Inheritance and allelic relationships among Russian wheat aphid resistance genes in winter wheat

The Russian wheat aphid (RWA), Diuraphis noxia (Mordvilko), has caused extensive economic damage to wheat (Triticum aestivum L.) and barley (Hordeum vulgare L.) in the western Great Plains and intermountain area of North America during 1986-1994. Genetic information is necessary to efficiently develop adapted resistant cultivars. Genetic studies were conducted to determine the mode of inheritance and allelic relationships among four resistant winter wheat accessions, PI 372129, PI 243781, PI 262605, and PI 294994. To determine the mode of inheritance, the resistant lines were crossed to the susceptible winter wheat cultivar Carson, and F-1, F-2, and BC1F1 seedlings were screened for RWA reaction. To determine allelism, the resistant lines were intercrossed and crossed to two other known RWA-resistant wheat accessions, PI 137739 (Dn1) and PI 262660 (Dn2), and the F-2 seedlings were evaluated. Resistance in PI 262605 and PI 294994 was controlled by one and two dominant genes, respectively, and the resistance gene in PI 262605 was allelic to Dn1 and Dn2. In contrast to an earlier report, Dn1 and Dn2 are probably allelic since no segregation was observed among F-2 seedlings from this cross, and progeny of crosses with four other accessions gave similar ratios. Resistance in PI 372129 and PI 243781 is conditioned by single dominant genes that are independently inherited. The gene symbols Dn4 and Dn6 are recommended for the resistance genes in PI 372129 and PI 243781, respectively.