Quantitative Trait Loci for Antibiosis Resistance to Corn Earworm in Soybean

In more than 25 yr since the discovery of soybean [Glycine max (L.) Merr.] resistance to defoliating insects, attempts to introgress this trait into elite germplasm have been relatively unsuccessful. Resistance to defoliating insects in soybean is expressed as a combination of antibiosis (toxicity) and antixenosis (nonpreference). Both of these resistance modes are inherited quantitatively in soybean. The objectives of this study were (i) to use restriction fragment length polymorphism (RFLP) maps to identify quantitative trait loci (QTLs) in soybean for antibiosis against corn earworm (CEW) (Helicoverpa zea Boddie), (ii) to determine the relative magnitude, gene action, and genomic locations of these QTLs, and (iii) to compare them to previously detected soybean antixenosis QTLs. Restriction fragment length polymorphism maps were constructed in three soybean F₂ populations segregating for antibiosis against CEW: `Cobb' × PI171451, Cobb × PI227687, and Cobb × PI229358. Antibiosis was measured as larval weight gain in a detached leaf assay. The RFLP data were associated with insect bioassay data to detect QTLs for antibiosis in each cross. Variance component heritability estimates for antibiosis in the three crosses were 54, 42, and 62% in Cobb × PI171451, Cobb × PI227687, and Cobb × PI229358, respectively. An antibiosis QTL on Linkage Group (LG) M was detected in both Cobb × PI171451 and Cobb × PI229358 (R ² values of 28 and 22%, respectively). An antixenosis QTL was also significant at this location in the same two crosses. This was the only insect-resistance QTL that was detected for both antibiosis and antixenosis. Antibiosis QTLs were also detected on LGs F and B2 in Cobb × PI227687 (R ² = 33 and 12%, respectively), and LGs G and J in Cobb × PI229358 (R ² = 19% for each). Antibiosis was conditioned by the PI (resistant parent) allele at the QTLs on LGs G, M, and B2, whereas the susceptible parent, Cobb, provided antibiosis alleles at the QTLs on LGs F and J.