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School of Medicine, Fujita-Gakuen Health University, Toyoake, Aichi-ken, 470-11 Japan
To make clear the relationship between chromosomes and genetic linkage groups
of rice, Dr. R.J. Singh and I have worked together at the IRRI for about three
weeks in June 1986. We observed pachytene chromosomes to identify the trivalent
chromosomes of IR36 Triplo series, which were numbered according to their
pachytene length by Khush et al. (1984). The numbers did not agree well with
those for the trisomic series of Nipponbare established in Japan (Kurata et al.
1981; Kurata 1984; Iwata et al. 1984), when compared on the basis of
association with linkage groups. The general aim of our work was to resolve the
discrepancies between the two series.
To avoid confusion, we adopted the numbering of pachytene chromosomes given by Shastry et al. (1960). We have agreed in recognizing that the extra chromosomes of Triplos 1,2,5,7,9 and 10 are chromosomes 1,2,5,7,9 and 10, respectively. With regard to the remaining six Triplos, Dr. Singh insisted that his previous identification was correct, but my identification differed from his and time was not enough to reach a common conclusion. In this note, I will report my observations on the six Triplo lines which remain unsettled.
Pachytene chromosomes are each characterized morphologically, but their characteristics are not distinct enough to enable an observer to identify a single chromosome separately from others. For identifying a given chromosome, it is necessary to determine not only its characteristics but also those of all others so as to identify all 12 chromosomes one by one.
The relative length and arm ratio of pachytene chromosomes measured in IR36 and its six Triplo lines are given in Tables 1 and 2, respectively. For the two nucleolar chromosomes, 9 and 10, reliable measurements of arm ratio were not obtainable. All the chromosomes showed almost the same characteristics as those of cultivar Nipponbare observed previously (Kurata et al. 1981; Kurata 1984), except that IR36 had two nucleolar chromosomes. Each chromosome was identifiable by these characteristics. In Fig. 1-5, the chromosomes are each numbered and the position of centromere is shown by an arrow. Trivalent chromosomes are shown by a larger arrowhead and their univalent portion by broken lines with small arrowheads. My observations of the six Triplos are described as follows:
Triplo 3. The trivalent was of medium length (Fig. 2), and its relative length
and arm ratio were close to those of chromosome 6 (Tables 1 and 2). All
bivalents chromosomes were also identified by comparing them with those of
IR36. The trivalent was then identified to be chromosome 6.
Triplo 4. In a nucleus observed, all 12 chromosomes were separable as shown in Fig. 1, and were identified by their relative length, arm ratio and other features. The trivalent of this line was identified as chromosome 3; chromosome 4 characterized by its subtelocentric feature was a bivalent.
Triplo 6. The trivalent was a short chromosome having a heavily stained short arm (Fig. 4). After the identification of all other bivalents, it was considered to be chromosome 11.
Triplo 8. The trivalent was a short metacentric chromosome and appeared to be chromosome 12 (Fig. 3).
Triplo 11. The trivalent was a relatively short and totally heavily stained metacentric chromosome. It may be considered as chromosome 8.
Triplo 12. The trivalent was a relatively long subtelocentric chromosome showing the characteristics of chromosome 4. This was observed in several nuclei in which individual chromosomes were recognizable clearly (Fig. 5). In those nuclei, other bivalent chromosomes were also identified one by one. Their relative length and arm ratio showed good agreement with thos of IR36 (Tables 1 and 2).
According to Khush et al. (1984), in crosses with gene-marker lines, Triplo 3 showed a trisomic ratio for gene Cl (clustered spikelets) belonging to linkage group I, which is known to be associated with chromosome K6 of the Nipponbare trisomic series (Iwata et al. 1984). Similarly, Triplo 4 showed trisomic ratios for genes ch\1\ (chlorina-1) and dl(drooping leaf) of linkage group XI, which is associated with chromosome K3 of the Nipponbare series. Triplo 12 showed a trisomic ratio for gene lg (liguleless) of linkage group II, which is associated with chromosome K4 of the Nipponbare series (Table 3). The karyotypes of Nipponbare and IR36 are quite similar and there is no evidence for their significant difference. Therefore, the result of these genetic experiments may serve as a supporting evidence for the cytological identification that the extra chromosomes of Triplos 3,4 and 12 are chromosomes 6,3 and 4, respectively.
On the other hand, when a trivalent chromosome was identified without complete identification of all other bivalents in the nucleus, and when the judgement was not reconfirmed by repeated observations in a number of nuclei, the result could not be considered very reliable. For this reason, even though Dr. Singh and I have agreed, re-examination may be suggested as to Triplos 5 and 7. Also the distinction between relatively small chromosomes link 7,8 11 and 12 is not always concrete, although chromosomes 9 and 10 attached to the nucleolus cannot be mistaken. Accordingly, my identification as to Triplos 6,8 and 11 reported in this note may be taken as tentative.
Table 1. Relative length of pachytene chromosomes obtained in IR36 and its
six Triplo lines (in %, mean for 3 or more measurements)
Table 2. Arm ratio (long/short) of pachytene chromosomes obtained in
IR36 and its six Triplo lines (not measurable precisely in nucleolar chromosomes, nos. 9 and 10)
Fig. 1. Individual pachytene chromosomes observed in Triplo 4. Chromosome 3
forms a trivalent.
Fig. 2. Chromosome 6 forming a trivalent in Triplo 3
Fig. 3. Chromosome 12 forming a trivalent in Triplo 8.
Fig. 4. Chromosome 11 forming a trivalent in Triplo 6.
Fig. 5. Chromosome 4 forming a trivalent in Triplo 12.
Table 3. Extra chromosomes identified in respective Triplo lines
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Triplo no. Extra chromosome Corresponding Nipponbare Corresponding
no. series b linkage group
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1 1a K1 III
2 2a : 3c K2 X
3 6 K6 I
4 3 : 2c K3 XI, XII
5 5a K9 VI, IX
6 11 K5 d-33
7 7a K11 IV
8 12 K7 su
9 9a K10 V, VII
10 10a K12 fgl
11 8 K8 VIII
12 4 K4 II
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a Agreement with Dr. R.J. Singh
b Kurata et al. 1981
c Change proposed by Oka and Wu (this issue)
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Iwata, N., H. Satoh and T. Omura, 1984. The relationships between chromosomes identified cytologically and linkage groups. RGN1: 128-130.
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., 1984. "Analysis of chromosomes of rice, with special reference to the identification of extra chromosomes of trisomics and chromosomal differentiation among genomes". Recent Advances in Breeding Research 25: 93-97. (in Japanese)
Kurata, N. and T. Omura, 1987. Karyotype analysis in rice, 1. A new method for identifying all chromosome pairs. Jpn. J. Genet. 53: 251-255.
Kurata, N., N. Iwata and T. Omura, 1981. Karyotype analysis in rice, II. Identification of extra chromosomes in trisomic plants and banding structure on some chromosomes. Jpn. J. Genet. 56: 41-50.
