| Dosage Effect of the spelta Gene q1*
M. MURAMATSU Curtis Hall, University of Missouri, Columbia, Missouri, U.S.A. Spontaneous or artificial speltoid mutation in the hexaploid common wheat, Triticum aestivum, is considered, in almost all cases, to be a deficiency for a segment including the speltoid-suppressing or squarehead gene Q on chromosome 5A (IX). It has been suggested that T. spelta, which resembles the speltoids, also originated as a deficiency for Q. Sears (unpublished) obtained the dosage series of the spelta gene q1 (an allele of Q) up to 4 doses and found that q1 behaved as a null allele. As Sears (1954, 1956) pointed out, however, if the Q gene arose by a mutation from q1, the latter could not be a deficiency. In this case, the most likely explanation for the null effect of q1 would be that it is an amorph or that it is a hypomorph with an effect so feeble that four doses of q1 scarcely have a recognizable effect on the phenotype. It is therefore of interest to obtain a plant with five or more doses of q1. Such a plant was discovered in the process of combining monosome 5B (V) with tetrasome 5A (IX) q1q1q1q1 from T. spelta. From a tri-5A (q1q1q1) monotelo-5B plant, 16 offspring were obtained. Fifteen of them showed the expected spelta expression, but one was squarehead. It had short culms, and the tips of the spikes were compact and infertile. That this plant was not a contaminant was clear from the fact that it had a pair of telocentrics. It also apparently had an isochromosome for the long (q1-carrying) arm of chromosome 5A plus three normal 5A's. From this plant, 29 seeds were sown, and 23 of the resulting offspring were analyzed cytologically. The chromosome constitution varied from 21II (two doses of q1) including one ditelosome to 20II + 1IV + 1I iso (six doses of q1) including one ditelosome. The plants with five doses of q1 showed the same phenotype as the parent. All the plants which had from two to four doses of q1 showed spelta expression. The plant with six doses of q1 had a spike still more compact than those with five doses. Evidently q1 is an allele which has an effect similar to that of Q but of lesser degree. Since q1 is an active gene, not merely a deficiency, T. spelta can not have originated from T. aestivum in the way that speltoids arise. The fact that q1 is a hypomorph strengthens the view of Kuckuck that aestivum (i.e., Q) originated from spelta through a duplication of q1. |
| * This work was supported by a grant from the National Science Foundation to Dr. E. R. Sears. |