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Institute of Botany, Academia Sinica, Nankang, Taipei, Taiwan 115, China
The new set of primary trisomics established in cultivar IR 36 (Khush et al.
1984) is proving useful for cytogenetic studies of rice. After the
International Rice Genetics Symposium held at IRRI in May 1985, we analyzed
the karyotype of IR 36 to facilitate identifying the extra chromosomes of each
trisomic. The analysis was carried out at pachytene stage of pollen-mother
cells (Wu 1967) and at mitotic prometaphase (Kurata and Omura 1978), assisted
by a digitizer and on-line computation.
Cultivar IR 36 has two nucleolar chromosomes as has been reported earlier (Khush et al. 1984). The short-arm ends of both are attached to the nucleolus (Fig. 1). When arranged according to the descending order of length, the two nucleolar chromosomes were designated as 8 and 10, repectively (Figs. 1 and 2; Table 1). Rice chromosomes can also be characterized by their arm ratios. Chromosome 2 is more metacentric than chromosome 3. The same is true in the cases of 6 and 5, 9 and 7, and 8 and 10 (Table 2).
The distribution of heterochromatin is another criterion for identifying the chromosomes. The short arm of chromosome 4 is almost entirely heterochromatic. It is somewhat lesser in the short arms of chromosomes 5 and 9 (Figs. 1 and 2). On the other hand, in larger chromosomes such as 1,2 and 3, the heterochromatin is obviously insignificant as compared with that of smaller chromosomes.
In order to determine the extent of reliability of chromosome numbering system based on length, the data of relative length (percent of total length) were subjected to statistical analysis. Duncan's new multiple range test (Steel and Torrie 1960) revealed that differences in relative length were significant among all 12 pairs except that chromosomes 8 and 9, as well as 10 and 11 were indistinguishable by length at both the pachytene and somatic prometaphase stages. At prometaphase, chromosomes 2 and 3 were also indistinguishable by length (Table 1). Comparison between the pachytene and prometaphase stages by t test showed that significant differences existed in certain chromosomes in both relative length and arm ratio.
Based on the data we obtained, it seems feasible to identify the extra chromosomes involved in the new trisomic set by examining their size, arm ratio and/or heterochromatin distribution, although some minor compromize among workers may be necessary.
Table 1. Length measured (in micron) and relative length (in percent of total length) of pachytene and somatic prometaphase chromosomes.
==============================================================================
No. Pachytene (25 cells) Prometaphase (35 cells)
--------- ------------- ---------------------------- t for
Absolute Relative Absolute Relative relative
length(u) length (%) length (u) length (%) length
-------- ---------- ---------- ----------- (df=58)
Mean SD Mean SD Mean SD Mean SD
==============================================================================
1 38.50 9.03 13.58 0.87 6.44 1.54 13.81 0.97 0.95
2 35.97 5.01 12.15 0.74 5.75 1.51 11.70 0.93 2.01*
3 32.25 4.40 10.82 0.78 5.37 1.33 11.12 0.75 1.50
4 26.20 3.36 8.80 0.54 4.28 0.80 9.16 0.45 2.86**
5 24.12 3.35 8.06 0.37 4.17 1.03 8.71 0.50 5.51**
6 22.84 2.96 7.59 0.35 3.81 1.04 7.91 0.41 3.20**
7 21.48 2.73 7.19 0.31 3.55 0.95 7.26 0.60 0.54
8n 20.34 2.59 6.82 0.37 3.12 0.76 6.69 0.32 1.46
9 19.97 2.70 6.70 0.38 3.07 0.63 6.51 0.51 1.56
10n 19.03 2.26 6.39 0.37 2.92 0.52 6.02 0.47 3.25**
11 18.51 2.43 6.21 0.37 2.65 0.56 5.79 0.49 3.56**
12 16.68 2.41 5.68 0.39 2.53 0.56 5.33 0.42 3.33**
==============================================================================
n-Nucleolar chromosome*,** Significant at 5% and 1% levels, respectively.
Vertical bar shows that the difference in relative length is not significant (at 1% level) after Duncan's new multiple range test.
Table 2. Arm ratios observed in pachytene and somatic prometaphase chromosome
numbered according to the descending order of length.
==============================================================================
No. Pachytene (19-22 cells)a Prometaphase (16-22 cells)a
---------------------------- ------------------------------ t(df=33)a
Mean SD Mean SD
==============================================================================
1 1.79 0.28 1.81 0.26 0.23
2 1.17 0.15 1.28 0.18 2.00
3 1.93 0.49 1.88 0.28 0.36
4 2.93 0.60 3.72 0.46 4.29**
5 1.57 0.36 1.78 0.22 2.00
6 1.35 0.31 1.21 0.15 1.67
7 2.12 0.58 1.95 0.33 1.04
8n 1.69 0.44 2.88 0.87 5.22**
9 1.32 0.19 1.39 0.15 1.27
10n 2.44 0.62 3.05 1.15 2.00
11 1.31 0.28 1.79 0.27 5.05**
12 1.21 0.12 1.38 0.22 3.09**
==============================================================================
n-Nucleolar chromosome*,** Significant at 5% and 1% levels, respectively.
a-Number of measurements differed according to chromosomes.
Smallest numbers were used in t test.
Fig. 1. Pachytene chromosomes from a PMC of cultivar IR 36, numbered in the
descending order of length.
Fig. 2. Prometaphase chromosomes from a root-tip cell of cultivar IR 36.
References
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. and T. Omura, 1978. Karyotype analysis in rice, 1. New method for identifying all chromosome pairs. Jpn. J. Genet. 53: 251-255.
Steel, R.G.D. and J.H. Torrie, 1960. Principles and Procedures of Statistics. McGraw-Hill, New York.
Wu, H.K., 1967. Note on preparing of pachytene chromosomes by double mordant. (in Chinese with Eng. Summary) Scientic Agriculture (Taiwan) 15:40-44.
