| Cytogenetic studies of X-ray induced erect-type mutants
in common wheat. II. Isamu UCHIKAWA Laboratory of Cytogenetics, Biological Institute Ehime University, Matsuyama, Japan 3. Type III erect-type mutants The percentage of the occurrence of the mutants of this type in X2 was only 0.38%. The heterozygotes were still more shorter in height and spike-1ength, and larger in straw-diameter, spike-width, leaf-width and spike-density than those of Type II heterozygotes. The seed germination rate and seed fertility were remarkably lower than those of Type II. The homozygotes had more intensified characters than those of the heterozygotes. The heterozygotes gave normals, heterozygotes, homozygotes and a few unexpected plants (Type IV erect-type heterozygote and lax-eared) in a ratio of 1 : 1 : 0.7 : 0.05 in the following generation. The heterozygotes had 2n=42 chromosomes including one large V-shaped isochromosome. In meiosis of PMC's the majority of the cells showed 21II including one heteromorphic pair (61.17%) or 20II+2 unequal univalents (37.71%), besides a few of 1III+19II+1I (0.44%) and 1IV+18II + 2I (0.68%). In the heteromorphic pair or two unequal univalents, one smaller partner was middle-sized with a submedian attachment chromosome, and the other one was remarkably larger in size, about twice as long as the long arm of the smaller partner. From these cytogenetic facts, it is assumed that the Type III erect-type mutants is originated by the duplication of the long arm of a chromosome, which bears the erect-type promoting gene (ert) or genes, and the isochromosome formation might be brought about by the misdivision of a chromosome. The isochromosome bears the erect-type promoting gene in double dose. The abnormal segregation ratio (1 : 1: 0.7) can be explained by the competition between the male normal gametes with 21 chromosomes and the male mutant gametes with 20+1 isochromosomes, in which the normal gametes are about 10 times as effective as those of the mutant gametes, and a slight degree of the zygotic elimination of homozygotes. |
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