| Identification of the wheat chromosome carrying leaf-rust
resistance from Aegilops umbellulata E. R. SEARS U. S. D. A. and Missouri Agricultural Experiment Station, Columbia, Mo., U.S.A. The strain of wheat designated T47 (sometimes referred to under the name Transfer) differs from the variety Chinese Spring only by carrying a small intercalated piece of one of the chromosomes of Aegilops umbellulata. This introduced segment provides resistance to leaf rust, Puccinia recondita. Since the strain has now been distributed to wheat breeders in nine different countries and in 13 different states of the U.S.A., information as to which chromosome carries the introduced segment may be of interest. Data obtained by means of routine monosomic analysis failed to identify the chromosome. The 32 : 2 segregation that resulted in F2 from the cross of T47 with monosomic 6B (X) differed significantly from 3 : 1, but one of the two susceptible individuals proved to be disomic 6B, and one of the resistant segregates was nullisomic 6B. Barring error, this was impossible if chromosome 6B actually carried the resistance. To check chromosome 6B further, a cross was made of T47 onto a plant disomic for the Aegilops chromosome and monosomic for chromosome 6B. This material had been synthesized some years before in order to test the possibility of substituting the Aegilops chromosome for chromosome 6B. Similar Aegilops substitutions were available for chromosomes 3B (III), 4B (VIII), 7B (VII), and 2D (XX), and these lines were also crossed by T47. From these crosses all the offspring carried the Aegilops chromosome and the T47 translocated chromosome, and all those studied were deficient (monosomic) for one particular wheat chromosome. Where chromosome 3B, 4B, 7B, or 2D was involved, maximum pairing proved to be 1III19II1I, the trivalent consisting to the Aegilops chromosome, the T47 translocated chromosome, and the corresponding wheat chromosome. The univalent was the chromosome under test. With chromosome 6B a different pairing pattern was found - a maximum of 21 pairs and never a trivalent. The translocated chromosome must therefore be 6B. There being no normal 6B present in this F1, the translocated 6B could only pair with the Aegilops chromosome, and no trivalent was possible. The frequency of pairing of the translocated 6B with the Aegilops chromosome was low-only 11 pairs in 100 cells. This confirms the conclusion previously made that the translocated segment is small. When normal 6B was also present, 23 of the 249 microsporocytes examined had a trivalent-approximately the same frequency of pairing as with normal 6B absent. Thus it appears that the small segment of Aegilops chromatin inserted in chromosome 6B can pair with the homologous portion of the Aegilops chromosome almost equally well whether the wheat portion of the translocated chromosome is paired or not. The failure of monosomic analysis to identify chromosome 6B as the one concerned was evidently due to contamination at the time of pollination of mono-6B by T47. Two F1 plants were used, one of which must have come from a contaminant pollen grain, presumably from resistant translocation line T41 or T44, both of which were being used in crosses at the time. Excluding the data from mono-6B, 479 resistant and 180 susceptible plants were observed in F2-a resonable fit to a 3 : 1 ratio. |