Fig. 1. Somatic and pachytene chromsomes of a lg pseudo-dominant plant. a: somatic chromosomes, arrow shows chromosome 4 and arrowhead shows a fragment chromosome. b: pachytene chromosomes. Arrowhead shows a fragment chromosome.
Faculty of Agriculture, Kyushu University, Fukuoka, 812 Japan
1) Present address: Rice Genome Research Program, Society for Techno-innovation of Agriculture, Forestry and Fisheries (STAFF) Institute, 446-1, Ippaizuka, Kamiyokoba, Tsukuba, 305 Japan
2) Plantech Research Institute, c/o MC-Research, 1000 Kamoshita-cho, Midori-ku, Yokohama, 227 Japan
Induced deficiencies have been successfully utilized for centromere mapping and linkage-map orientation in different organisms (Singh 1993). We have conducted an experiment to induce segmental deficiencies in rice by using irradiated pollen (Wang et al. 1988, 1991). A deficiency induced in chromosome 4 is reported in this paper. In 1988, we found an lg pseudo-dominant plant among 47 F`1` plants from a cross between irradiated Taichung 65 and a homologous strain carrying lg (chromosome 4), wx (chromosome 6) and la (chromosome 11). The pseudo-dominant F`1` plant showed normal phenotypes for gene la and wx and the mutant phenotype for lg. It was liguleless, and showed dark green leaves, imperfectly emerged panicles and sterility.
The F`1` plant was investigated for somatic chromosome in the root-tip cells. There were 23 ordinary and one fragment chromosome (Fig. 1a). One of chromosome 4, easily detected for its subtelocentric feature (Kurata and Omura 1978), was missing. In view of dark staining of the fragment chromosome, it seemed to be derived from the short arm or from the centromeric heterochromatin of the long arm of chromosome 4. This suggests that gene lg is located on the long arm of chromosomes 4. Ikeda et al. (1992) have also located lg on the long arm of chromosome 4 by using an acrotrisomic strain.
Pachytene chromosomes of the pseudo-dominant plant were studied with a large number of PMCs to determine the length of missing part more precisely. In every pachytene nucleus, a small piece of darkly stained fragment was observed, which was a univalent chromosome (Fig. 1b). This also indicates that the fragment corresponds to either a proximal half of the short arm or the centromeric heterochromatin of the long arm of chromosome 4. The fragment chromosome never paired with the normal one, giving rise to the configuration of 11 II+2I even in the pachytene nuclei (Fig. 1b). This would result from an early disjunction because of the small size of the fragment.
To examine transmission of the deficiency to progeny, pollination was made using the pseudo-dominant plant as the female parent. A total of 50 F`1` seeds were obtained. However, all F`1` plants from the seeds were normal in morphology and seed fertility, indicating that the deficiency was not transmitted through the megaspore to progenies. Apparently, it cannot be transmitted through pollen grains.
Fig. 1. Somatic and pachytene chromsomes of a lg pseudo-dominant plant.
a: somatic chromosomes, arrow shows chromosome 4 and arrowhead shows a
fragment chromosome. b: pachytene chromosomes. Arrowhead shows a fragment
chromosome.
We thank Dr. N. Kurata of Rice Genome Research Program for her critical review of cytological data and discussion.
References
Ikeda, K., H. Furuumi, A. Yoshimura, H. Yasui and N. Iwata, 1992. Production and identification of acrotrisomics in rice. RGN 9: 42-45.
Kurata, N., and T. Omura, 1978. Karyotype analysis in rice I. A new method for identifying all chromosome pairs. Jpn. J. Genet. 53: 251-255.
Singh, R. J., 1993. Plant Cytogenetics. CRC Press, 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.