Department of Agronomy, National Chung-Hsing University, Taichung, Taiwan 40227, ROC
Distorted F`2` ratios are often observed for mrphological markers in crosses between distantly related rice varieties. The Acp-1 locus located on chromosome 12 (Ishikawa et al. 1987) has alleles designated as -4 (= 1) and +9 (=2) which are frequently found in Indica and Japonica cultivars, respectively, and other alleles distributed in the common wild rice.
The F`2` segregation pattern for Acp-1 was observed in a cross between TCS3 (Indica) and CH106 (Japonica), at three locations in Taiwan (Table 1). Two replications were made at each location. At all locations, the Indica-type segregants (Acp-1-4) were more frequent than 25%, and the Japonica-type segregants (Acp-1+9) were fewer than 25%. The frequency of heterozygotes (F`1` type) agreed with expected 50%. The occurrence of similar patterns at the three locations differing in environmental conditions suggests that segregation distortion is genotype-inherent and not influenced much by environment.
Table 1. F`2` segregation patterns for Acp-1 alleles in TCS3x CH106, observed
at three locations in Taiwan
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Acp-1-4 F`1` type Acp-1+9 Total Chi2
Location Replication No. % No. % No. % no. (1:2:1)
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Hsinchu 1 19 24.1 51 64.5 9 11.4 79 10.4**
(North) 11 26 41.9 33 53.3 3 4.9 62 20.9 * *
Sum 45 31.9 84 59.6 12 8.5 141 25.4 * *
Taichung 1 26 34.7 40 53.3 9 12.8 75 8.9*
(Middle) 11 28 42.4 31 47.0 7 10.6 66 13.6**
Sum 54 38.3 71 50.4 16 11.3 141 21.7**
Pingtung 1 19 35.2 31 57.4 4 7.4 54 11.7**
(South) 11 33 34.0 51 52.6 13 13.4 97 9.2*
Sum 52 34.4 82 54.3 17 11.3 151 19.6
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*,** Significant at 5% and 1% levels, respectively.
Table 2. Segregation for Acp-1 alleles observed in B`1` generation
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Cross comination No. of plants Total Chi2 Transmission
Acp-1-4 Acp-1+9 (1:1) rate of Acp-1+9
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TCS3/(TCS3/CH106) 18 3 21 10.7** 0.14
CH106/(TCS3/CH106) 28 5 33 16.0** 0.15
(TCS3/CH106)/CH106 27 31 58 0.97 0.53
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** Significant at 1% level.
When the F`1` plants of TCS3XCH106 were used as the female parent for
backcross with CH106, the B`1` ratio was 1:1 (Table 2). When the F`1` plants
were used as the male parent, transmission of the Indica allele was higher
than that of the Japonica allele. This suggests that distortion is due to
competition of pollen grains for fertilization. Presumably, there is a
gametophyte gene controlling pollen-tube growth.Assuming that a gametophyte gene suppressing Japonica pollen is linked with Acp-1, the recombination value between Acp-1 and the gametophyte gene (p) was estimated from the data for 30 F`2` lines raised from heterozygous F`2` plants. The frequency of the Japonica-type allele (Acp-1+9) was lower than 25% (lowest being 4%) in 25 lines, normal (about 25%) in 4 lines, and higher than 25% (about 40%) in one line. These numbers, i.e., 25 (=a), 4 (=b) and 1 (=c), were put into the formula proposed by Nakagahra et al. (1972), recombination fraction was computed as: p=(2c+b)/2(a+b+c), with its standard deviation s`p`= sq rt(p(1-p)/2(a+b+c)). As the result, p=0.10+/-0.016 was obtained. The gametophyte gene assumed may be designated as ga-13 tentatively. (cf. RGN 10, p. 10).
References
Ishikawa, R., T. Kinoshita and H. Morishima, 1987. Trisomic analysis of genes for isozymes: Location of Cat-1, Acp-1 and Pox-2 on chromosomes. RGN 4: 75-76.
Nakagahra, M., T. Omura and N. Iwata, 1972. Gametophyte genes and their loci on the eleventh linkage group of cultivated rice. Japan. J. Breed. 22: 305-312.