| Cytogenetic studies of X-ray induced erect-type mutants
in common wheat. I. Isamu UCHIKAWA Laboratory of Cytogenetics, Biological Institute Ehime University, Matsuyama, Japan In the year 1961 the author found a new type mutants in X2 generation of X-ray treated normal common wheat. The most conspicuous features of these mutants were dense erect spikes, tough and short straws and stiff erect leaves. The mutants with these characteristics were called "erect-type" by the author. The erect-type mutants are classified into four groups from the cytogenetic point of view : the mutants with somewhat thick and short straw and uniformly dense spike (Type I), those with remarkably short and thick straw and uniformly dense thick spikes (Type II), those with remarkably tough, thick short straws and dense squarehead spikes (Type III) and those with thin but stiff short straws and short dense spikes (Type IV). The last one is frequently segregated in the offspring of Type III heterozygotes, besides those directly arose in X2 generation. These erect-type mutants, however, are clearly distinguished from the compactoids (UCHIKAWA 1955, 1960) and dense-eared mutants (UCHIKAWA 1963) in their morphological and cytological features. 1. Type I erect-type mutants The mutants of this type directly arose as heterozygotes or chimeral plants in the X2 offspring of X-ray treated normal plants. The arising rate was 0.84% in X2. The heterozygotes were fairly shorter in height and spike length, and larger in straw-diameter and spike-density than those of normal plants. The leaves of this mutant were erect. The germination rate and seed fertility were slightly inferior to those of the normal plants. The homozygotes of this type had more intensified characters than those of the heterozygotes. The heterozygotes gave normals, heterozygotes and homozygotes in the ratio 1 : 1.9 : 0.7 respectively in the next generation. The heterozygotes and homozygotes of this type uniformly had 2n=42 chromosomes. In meiosis, the heterozygotes formed 21II association most frequently in PMC's (91.17%), besides a few of 20II+2I (8.06%), 1III+19II+1I (0.32%) and 1IV+19II (0.45%). The homozygotes also formed 21II in the majority of PMC's (95.1%), and in rare occasions the chromosome associations 20II+2I (4.15%), 1III+19II+1I (0.32%) and 1IV+ 19II (0.52 %) were seen. From these cytogenetic results, the author assumed that the Type I erect-type mutants may have been originated by gene mutation or by the occurrence of minute, microscopically undetectable, duplication in one of the chromosomes, which carried an erect-type promoting gene ert (or genes) loci region. The fact that the obtained segregation ratio was near the value of the simple Mendelian ratio and the results concering the erectoid mutation of barley obtained by GUSTAFSSON (1947), HAGBERG (1958) and the others supported the gene mutation assumption but the writer canot entirely deny some possibility of the occurrence of minute duplication from the fact that the 20II + 2I chromosome association was seen of comparatively high freqency. |
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