II. 10. A semi-dominant short awn mutant in Morex.
R.T. Ramage. Agricultural Research Service, U. S. Department of Agriculture and University of Arizona, Tucson, Arizona 85721, U.S.A. "R"
Mutants in which the heterozygote can be readily distinguished from either homozygote are most useful in genetic analysis, particularly if they can be classified before flowering and do not adversely affect vigor or fertility of plants carrying the mutant allele. The row-number locus on chromosome 2 (V-v) is one of a very few that have been reported in barley that meet these criteria. Several other mutants that are semi-dominant or incompletely dominant have been reported, but in most of them, the homozygote is lethal or sterile and the heterozygote has reduced vigor and/or fertility. For example, Takahashi, Hayashi, Konishi and Moriya (1973. Ohara Inst. Landw. Biol. Okayama Univ. 15, 3: 147-168) reported a mutant in which the homozygote was a lethal xantha and the heterozygote was a viable chlorina. Foster (1979. BGN 9: 23-24) reported a mutant in which the homozygote was a lethal albino and the heterozygote was a viable, but less vigorous light green. He also described a mutant for "grass dwarf" in which the homozygotes were severely stunted, had many tillers and were sterile. The heterozygotes were slightly less vigorous than normal and could, with some exceptions, be distinguished from normal plants. Tsuchiya (1962. Seiken Ziho 14: 21-34) reported an incompletely dominant mutant for short awns in which the heterozygote had medium length awns. Tsuchiya and Singh (1972. BGN 2: 99) reported that, by trisomic analysis, the short awn gene was not located on chromosomes 3, 4, 5, 6, or 7, suggesting that it is probably on either chromosome 1 or 2. In their analysis of the data, they combined the normal and heterozygous plants into one class to give a 3:1 ratio.
We are reporting a semi-dominant mutant for short awns that was induced in the cultivar 'Morex', C. I. 15773. Plants homozygous far the mutant have fewer tillers than Morex, but otherwise are relatively vigorous and fertile. The short awn mutant was crossed onto a male sterile mutant obtained from the same M2 population. The Fl plants had short to medium length awns that could be readily distinguished from both the short awn mutant and normal Morex. Except for having shorter awns, the Fl plants could not be distinguished from normal Morex. An F2 consisting of 23 short awn : 38 medium awn : 25 long awn plants was grown. These numbers fit a 1:2:1 ratio (X2 value of 1.256). The Fl and F2 indicate that the mutant is semi-dominant, at least in Morex background genotype.
Additional F2's and F3's and backcrosses of the Fl to both the mutant and normal Morex are being grown to confirm semi-dominance. The mutant will be crossed with the incompletely dominant short awn mutant reported by Tsuchiya to determine allelism. Also, the mutant has been crossed to a tester set of male sterile genes closely linked to the centromeres of each of the 7 chromosomes in an attempt to assign it to a chromosome. As the tester set contains several different genetic backgrounds, it will be possible to observe the effect of background genotype on the expression of the short awn mutant.