13. A terminal deficiency in rice chromosome II

        Zi-Xuan Wang1, Yasunori Sukekiyo2, Atsushi Yoshimura and Nobuo Iwata
        Faculty of Agriculture, Kyushu University, Fukuoka 812, Japan
        1) Present address: Rice Genome Research Program, STAFF Institute, 446-1
            Ippaizuka, Kamiyokoba, Tsukuba, lbaraki 305, Japan
        2) Present address: Plantech Research Institute, c/o MC-Research,
            1000-Kamoshita-cho, Midori-ku, Yokohama 227, Japan

     Many chromosome aberrants, including segmental deficiencies have been obtained in rice by using irradiated pollen (Wang et al. 1988, 1991). We report here a terminal-deficiency of rice chromosome 11.
    An la pseudo-dominant plant was found amongst the 245 F1 plants derived from a cross between a stock homologous for recessive genes lg (chromosome 4), st-I (formerly ws-I, chromosome 6) and la (chromosome II) and irradiated Taichung 65. The plant showed normal phenotype for lg and st-I, but mutant phenotype for la, which was pseudo-dominant. The plant showed lazy growth habit as the expression of the recessive gene la, had otherwise normal plant morphology and had a seed-fertility of 40%.
 
  Fig. 1. Somatic and pachytene chromosomes of an la pseudo-dominant plant, a: somatic chromosomes, arrow shows a fragment chromosome, b: pachytene chromosomes, arrow shows the centromere of the chromosome, which gives single strand chromosome in terminal part of the long arm.     Somatic chromosomes of the plant were investigated using the root tip cells. There were 23 chromosomes and 1 fragment chromosome (Fig. la). In order to know the exact length of the missing chromosome segment, we carried out pachytene analysis. It was found that about 75% of the long arm of chromosome was missing (Fig. 1 b). Thus, this is a terminal chromosome deficiency. Because the plant showed pseudo-dominant phenotype for la which is known to be on chromosome 11, the missing chromosome segment must include the region of la. Therefore, the deficiency could be identified genetically to be in rice chromosome 11.
    Chromosome behavior in meiosis was also examined extensively using a large number of pollen mother cells (PMCs). More than 300 PMCs were examined. The results showed that only chromosome configuration of 12 II was observed in metaphase I (MI), chromosome separations were normal in anaphase I (AI) and anaphase II (AII), and no other abnormal chromosome configurations, such as chromosome loop and chromosome laggards were observed. The normal chromosome separation in meiosis in the plant means that two kinds of microspores with n=12 and n=ll+l fragment were formed. These results also suggest that the plant had no other chromosomal rearrangements. In order to test transmission of the deficiency to progenies, artificial pollination was made with the vegetatively-propagated la pseudo-dominant plants as female parent. The selfed seeds of the pseudo-dominant plants were also collected. A total of 404 f1 seeds and more than three thousand selfed seeds from the pseudo-dominant plant were obtained during three years (1988, 1989 and 1990). However, all plants from the F1 seeds, as well as from the selfed seeds, were normal in plant morphology and seed fertility. These results indicate that the deficiency was not transmitted to progenies either through pollen or the egg. Induced deficiencies have been successfully utilized for centromere mapping and determining the orientation of the linkage maps in many species (Singh 1993). Similarly, the terminal deficiency reported here will be useful material for similar studies in rice. We are now attempting to determine the orientation of the RFLP linkage map of the chromosome 11 by using this terminal deficiency.
    We thank Dr. N. Kurata, Rice Genome Research Program, NIAR, Japan for her critical viewing of the cytological data.

References

Singh, R. J., 1993. Plant Cytogenetics. CRC Press, Boca Roton Ann Arbor, London and Tokyo.
Wang, Z. X., N. Iwata, Y. Sukekiyo, A. Yoshimura and T. Omura, 1988. A trial to induce chromosome
        deficiencies and monosomics in rice by using irradiated pollen. RGN 5: 64-65.
Wang, Z. X., N. Iwata, Y. Sukekiyo and A. Yoshimura, 1991. Induction of chromosome aberrants in rice
        (Oryza sativa L.) by using irradiated pollen. J. Fac. Agr. Kyushu Univ. 36: 99-108.