Quantitative trait loci for antibiosis resistance to corn earworm in soybean

In more than 25 yr since the discovery of soybean [Glycine mar (L.) Merr.] resistance to defoliating insects, attempts to introgress this trait into e lite 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 inherit ed quantitatively in soybean. The objectives of this study were (i) to use restriction fragment length polymorphism (RFLP) maps to identify quantitati ve trait loci (QTLs) in soybean for antibiosis against corn earworm (CEW) ( Helicoverpa tea Boddie), (ii) to determine the relative magnitude, gene act ion, and genomic locations of these QTLs, and (iii) to compare them to prev iously detected soybean antixenosis QTLs. Restriction fragment length polym orphism maps were constructed in three soybean F-2 populations segregating for antibiosis against CEW:'Cobb' x PI171451, Cobb x PI227687, and Cobb x P I229358. Antibiosis was measured as larval weight gain in a detached leaf a ssay. The RFLP data were associated with insect bioassay data to detect QTL s for antibiosis in each cross. Variance component heritability estimates f or antibiosis in the three crosses were 54, 42, and 62% in Cobb x PI171451, Cobb x PI227687, and Cobb x PI229358, respectively. An antibiosis QTL on L inkage Group (LG) M was detected in both Cobb x PI171451 and Cobb x PI22935 8 (R-2 values of 28 and 22%, respectively). An antixenosis QTL was also sig nificant at this location in the same two crosses. This was the only insect -resistance QTL that was detected for both antibiosis and antixenosis. Anti biosis QTLs were also detected on LGs F and B2 in Cobb x PI227687 (R-2 = 33 and 12%, respectively), and LGs G and J in Cobb x 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 a ntibiosis alleles at the QTLs on LGs F and J.