Molecular mapping and allelic relationships of Russian wheat aphid-resistance genes

The Russian wheat aphid (RWA), Diuraphis noxia (Mordvilko), is a serious economic pest of wheat (Triticum aestimin L.) worldwide. Although resistant wheat cultivars provide good control of RWA, genetic characterization is critical to the continual use of aphid-resistance genes. The objective of this study was to determine the chromosome location and the genetic relationships of the RWA-resistance genes Dn1, Dn2, Dn4, Dn5, Dn6, Dnx, and several uncharacterized Dn genes. Molecular mapping demonstrated that Dn genes in four uncharacterized wheat lines (PI 47545, PI 222666, PI 222668, and PI 225245) as well as DO, Dn2, Dn5, Dn6, and Dnx are tightly linked to microsatellite markers Xgwm44 and Xgwm111. Deletion bin mapping assigned Xgwm44 physically in the region distal to the breakpoints 7DS-4 (FL 0.61) and Xgwm111 distal to 7DS-5 (FL 0.36) of wheat chromosome arm 7DS. Allelism tests detected no segregation for susceptibility among TC1F1 plants from the test crosses or among F-2 plants derived from intercrosses between wheat lines containing each of these known Dn genes. Both marker-linkage analyses and allelism tests revealed that all these Dn genes are clustered in a region linked to Xgwm111 and are either alleles of a single locus or are closely related members of a Dn gene family. Dn4 and the uncharacterized Dn gene in PI151918 are either allelic or linked on wheat chromosome 1DS. The linked markers and genetic relationships of these Dn genes will greatly facilitate their use in wheat breeding and the deployment of aphid-resistant cultivars.