean looper; SSR, simple sequence repeat. R e p ro d u c e d fr o m C ro p S c ie n c e . P u b lis h e d b y C ro p S c ie n c e S o c ie ty o f A m e ri c a . A ll c o p y ri g h ts re s e rv e d . 1094 Published online March 27, 2006