College of Agriculture, University of the Ryukyus, Senbaru, Nishihara, Nakagami-gun, Okinawa, 903-01 japan
Since twelve linkage groups corresponding to the haploid chromosome
number of rice were proposed by Nagao and Takahashi (1963), the relationships
between linkage groups and chromosomes have been examined by the use of
segmental interchanges and primary trisomics (Iwata and Omura 1971a,b, 1975,
1967; Sato 1976, Sato et al. 1982). Interchange homozygotes used in these
experiments were all determined cytologically and the chromosome numbering
system proposed by Nishimura (1961) was followed. The results showed that
three linkage groups, VI, IX and XII were associated with the 2nd
choromosome, two groups, V and VII, were assigned to the 1st chromosome, and
that the remaining seven linkage groups corresponded to seven other
chromosomes, respectively. Accordingly, three linkage groups remained to be
established. A new linkage group corresponding to the 7th chromosome was
recently identified (Sato and Shinjo, in preparation). The remaining two
groups will be established in near future. Several genes have been found to
be located on the 4th and 12th chromosomes, to which no linkage group had
been assigned (Yoshimura et al. 1982).
The construction of cytological maps is a problem left for studies in the future. The point of interchange is a useful cytological market. To determine the point of interchange and the position of centromere, Sato et al.(1980) observed pachytene chromosomes of hybrids between two reciprocal translocation homozygotes with different interchanged segments on the same chromosome, which show a configuration of six paired chromosomes, in comparison with those of their simplex heterozygotes. By examining the ratios of interchanged segments, the breakage points could be located on respectively chromosomes. The centromeres were represented by a small block of heterochromatin in the pachytene chromosomes, although it was not easy to determine the position when two or more heterochromatin blocks occurred on a single chromosome. Yet, a study of configurations of six paired chromosomes could bring about a provisional determination of the positions of centromeres. Thus, the cytological map for nine chromosomes indicating the point of interchange, centromere and several marker genes were constructed (see Fig. 2, (B) Current Linkage Map, p.19).
References
Iwata, N. and T. Omura, 1971a. Linkage analysis by reciprocal translocation method in rice plants (Oryza sativa L.), I. Linkage groups corresponding to the chromosome 1, 2, 3 and 4. Jpn. J. Breed. 21: 19-28 (Jap./Eng.).
----- and ----- 1971b. Ditto, II. Linkage groups corresponding to the chromosomes 5, 6, 8, 9, 10 and 11. Sci. Bull. Fac. Agr., Kyushu Univ. 25: 137-153 (jap./Eng.).
----- and ----- 1976. Studies on the trisomics in rice plants (Oryza sativa L.), IV. On the possibility of association of three linkage groups with one chromosome. Jpn. J. Genetics 51: 135-137.
Nagao, S. and M. E. Takahashi, 1963. Genetical studies on rice plant, 27. Trial construction of twelve linkage groups in Japanese rice. J. Fac. Agr., Hokkaido Univ. 53: 72-130.
Sato, S., 1976. Linkage analysis of rice plant by the use of reciprocal translocation lines. Bull. Coll. Agr., Univ. Ryukyus 23: 73-104 (Jap./Eng.).
-----, T. Kinoshita and M. E. Takahashi, 1980. Genetical studies on rice plant, 71. Location of centromere and interchange breakpoints in the pachytene chromosome of rice. Jpn. J. Breed. 30: 387-398.
Sato, S., K. Muraoka and Y. Sano, 1982. Reconstruction of a linkage group corresponding to Nishimura's second chromosome in rice, Oryza sativa L. Jpn. J. Breed. 32: 232-238.
Yoshimura, A., N. Iwata and T. Omura, 1982. Linkage analysis by reciprocal translocation method in rice (Oryza sativa L.), III. Marker genes located on chromosomes 2, 3, 4 and 7. Jpn. J. Breed. 32: 323-332.