| Radiation-induced mutants in a Japanese wheat variety,
Shinchunaga S. ICHIKAWA Laboratory of Genetics, Kyoto University, Kyoto, Japan Irradiation with 10 to 40 kr of gamma rays from 60Co on the dry seeds of Shinchunaga, a Japanese variety of common wheat, induced 248 kinds of mutants. For convenience' sake, these mutants were classified into six groups, speltoid, compactoid, squareheaded, lax-spiked, dense-spiked, and other mutants from the spike shapes, but other mutant characters were observed always in combination with the above mutants. Cytogenetical investigations have been performed on these mutants up to the X8 generation. Speltoids were caused by a loss of gene, called Q, on the long arm of 5A chromosome. Namely, they have deficiencies of various sizes on the long arm, or are missing the whole arm or the whole chromosome. Only one speltoid mutant was considered to be near to the A-type speltoid of gene mutation type. Most compactoids, sub-compactoids and the related squareheadeds were due to an increase of the Q gene. Namely, the increase of the whole arm and of the whole chromosome occasionally accompanied with the decrease of the homoeologous chromosome were observed in these mutants. Besides, the sub-compactoid and squareheaded mutants of a strain were considered to be pentasomic and tetrasomic for either of 5B or 5D chromosomes, respectively. Some of squareheadeds which do not segregate compactoids were caused by a loss of the short arm of 5A chromosome or by deficiencies on the arm. Several lax-spiked mutants were certified to be missing 6D chromosome or the homoeologous chromosomes or one of the arms of these chromosomes. Some other lax-spikeds were also missing a chromosome of different homoeologous group. A part of dense-spikeds were nullisomic for the chromosomes of homoeologous group 3, or were missing one of the arms of these chromosomes. Among many other mutants too, various chromosomal aberrations were observed. It is conclusive from the present investigations that the majority of radiation-induced mutants are caused by deficiencies or other chromosomal aberrations rather than by gene mutations, in contrast with spontaneous mutants. |