9. A trial to induce chromosome deficiencies and monosomics in rice by using irradiated pollen

Faculty of Agriculture, Kyushu University, Hakozaki, Fukuoka, 812 Japan

We are engaged in an experiment to obtain segmental deficiencies in specific chromosome regions by means of the "pseudo-dominant technique", i.e., stocks carrying one or more recessive marker genes are pollinated with irradiated pollen of a stock carrying normal alleles at the corresponding loci. The marker lines with two or more recessive genes we have maintained were used as the female parent. Taichung 65 was used as the pollen parent, and one day before pollination the Taichung 65 plants were exposed to gamma rays at a 5KR or 10KR dosage. The F\1\ plants were raised and examined. The "pseudo-dominant" plants, expressing a given trait of the parental marker line, were selected for cytological examination. Some of the weak plants and morphological variants were also retained for study.

In the 1985-1987 experiment, about 10,000 F\1\ seeds were sown and a lot of chromosomal variants, morphological variants and "pseudo-dominant" plants were obtained and maintained vegetatively (Table 1). Monosomic plants with 2n=23, plants with 2n=23+1 fragment, and some "pseudo-dominant" plants were studied cytologically. In Fig. 1 A, B and C are shown the root-tip chromosomes of a "pseudo-dominant" plant (A), a plant with 2n=23+1 fragment (B), and of a monosomic plant (C), respectively.

These chromosomal variants would be useful in cytological and genetical studies. Our plan is to raise a large F\1\ population to select more chromosomal deficiencies and to accomplish a monosomic series. The use of irradiated pollen is an effective method for obtaining such plants.

We are also engaged in irradiation of trisomic plants to induce monotelo- trisomics. Plants with 24 normal and one telosomic chromosome, which are similar to the parental trisomics morphologically, have been selected.

Table 1. The induction and morphological features of monosomics and plants with a segmental deficiency, obtained from the F\1\ of marker lines pollinated with gamma-rayed Taichung 65 pollen.

Fig. 1. Root tip cell chromosomes of variants induced by gamma irradiation of pollen.

A: Chromosomes of a 1a-pseudo-dominant plant. Arrow shows an acrocentric chromosome.

B: Chromosomes of a plant with 2n=23+1 fragment. Arrow shows the fragment chromosome.

C: Chromosomes of a monosomic plant.