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Epistatic interaction among rust resistance genes in common bean
M. PASTOR-CORRALES (1), G. Valentini (2), Q. Song (3), P. Cregan (3). (1) USDA ARS, Beltsville, MD, U.S.A.; (2) Universidade Estadual de Maringá, Maringa, Brazil; (3) USDA/ARS, Beltsville, MD, U.S.A.

Resistance in common bean to the rust pathogen is conferred by single and dominant genes. The resistant phenotype of these genes is expressed either as a hypersensitive reaction (HR), visualized as necrotic spots without sporulation or as tiny uredinia (R). Epistatic interaction, in which one gene masks the phenotype of another gene, has been mentioned often in the literature for some rust resistance genes, however little is known about these interactions. We conducted a study using phenotypic and genetic markers to elucidate the interaction between the <i>Ur-4</i> and <i>Ur-5</i> rust resistance genes. A total of six F<sub>1</sub> plants and 182 F<sub>2</sub> plants from a Mexico 309 (<i>Ur-5</i>) x Early Gallatin (<i>Ur-4</i>) cross plus the two parents, and 10 check cultivars, were inoculated with races 40, 44, 53 and 108 of <i>U. appendiculatus</i>. Races 53 and 108 were used to detect the presence of <i>Ur-5</i> and <i>Ur-4</i>, respectively, and races 40 and 44 to determine the plausible epistatic interaction between these genes. We also used simple sequence repeat genetic markers that were previously identified as closely linked to the <i>Ur-4</i> and <i>Ur-5</i> genes. Inoculation of the 182 F<sub>2</sub> plants produced a segregation ratio of 12R: 3HR: 1S. The molecular analysis on the 182 F<sub>2</sub> plants using the SSR markers gave the same results as the phenotypic evaluation, except for one plant. These results indicate a dominant epistatic interaction, where the <i>Ur-5</i> gene is epistatic to <i>Ur-4</i> and the dominant allele of <i>Ur-5</i> hides the effects of <i>Ur-4</i>.

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