| Radiobiological studies in plants, VII Chromosome aberrations induced by gamma-irradiation in a Japanese wheat variety1),2) I. NISHIYAMA and S. ICHIKAWA Laboratory of Genetics, Faculty of Agriculture, Kyoto University, Kyoto, Japan In the fourth paper of this series the authors reported the effects of gammairradiations on oats and wheat, especially in regard to the occurrences of chlorophyll mutations in the progenies of irradiated oats. No chlorophyll mutant was observed in wheat, T. vulgare VILL. var. Shinchunaga, so far examined (NISHIYAMA and ICHIKAWA 1961a). However many morphological and physiological mutants were often found in the X2 and the later generations of the same irradiated wheat. From the genetic behaviors of the mutant characters it was assumed that most of them had been induced through chromosome aberrations (NISHIYAMA and ICHIKAWA 1961b). A part of results from cytological and genetic observations on these mutants will be briefly reported in this paper. Results and discussion: Many of the X2-plants derived from 118 X1-spikes and of their progenies (X3-X5) showed aberrant change in theirs morphological or physiological characters. These plants were cytologically investigated in the meiosis of the pollen mother cells. Out of 44 X2-lines investigated cytologically, 34 showed abnormal meiotic configurations, such as multivalents, univalents, heteromorphic bivalents, aneuploidy, partial asynapsis, or mixoploidy in their progenies. Total number of plants examined cytologically amounted to 546, of which 342 (63%) showed chromosome aberrations. It is noteworthy that the plants with 38-45 somatic chromosomes were found in the progenies of the irradiated wheat. Excluding the plants which showed partial asynapsis or mixoploidy, about 50% of plants has 42 chromosomes, 21% had 41 chromosomes, 9% 40 chromosomes, 9% 43 chromosomes, 7% 44 chromosomes, and the remainder 38, 39 or 45 chromosomes. Some different types of meiotic configurations were sometimes found in the plants with the same chromosome number. From this fact it was assumed that these plants had different chromosome constitutions although they had the same number of chromosomes. For instance, the plants with 41 chromosomes could be classified into monosomics (20II + 1I in most cases), nulli-trisomics (1III+ 19II in most cases), and monosomics with one or two complex pairings of chromosomes (1IV + 18II + 1I, 2IV + 16II + 1I, etc.). Total 204 X3-X5-segregates from 31 X2-lines had the same number of chromosomes as the original variety, showing 21 normal bivalents (42 somatic chromosomes). However, aberrant configurations of chromosomes such as one or two complex pairings of four or sometimes more chromosomes were occasionally found in the progenies of many X2-lines. Further observations showed the occurrences of mono-trisomics (1III + 19II + 1I) and nulli-trisomics (1III + 19II) in a few cases. Besides, there were observed mixoploid segregates and partially asynaptic plants, the details of which will be mentioned later. |
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