Fig. 1. Regional relationship of RFLP linkage groups (Saito et al- 1991) on pachytene chromosomes 1, 3, 4, 9 and 12. Dotted bars on RFLP linkage groups indicate the inferred ranges of centromere positions.
Faculty of Agriculture, Kyushu University, Fukuoka 812, Japan
Linkage studies of rice have developed rapidly, particularly by the use of restriction fragment length polymorphism (RFLP). The completion of rice linkage map opens new approaches in the field of rice genetics and breeding. However, the orientation of the linkage groups on chromosomes, the arm location of the marker genes, and the position of centromeres remain unclear. We report here the integration of the RFLP linkage maps with pachytene chromosomes.
Fig. 1. Regional relationship of RFLP linkage groups (Saito et al- 1991) on
pachytene chromosomes 1, 3, 4, 9 and 12. Dotted bars on RFLP linkage groups
indicate the inferred ranges of centromere positions.
Peciprocal translocations were used for cytogenetically locating the interchange breakpoint as the landmark on pachytene chromosomes. Cross-shaped quadrivalent was observed in the reciprocal translocation heterozygote at the pachytene stage, in which the center of the cross shape represents the breakpoint. Acetic carmine smear method described by Wu (1967) was employed in this analysis. Tertiary trisomics were used to locate the breakpoints on RFLP linkage groups. Up to the present time, 25 strains of Japonica tertiary trisomics have been produced by three-way cross: primary trisomics/ reciprocal translocations// normal disomics (Nonomura et al. 1994). Japonica-Indica hybrid tertiary trisomics were developed from them, and analyzed for RFLP gene dosage described by Young et al. (1987). RFLP clones mapped by Saito et al. (1991) were used in this study. Since these tertiary chromosomes were derived from the interchange chromosomes of reciprocal translocations, it was easy to compile the positional information of the breakpoints on RFLP linkage map and pachytene chromosomes.
The orientation of linkage groups, the arm location of marker genes, and the inferred ranges of centromeres were decided on chromosomes 1, 3, 4, 9 and 12 (Fig. 1). Although only one breakpoint was located on chromosome 3, the orientation of the linkage group could be determined based on the RFLP gene dosage analysis. The Japonica-Indica hybrid of TT102 (3-12), in which an extra chromosome derived from RT102, showed the dosage effect at the RFLP locus of XNpb 348, but not at XNpb 129, 234, 51 and 249 on chromosome 3 (data not shown). Thus, the breakpoint of RT 102 (3-12) was considered to exist within the range from XNpb 129 to 348. On the other hand, this hybrid exhibited gene dosage effect at XNpb 316, 124-1 and 193 but not at XNpb 261, 336 and 148 on chromosome 12 (data not shown). The tertiary chromosome of TY102 is composed of an entire short arm, a centromere and an interstitial segment of the long arm of chromosome 12, and a distal end of the long arm of chromosome 3, so that the segment from XNpb 348 to 173 corresponds to the end of long arm of chromosome 3. Two breakpoints of RT 2(1-9a) and RT 4(9-12) were arranged in reverse order between the linkage group and pachytene chromosome of chromosome 9. This discrepancy remains unsolved.
References
Nonomura, K. I., A. Yoshimura, T. Kawasaki and N. Iwata, 1994. Production of tertiary trisomics in rice (Oryza sativa L.). Breeding Science 44: 137-142.
Saito, A., M. Yano, N. Kishimoto, M. Nakagahra, A. Yoshimura, K. Saito, S. Kuhara, Y. Ukai, M. Kawase,T. Nagamine, S. Yoshimura, 0. Ideta, R. Ohsawa, Y. Hayano, N. Iwata and M. Sugiura, 1991. Linkage map of restriction fragment length polymorphism loci in rice, Japan. J. Breed. 41: 665-670.
Wu, H. K., 1967. Note on preparing of pachytene chromosomes by double mordant. Sci. Agr. (Taiwan) 15: 40-44. (in Chinese with English summary)
Young, N. D., J. C. Miller, and S. D. Tanksley, 1987. Rapid chromosomal assignment of multiple genomic clones in tomato using primary trisomics. Nucl. Acids Res. 15: 9339-9348.