Genetic Relationships Between Alleles of the Rp1 Rust Resistance Locus of Maize. Scot H. Hulbert. Department of Plant Pathology, Throckmorton Hall, Kansas State University, Manhattan 66506-5502 U.S.A. Mehmet A. Sudupak, and Kyung S. Hong. Department of Plant Pathology, Throckmorton Hall, Kansas State University, Manhattan 66506-5502 U.S.A. MPMI 6:387-392. Accepted 9 March 1993. This article is in the public domain and not copyrightable. It may be freely reprinted with customary crediting of the source. The American Phytopathological Society, 1993.

Rp1 is a complex resistance locus of maize. Different Rp1 genes confer resistance to different common rust isolates and vary in their meiotic stability. Susceptible derivatives from Rp1 heterozygotes, and those with the combined resistance of both parents, are commonly associated with a single nonparental combination of flanking markers—indicating they arise by simple crossing-over, and that one of the genes maps distally or proximally to the other. Susceptible derivatives from lines homozygous for unstable Rp1 genes exhibit both nonparental combinations of flanking markers—indicating they arise by unequal crossing-over. We analyzed derivatives from 10 different test cross populations to determine if such complex types of recombination events can occur in heterozygotes. The Rp1 genes used to construct the heterozygotes were known to vary in their meiotic stability. Recombinants with both non-parental combinations of flanking markers were found from three of the heterozygotes. All three hybrids carried at least one unstable gene. The occurrence of these recombinants indicates Rp1 genes reside on duplicated sequences which, in at least some lines, can mispair during meiosis. Implications for allelism tests for resistance genes, for the generation of a Rp1 area genetic fine structure map, and for transposon tagging strategies are discussed.