| A novel behavior of a ring chromosome in common wheat
H. TSUJIMOTO Kihara Institute for Biological Research, Yokohama City University, Mutsukawa 3-122-20, Minami-ku, Yokohama 233, Japan The ring chromosome produced by the free end fusion following the chromosome breakages is reported in several eukaryotes, e.g., human, Drosophila and maize. In wheat, FRANKEL (1949) first reported a ring chromosome and proposed a mechanism of its production. The very unique behavior of a ring chromosome in somatic division was called the breakage-fusion-bridge cycle by MCCLINTOCK (1941). She observed in maize, chromosorne bridges formed by a ring chromosome in anaphase that produced two ring chromosomes of different sizes after breakage of the bridges and reunion of the free ends. The author found a plant having a ring chromosome in the self-pollinated progeny of a monosomic plant of common wheat. The behavior of this ring chromosome, different from that reported by MCCLINTOCK (1941), is described in this paper. Materials and Methods Monosomic series of a common wheat, Triticum aestivum (L.) Thell. cv. Chinese Spring produced by SEARS (1954) was provided by Dr. K. NISHIKAWA, Gifu University, Japan. A plant having the ring chromosome was found in the selfed progeny of mono-4B. Root tips of this plant and its progeny, which were pretreated in cold water (0C) for 24 hrs, were fixed in 1 : 3 acetic alcohol. Acetocarmine squash method was applied for the cytological observation of their mitosis. Results In the routine work of chromosome checking for maintenance of monosomic series, a plant carrying 40 meta-or submetacentric and one ring chromosome, i.e, 2n =40+1ring, was obtained in a self-pollinated progeny of mono-4B (Fig. 1A). Morphology of this plant was similar to that of nulli-4B, by having short and slender culms with narrow leaves, and small and compact spikes with short awns due to deficiency of the Hd gene on the chromosome 4B. The plant was semi-sterile. Karyotypes of ten plants in the self-pollinated progeny of this plant were observed. One, six and two plants carried 42, 41 and 40 meta- or submetacentric chromosomes, respectively. Only one plant had ring chromosomes in some cells. Two root tips of this plant were available for observation. In one root tip, 16 cells were observed, which had only 41 rod chromosomes in total. In the other root tip, 21 cells were observed, 17 of which carried one ring and 40 rod-shaped chromosomes. The remaining four cells had no ring but 41 rod chromosomes. No size difference was found among the ring chromosomes. The rod chromosomes did not show any visible aberration, and no telosomes or dicentric chromosomes were observed. Among 16 anaphase or telophase cells observed, 14 showed 1 : 1 separation of the equal-sized ring chromosomes, whereas two had chromosome bridges. Fig. 1B and C show two anaphase cells in which a double-sized ring chromosome formed bridges ; one of them was elongated O-shaped and the other 8-shaped. A single plant carrying 42 rod-shaped chromosomes derived from the original ringchromosome carrier died before heading. Its sib plants having 41 rod-shaped chromosomes were similar to nulli-4B in morphology, like the parental ring-chromosome carrier. |
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