| F1 monosomic analysis involving a smooth-awn
durum wheat R. E. ALLAN and O. A. VOGEL Crops Res. Div., Wash. Agr. Exp. Sta., Pullman, Washington, U.S.A. Morphological examination of F1 plants monosomic for chromosomes I through XIV from crosses of smooth-awn durum selection 396 with the Chinese series did not reveal the chromosome or chromosomes possessing the recessive smooth-awn factor(s) of the durum. Awns or awnlets of F1 monosomic for chromosomes of the A and B genome except chromosomes II and XIII were barbed to a degree similar to their respective disomic sibs. No awns were produced on plants monosomic for chromosomes II or XIII. At least three explanations are possible for the failure to locate the chromosome(s) concerned with the smooth-awn trait. The smooth-awn factor(s) may be located on chromosome II or XIII and the completely awnless condition of these monosomics makes detection impossible. A second alternative is that a factor located in the D genome of Chinese Spring may inhibit the expression of the recessive smooth-awn factor(s) in the A or B genome. This appears possible because awns of Aegilops squarrosa selections are coarsely barbed which suggests that the D genome possesses factors influencing barbing. A third explanation could be that the recessive smooth-awn character is incapable of expression in the hemizygous condition. Chinese Spring possesses well-developed barbs on the keels of its outer glumes. Smooth-awn selection 396 has keels free of barbs but slightly scabrous. Keels of all F1 monosomics of the A and B genomes were as barbed as their respective disomic sibs except for plants monosomic for chromosomes II and XIII. Plants monosomic for these chromosomes had fewer and less developed barbs than their disomic sibs. Apparently chromosomes II and XIII carry recessive factors conditioning barbing of keels. It is not known whether barbing of keels and awns is a pleiotropic condition. Awn development of the F1 monosomic plants revealed that smooth-awn durum selection 396 possesses the recessive alleles of Hd and B2 factors located, respectively, on chromosomes VIII and X of Chinese Spring. In addition to these known factors, selection 396 must possess a recessive factor (aa) for awn development on chromosome V, as plants monosomic for V had well-developed awnlets. F1 plants nullisomic for chromosome XVI exhibited a high degree of awn development compared with plants with 35 chromosomes. Chinese Spring apparently has an awn-inhibiting factor (BB) located on chromosome XVI that suppresses one or more of the awn-developing factors derived from selection 396. Under greenhouse conditions at Pullman, Washington, the culms of smooth-awn durum selection 396 are covered by a heavy waxy bloom, whereas culms of Chinese Spring are almost waxless. F2 populations of the cross Chinese Spring x selection 396 indicated that the waxy condition is controlled by a single dominant factor. Except for F1 plants monosomic for chromosome XIII, all A and B genome F1 monosomics had waxy culms. Plants monosomic and disomic for chromosome VIII were less waxy than other plants, but not as waxless as F1 plants monosomic for chromosome XIII. If selection 396 possesses a dominant factor (Wx) for waxy culms, it apparently does not express itself in the hemizygous condition. Plants of the Wx wx genotype have a waxy phenotype, whereas plants of Wx- - genotype, from which the Wx allele is absent, as well as wx wx plants, have waxless phenotypes. Perhaps Wx equals two units of a precursor for waxiness and wx equals one. If at least three units are needed to bring about the waxy condition, then Wx wx plants would form wax but Wx- - and wx wx plants, having only two units, would not develop wax. Date of anthesis was strikingly late for plants monosomic for chromosome XIII; these monosomes flowered 11 days after their disomic sibs. Future plans for the study of the smooth-awn trait involve the production of an amphidiploid between the durum and Aegilops squarrosa so that the effect of the D genome on expression of this trait can be studied. |