10. Production and identification of acrotrisomics in rice

Plant Breeding Laboratory, Faculty of Agriculture, Kyushu University, Fukuoka, 812 Japan

Acrotrisomics, a cytogenetical variant with an acrocentric chromosome in addition to normal chromosome complement, is one of the efficient tools for chromosome mapping. In rice, there was no report on acrotrisomics. In this paper, we isolated rice acrotrisomics as a mapping tool through the pollination of irradiated pollens to primary trisomics.

Two types of Japonica primary trisomics, Triplo 4 with liguleless gene (lg) as nulliplex and Triplo 7 with long empty glumes gene (g-1) as nulliplex, were pollinated with irradiated pollens of disomic plants. In the F1 progenies, acrotrisomics like plants were selected based on the phenomenon of pseudodominance and morphological feature. Among the selected plants, acrotrisomics was identified by microscopic observation of somatic chromosomes and RFLP gene dosage analysis of acrotrisomic F1 hybrids crossed with an Indica variety IR24. Six RFLP markers mapped on chromosomes 4 and 7 (Saito et al. 1991) were used for Southern hybridization.

Among 353 F1 plants obtained from pollination with irradiated pollen to liguleless Triplo 4, we selected 15 plants showing pseudodominance and further analyzed one late heading plant. The extra chromosome of the plant lacked less stained distal segment of long arm of chromosome 4 (Fig. 1). RFLP gene dosage effects were observed at the locus of XNpb203 but not at the loci of XNpb114 and 120 on chromosome 4 (Fig. 2). These results suggested that the breakpoint of the acrocentric chromosome were between XNpb203 and 114 and the chromosome was deficient in the loci of XNpb114, XNpb120 and lg of long arm of chromosome 4. We designate this acrotrisomics as Acro4 The present results and linkage informations (Ideta et al. 1992) make it possible to determine the orientation of linkage groups. From short arm towards long arm on chromosome 4, the arrange-

Fig. 1. Somatic chromosomes of Acro4S^4L at prometaphase. An arrow indicates acrocentric chromosome.

Fig. 2. RFLP gene dosage analysis of Acro4S^L4 by using the acrotrisomic F1 hybrids and RFLP clones mapped on chromosome 4. In autoradiogram, P\1\: Japonica, P\2\: Indica, D: disomic F1, PT: primary trisomic F1, AT: acrotrisomic F1. Black bar represents the extra chromosome segment and hatched bar indicates inferred region of the acrocentric chromosome.

Fig. 3. RFLP gene dosage analysis of Acro7 by using the acrotrisomic F1 hybrids and RFLP clones mapped on chromosome 7. In autoradiogram, P\1\: Japonica, P\2\: Indica, D: disomic F1, AT: acrotrisomic F1. Black bar represents the extra chromosome segment and hatched bar indicates inferred region of breakpoint of the acrocentric chromosome.

ment of conventional and RFLP markers are XNpb203-XNpb49-d-11-XNpb114- XNpb120-lg-Pl-d-2.

Among 276 F1 plants obtained from pollinating irradiated pollens to Triplo 7 with long empty glumes, we selected and analyzed one plant which did not show pseudodominance but had round grain and high sterility. RFLP gene dosage effects were detected at the locus of XNpb338 but not at the loci of XNpb20 and 22 on chromosome 7 (Fig. 3). These results suggested that the breakpoint of the acrocentric chromosome were between XNpb338 and 20 and the chromosome possessed the loci of gel to XNpb338 but not the loci of XNpb20 to 22 of chromosome 7. Since the position of breakpoint has not been determined, we tentatively designate this acrotrisomics as Acro7. Detailed microscopic observation of the acrocentric chromosome is under way.

Ideta, O., A. Yoshimura, T. Matsumoto, H. Tsunematsu and N, Iwata, 1992. Integration of conventional and RFLP linkage maps in rice. I. On chromosomes 1, 2, 3 and 4. Rice Genetics Newsletter 9 (in press).

Saito, A., M. Yano, N. Kishimoto, M. Nakagahra, A. Yoshimura, K. Saito, S. Kuhara, Y. Ukai, M. Kawase, T. Nagamine, S. Yoshimura, O. Ideta, R. Ohsawa, Y. Hayano, N. Iwata and M. Sugiura, 1991. Linkage map of restriction fragment length polymorphism loci in rice. Jpn. J. Breed. 41: 665-670.