| Discussion According to MCCLINTOCK (1941), a dicentric, double-sized ring chromosome produced by sister chromatid exchange between replicated ring chromosomes forms chromosome bridges in mitotic anaphase. These bridges are broken and the resulted chromosomes produce new ring chromosomes by the fusion between active, free chromosome ends. The ring chromosomes successively produce new ring chromosomes of different sizes by the breakage-fusion-bridge cycle. TSUNEWAKI (1959) observed this cycle in Agropyron glaucum, a related species of wheat. TSUJIMOTO & TSUNEWAKI (1985) found ring chromosomes in the offspring of the gametocidal-gene carriers of common wheat, which also changed their size during successive root-tip mitoses. Behavior of the present ring chromosome found in the offspring of mono-4B differed from those reported in the above cases. Although the present ring chromosome formed bridges in mitotic anaphase and was broken, the resulted rod-shaped chromosomes seemed not to produce new ring chromosomes by the fusion of broken chromosome ends. Occurrence of plants or cells carrying only the rod-shaped chromosomes in the offspring of the ringhromosome carrier supports it. The facts that the broken chromosome ends do not fuse and that no aberrant chromosomes are produced may indicate that the present ring chromosome has a point at which the chromosome breaks preferentially and the resulting free ends heal before fusion takes place. Awnedness of this line shows deletion of a chromosome segment carrying the Hd gene that locates very close to the centromere (7.7+ or -3.7%, RAO 1981). Thus, the present ring chromosome seems to lack most part of the short arm of chromosome 4B. In fact, the ring- chromosome carrier and monosomic plants derived from it had a morphology similar to nulli-4B. Acknowledgement The author thanks Dr. K. TSUNEWAKI, Professor, the Laboratory of Genetics, Faculty of Agriculture, Kyoto University for his critical reading of and valuable advice to the manuscript. References FRANKEL, O.T. 1949. A self-propagating structural change in Triticum. Heredity 3 : 163-194. MCCLINTOCK, B. 1941. The association of mutants with homozygous deficiencies in Zea mays. Genetics 26 : 542-571. RAO, M.V.P. 1981. Telocentric mapping of the awn inhibitor gene Hd on chromosome 4B of common wheat. Cer. Res. Comm. 9 : 335-337. SEARS, E.R. 1954. The aneuploids of common wheat. Mo. Agr. Exp. Sta. Res. Bull. 572. pp. 58. TSUJIMOTO, H. & K. TSUNEWAKI. 1985. Hybrid dysgenesis in common wheat caused by gametocidal genes. Jpn. J. Genet. 60 : 568-578. TSUNEWAKI, K. 1959. A ring chromosome in an F1 hybrid between wheat and Agropyron. Can. J. Bot. 37 : 1271-1276. |
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