| F1 Monosomic analysis of resistance in common
wheat to the greenbug (Schizaphis graminum Rond.) Byrd C. CURTIS*, A. M. SCHLEHUBER and C. L. MOORE Agronomy Department Oklahoma State University Stillwater, Oklahoma, U.S.A. The specific chromosome carrying the recessive greenbug resistant gene gb, was not determined in repeated tests of monosomic analysis. In tests conducted in 1957 and repeated in 1964, the F1 plants between DS28A, C.I. 13833, a greenbug resistant hard red spring wheat, crossed with the 21 different Chinese Spring monosomics (greenbug susceptible), were susceptible (killed) while the DS28A parent showed good resistance. One explanation for failure to locate the chromosome carrying the greenbug resistant trait is that the recessive resistant gene may be ineffective in a single dose. Such hemizygous ineffective genes are known in wheat (viz. SEAR'S sphaerococcum gene). In each of the F1 populations about 75 percent of the plants should have been monosomic. Thus, in the critical chromosome family about 75 percent of the plants should have been resistant had a single dose of the gb gene been effective. Another explanation is that resistance may be conditioned by more than one gene pair. The latter alternative is difficult to accept because the authors have found that the resistant trait responded like a simple recessive in a large number of F1, F2, F8, and backcross populations that irivolved several susceptible wheat varieties. In addition, if more than one gene pair conditions resistance there should have been different degrees of resistance among resistant progenies. This has not been the case. All resistant F2 plants as well as subsequent selections from these plants have proved to have resistance equal to that of DS28A. |
| * Now with the Agronomy Department, Colorado State University, Fort Collins, Colo. U.S.A. |