A Single Dominant Gene for Resistance to the Soybean Aphid in the Soybean Cultivar Dowling

The soybean aphid (Aphis glycines Matsumura), a new pest of soybean [Glycine max (L.) Merr.], rapidly spread throughout North America after its arrival in 2000 and caused millions of dollars in economic losses. At present, the application of insecticides is the only means to control the soybean aphid. However, genetic resistance to the aphid was recently discovered in soybean germplasm and the soybean cultivar Dowling was identified as having strong antibiosis-type aphid resistance. The objective of this study was to determine the inheritance of resistance to the soybean aphid in Dowling. Resistance in F1, F2, and F2–derived F3 (F2:3) families from crosses between Dowling and the two susceptible soybean cultivars Loda and Williams 82 was analyzed. All F1 plants were resistant to the aphid. Heterogeneity of segregation of F2 plants in 14 Dowling 3 Loda F2 families was nonsignificant (P 5 0.16), and pooled F2 data, with 132 resistant to 45 susceptible plants, fit a 3:1 ratio (P 5 0.90). F2 plants from Dowling 3 Williams 82 segregated 135 resistant to 44 susceptible, also fitting a 3:1 ratio (P 5 0.89). Segregation among the F2:3 families fit a 1:2:1 monogenic inheritance pattern. These results indicated that a single dominant gene named Rag1 controlled resistance in Dowling. The monogenic dominant nature of resistance will enable breeders to rapidly convert existing susceptible cultivars to resistant cultivars using backcrossing procedures. SINCE ITS ARRIVAL FROM ASIA in 2000 (Hartman et al., 2001), the soybean aphid has spread throughout soybean production areas in North America (Ragsdale et al., 2004). High aphid populations reduce soybean production directly by causing severe plant damage during feeding, including leaf distortion, stunting, and desiccation. Soybean plants are indirectly affected by the growth of black sooty mold fungus on aphid honeydew that inhibits plant photosynthesis and through the vectoring of serious soybean viruses such as Soybean mosaic virus (Hartman et al., 2001). In 2003, extensive economic losses caused by the soybean aphid occurred in soybean in several midwestern states. An estimated 1.6 million ha damaged was reported in Minnesota resulting in a loss of US$80 million (Associated Press, 2003). In Illinois, about 0.5 million ha were damaged with an estimated loss of US$45 million (Steffey, 2004). Presently, the only means that soybean growers have to control the soybean aphid is with the application of registered insecticides. During the 2003 soybean aphid outbreak, nearly 3 million hectares of soybeans in the USAwere sprayed to control the soybean aphid (Landis et al., 2003). From $9 to 12 million was spent on insecticide applications in Illinois alone in 2003 (Steffey, 2004). Plant insect resistance is an important component of an integrated pest management program that utilizes several types of control methods to control insects, including insecticide application and cultural management practices (Auclair, 1989; Harrewijn and Minks, 1989). Plant insect resistance is also the most cost effective and environmentally safe way to control insects such as the soybean aphid (Luginbill, 1969). Plant resistance to the soybean aphid was recently discovered in soybean germplasm (Hill et al., 2004). Resistance in the cultivar Dowling had strong antibiosis that limited aphid colonization on plants in non-choice tests. In field experiments, Dowling performed as well without treatment with the systemic insecticide imidacloprid [(EZ)-1-(6-chloro-3-pyridylmethyl)-N-nitroimidazolidin-2-ylideneamine] (Marathon, Olympic Horticultural Products, Mainland, PA) as it did with the treatment, and effectively controlled aphid population development during all soybean growth stages. Detailed analysis of the effects of antibiosis on aphid biology indicated that the resistance inDowling significantly reduced aphid survival, longevity, fecundity, and development (Li et al., 2004). Resistance to insects is governed by genetic mechanisms like other plant traits (Auclair, 1989). Knowledge of the inheritance of insect resistance, as with any other economic plant trait, facilitates the design of appropriate breeding procedures to develop resistant cultivars and is useful for the identification of biotypes of insects that may already exist or develop over time (Smith, 1989). Qualitative, or simply inherited, traits require different breeding methods than quantitative traits controlled by many genes. The objective of this study was to determine the inheritance of the aphid resistance in Dowling soybean. MATERIALS AND METHODS Crosses were made between the soybean aphid resistant cultivar Dowling and two susceptible soybean cultivars, Loda and Williams 82, inside a plant growth chamber (Conviron, model no. CMP4030, Winnipeg, MB, Canada) at 278C, 70% relative humidity, under a 12-h photoperiod provided by a mixture of incandescent and fluorescent lighting giving 300 mmol m s PAR irradiation. All crosses were made in one direction; pollen from Loda and Williams 82 flowers was transferred onto stigmata of Dowling flowers. Seed produced from individual crosses was harvested and planted separately for F2 seed production in a greenhouse maintained at 288C CurtisB.Hill,Dep. ofCropSciences,Univ. of Illinois, 1101West Peabody Drive, Urbana, IL 61801; Glen L. Hartman, USDA-ARS and Dep. of Crop Sciences, Univ. of Illinois, 1101West PeabodyDrive, Urbana, IL 61801;YanLi, 6029S.KimbarkAve.Apt. 1,Chicago, IL60637.Tradeand manufacturers’ names are necessary to report factually onavailable data; however, theUSDAneither guarantees nor warrants the standard of the product, and theuseof thenameby theUSDAimplies noapproval of the product to the exclusion of others that may also be suitable. Received 17 Nov. 2005. *Corresponding author ([email protected]). Published in Crop Sci. 46:1601–1605 (2006). Crop Breeding & Genetics doi:10.2135/cropsci2005.11-0421 a Crop Science Society of America 677 S. Segoe Rd., Madison, WI 53711 USA 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 . 1601 Published online May 18, 2006