Two primary trisomic series were employed for associating linkage groups of rice with respective chromosomes. Khush et al. (1984) used primary trisomics of an Indica variety IR36 and associated the linkage groups with 12 chromosomes. The extra chromosomes of the IR36 trisomics were identified at pachytene stage of meiosis. Iwata and Omura (1984) associated the linkage groups with respective chromosomes using the primary trisomics of a Japonica variety Nipponbare. The extra chromosomes of the Nipponbare trisomics were identified at mitotic prophase. When the results of the two studies were compared, linkage group-chromosome associations did not agree in all cases.
Meanwhile, on the basis of discussions among geneticists in Japan, IRRI and elsewhere, it was decided to hold an International Rice Genetics Symposium, which was held at IRRI in May 1985. Problems related to gene symbolization and chromosome numbering system in rice were discussed during the symposium. An interim committee chaired by Professor C. M. Rick and with Sir Ralph Riley, N. lwata, G. S. Khush, T. Kinoshita, N. Kurata, H. I. Oka, R. Seetharaman and H. K. Wu as members was established to examine the chromosome numbering system in rice. The Committee recommended that chromosome numbering system based on the length at pachytene stage of meiosis as proposed by Shastry et al. (1960) should be followed. This recommendation was accepted by the general body.
Discussions among participants of the Symposium also led to the birth of Rice Genetics Cooperative for promoting international collaboration in rice genetics. Rice Genetics Cooperative gave a top priority to resolve the discrepancies in the chromosome-linkage group associations determined by Khush et al. (1984) and Iwata and Omura (1984). It was decided that the IR36 trisomics should be reexamined to see if any error was made in the identification of extra chromosomes of some trisomics. Consequently , Dr. R. J. Singh representing the group of Dr. G.S. Khush and Dr. N. Kurata representing the group of Dr. Iwata spend three weeks at IRRI in June 1986 and examined the IR36 trisomics together. They confirmed the earlier identification that trisomics 1, 2, 5, 7, 9 and 10 have chromosomes 1, 2, 5, 7, 9 and 10, respectively. However, no agreement was reached regarding the identification of extra chromosomes of the remaining 6 trisomics due to the paucity of time. Dr. N. Kurata continued the examination of the 6 trisomics in Japan and Dr. H. K. Wu of Taiwan was also asked to examine these trisomics.
During the meetings of the Coordinating Committee of Rice Genetics Coope- rative held at IRRI on October 22-24, 1987 and attended by Drs. Y. Futsuhara, G. S. Khush, T. Kinoshita, T. Matsuo, H. I. Oka, T. Omura, D. Senadhira, H. K. Wu and T. Tsuchiya (invitee), new results were reviewed and it was agreed that the extra chromosome of trisomic 4 is number 3 and that of trisomic 8 is number 8. It was agreed to continue the examination of the remaining four trisomics (3, 6, 11 and 12). The results of investigations were reported by Kurata (1988) as well as by Wu and Chung (1988) which were based on observations of a large number of pachytene nuclei. Dr. G. S. Khush and his colleagues also continued the reexamination of IR36 trisomics.
A meeting to discuss this problem was held on August 26, 1989 at Tsukuba Japan, attended by Drs. Y. Futsuhara, G. S. Khush, T. Kinoshita, N. Kurata, H. I. Oka, T. Takeda and H. K. Wu. Although no new agreement was reached, it was agreed to continue effort to seek for a unified system of chromosome numbering. Dr. H. K. Wu reported his observations as shown by Chung and Wu (1988) that some plants of a part of IR36 trisomics have segmental interchanges involv-
Table 1.An agreed system of numbering rice chromosomes and their
relationships with linkage group.
________________________________________________________________
Trisomics Karyotype Trans-
_____________ ________________
Chromosome Khush Iwata Kurata Wu & amp; locations Linkage
Number et al. & amp; Omura & amp; Omura Chung Nishimura group
(1984) (1984) (1978) (1988) (1961)
________________________________________________________________
1 1 O K1 1 3 III
2 2 N K2 3 8 X
3 4 M K3 2 5 XI +XII
4 12 E K4 4 11 II
5 5 L K9 5 2 VI+IX
6 3 B K6 6 6 I
7 7 F K11 7(11) 10 IV
8 8 D K7 9 12 sug
9 9 H K10 8 1 V+VII
10 10 C K12 10 7 fgl
11 11 G K8 11(7) 9 VIII
12 6 A K5 12 4 d-33
________________________________________________________________
ing extra chromosomes. But this was not adopted by the group as there was no
supporting genetic evidence. All the available data were reviewed by Drs. N.
Iwata, G. S. Khush and T. Kinoshita again during Dr. Khush's visit to
Kurashiki City, Japan in January 1990. A tentative agreement was reached
regarding the identification of extra chromosomes of the remaining four
trisomics, taking the reports by Kurata (1988) and Wu and Chung (1988) into
consideration. Thus, it was agreed that the extra chromosomes of trisomics 3,
6, 11 and 12 are 6, 12, 11 and 4, respectively (Table 1).
This agreement was accepted during the meeting of ad hoc committee held on May 15, 1990 at IRRI, which was attended by Drs. Y. Futsuhara, N. Iwata, G. S. Khush, T. Kinoshita, H. I. Oka, K. Takeda, H. K. Wu, M. Yano, and A. Yoshimura. It was reported to the concluding session of the 2nd International Rice Genetics Symposium on May 18, 1990 and was approved by the general body. From now on, the linkage group corresponding to chromosome 1 will be designated as linkage group 1, and that corresponding to chromosome 2 as linkage group 2, and so on.
References
Chung, M. C. and H. K. Wu, 1988. Structural changes involving extra chromosomes in IR36 Triplo series and their identification. RGN 5: 47-54.
Iwata, N. and T. Omura, 1984. Studies on the trisomics in rice plants, VI. An accomplishment of a trisomic series in japonica rice plants. Jpn. J. Genet. 59(3): 199-204.
Khush, G. S., R. J. Singh, S. C. Sur and A. L. Librojo, 1984. Primary trisomics of rice: Origin, morphology, cytology and use in linkage mapping. Genetics 107: 141-163.
Kurata, N., 1988. Identification of trivalent chromosomes of IR36 triplo plants. RGN 5: 54-59.
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.
Nishimura, Y., 1961. Studies on the reciprocal translocations in rice and barley. Bull. Natl. Inst. Agr. Sci. D9: 171-235.
Shastry, S. V. S., D. R. Ranga Rao and R. N. Misra, 1960. Pachytene analysis in Oryza, I. Chromosome morphology in Oryza sativa. Ind. J. Genet. Plant Breed. 20: 15-21.
Wu, H. K. and M. C. Chung, 1988. Identification of extra chromosomes of six IR36 triplo lines. RGN 5: 41-46.
G. S. Khush
(in consultation with Y. Futsuhara, N. Iwata, T. Kinoshita and H. I. Oka)