Insect resistance. Russian Wheat Aphid (RWA) Resistance.
Contributed by Junhua Peng, Scott Haley, and Nora Lapitan
Russian wheat aphid (RWA) has rapidly become an important insect pest in the western United States. Direct and indirect economic losses in small grains incurred from reduced yield and increased production costs since its introduction in 1987 were estimated to be over $1 billion. This includes an estimated 8.1 million pounds of insecticides applied by US farmers at a cost of $95 million. The most effective means to control RWA is through deployment of resistant cultivars carrying one or more of the 10 known resistance genes to RWA. Field screening is extremely labor-intensive and greenhouse-based screening is limited to the cooler months of the year. Furthermore, combining more than one resistance gene in the same genotype, to potentially improve durability of resistance, is not possible with phenotypic evaluations. The use of molecular markers linked to the RWA resistance genes has the potential to solve some of these problems and to enable breeders to combine two or more RWA resistance genes efficiently in the same genotypes. Up to date, two RWA resistance genes, Dn2 and Dn4, have been studied more intensively than others and some tightly linked or flanking markers are available for marker-assisted selection of these two genes.
In June 2003 a new biotype virulent to both Dn2 and Dn4 was found in Colorado, you can read a report on the CSU's Wheat Breeding and Genetics Program website (if you cannot read the pdf file properly, right-click on the report link, select "Save target as..." and rename the file to download as "newrwa.pdf").
The Dn2 and Dn4 genes were first mapped using RFLP markers (1).
Resistance gene Dn4 was mapped to the short arm of chromosome 1D between RFLP markers Xabc156 (11 cM) and Xksud14 (1). Due to the complexity and usage cost, RFLP markers are actually not a practical tool in breeding. New PCR markers, i.e., microsatellites tightly linked to the gene have been recently identified (4, 5 and Arzani, Peng, and Lapitan unpublished, see methods for Dn4)
Dn2 is a dominant resistance gene and both RAPD derived SCAR markers (3) and microsatellite markers closely linked to this gene are identified (2, 4, 5). Thus two types of PCR markers, SCAR and microsatellite are available to this project (see methods for Dn2)
The original sources harboring Dn2 and Dn4 genes are PI 262660 and PI 372129, respectively (2, 3, 4, 5). In the current backcross breeding program of RWA resistance, the following donors and recurrent parents are used:
1. Genetic mapping of Russian wheat aphid resistance genes Dn2 and Dn4 in wheat. Ma, Z.Q.; Saidi, A.; Quick, J.S.; Lapitan, N.L.V. In: Genome, 1998, 41: 303-306. [abstract]
2. A microsatellite marker for tagging Dn2, a wheat gene conferring resistance to the Russian wheat aphid. Miller C. A.; Altinkut, A.; Lapitan N. L. V. In: Journal of Phytopathology, 2001, 149(11-12):641-648. [abstract]
3. Development of RAPD and SCAR markers linked to the Russian wheat aphid resistance gene Dn2 in wheat. Myburg, A.A.; Cawood, M.; Wingfield, B.D.; Botha, A.M. In:Theoretical and Applied Genetics, 1998, 96: 1162-1169 [abstract]
4. Microsatellite markers linked to six Russian wheat aphid resistance genes in wheat. Liu, X.M.; Smith, C.M.; Gill, B.S.; Tolmay, V.In: Theoretical and Applied Genetics, 2000, 102: 504-510 [abstract]
5. Identification of microsatellite markers linked to Russian wheat aphid resistance genes Dn4 and Dn6.. Liu, X. M.; Smith, C. M.; Gill, B. S. In:Theoretical and Applied Genetics, 2002, 104: 1042-1048. [abstract]