Transfer to hexaploid wheat of linked genes for adult-plant leaf rust and seedling stem rust resistance from an amphiploid of Aegilops speltoides × Triticum monococcum

A partially dominant gene for adult-plant leaf rust resistance together with a linked, partially dominant gene for stem rust resistance were transferred to the hexaploid wheat cultivar 'Marquis' from an amphiploid of Aegilops speltoides × Triticum monococcum by direct crossing and backcrossing. Pathological evidence indicated that the alien resistance genes were derived from Ae. speltoides. Differential transmission of the resistance genes through the male gametes occurred in hexaploid hybrids involving the resistant 'Marquis' stock and resulted in distorted segregation ratios. In heterozygotes, pairing between the chromosome arm with the alien segment and the corresponding arm of the normal wheat chromosome was greatly reduced. The apparent close linkage between the two resistance genes, 3 ± 1.07 crossover units, was misleading because of this decrease in pairing in the presence of the 5B diploidizing mechanism. The newly identified gene for adult-plant leaf rust resistance, located on chromosome 2B, is different from adult-plant resistance genes Lr12, Lr13, and Lr22 and from that in the hexaploid accession PI250413; it has been designated Lr35. It is not known whether the newly transferred gene for stem rust resistance differs from Sr32, also derived from Ae. speltoides and located on chromosomes 2B.Key words: hexaploid, Triticum, Aegilops, aneuploid, Puccinia graminis, Puccinia recondita.