National Institute of Genetics, Mishima, 411 Japan
Genes C and A are known to be basic in producing anthocyanins for coloration of different organs, and P is responsible for the distribution of pigment to the apiculus (Takahashi 1964). Series of multiple alleles are found at the C and A loci, which adjust the color tone from pink to dark purple. However, if the presence or absence of coloration only is considered, all varieties showing apiculus coloration at the flowering time equally have the three complementary genes, C, A and P. On the other hand, there are different genotypes among colorless varieties, as apiculus coloration does not occur when recessive allele is present at any one of the three loci.
Genes C, A and P belong to linkage groups I, III and II, respectively (RGN 1, 1984, p. 29). No inhibitor for these genes is known so far. The genotype for apiculus coloration of a given variety can be determined from the presence or absence of apiculus coloration in the F1 plants from crosses with strains having known genotypes. When necessary, F2 segregation patterns and linkage relations may be observed for confirmation.
The Asian common wild rice, 0. rufipogon, which is thought to be the wild progenitor of 0. sativa, shows apiculus coloration in most of its varieties, indicating that they all have genotype CAP. Exceptionally, a few of them with colorless apiculus were of cAP genotype probably as the result of introgression from cultivars, and a few others showed colorless apiculus plus colored stigma and had genotype CAp for apiculus coloration (Table 1).
Among 0. sativa cultivars, many are colorless and colored ones are found in land races which may be regarded as relatively "primitive". It was found that all colorless varieties of the Indica type had cAP. In contrast, in the Japonica type, the genotypes of colorless varieties were varied (Table 1).
From the frequencies of dominant and recessive alleles at the C, A and P
Table 1. Distribution of genotypes for apiculus coloration among
strains of wild and cultivated rices
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Colored Colorless No. of
Varietal group _______ ____________________________ strains
CAP cAP CaP caP CAp Cap
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0. rufipogon 45 2 2 49
(Asian, wild)
0. sativa,
Indica 19 45 64
Japonica Tropical 16 8 6 6 3 1 40
Temperate 8 1 7 8 2 26
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loci, Nei's (1975) average gene diversity, 
where x\ij\ is the frequency of
i-th allele at locus j and n is the number of loci, was computed in each
varietal group. The H value was 0.052 in 0. rufipogon and 0.314 in 0.
sativa. Among 0. sativa cultivars, the H value was much higher in the
Japonica (0.372) than in the Indica type (0.139; Table 2). Thus, in gene
diversity for apiculus coloration it may be said that
Wild<lndica<Japonica.
Table 2. Average gene diversity (H) obtained in different strain
groups
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Apiculus-coloration genes Isozyme genes
Varietal group ___________________________ ____________________
No. of No. of H No. of No. of H
strains alleles strains alleles
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0. rufipogon 51 5 0.052 56 40 0.476
0. sativa 130 6 0.314 64 23 0.337
Indica 64 4 0.139 32 23 0.286
Japonica 66 6 0.372 32 20 0.092
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This trend is opposite to that observed for isozyme alleles. In the H
values based on the frequencies of 40 alleles at 11 loci (Acp-1, Amp-3,
Cat-1, Est-2, Est-9, Pgd-2, Pgi-1, Pgi-2, Pox-1, Pox-2, and Sdh-1; Oka 1988,
p. 65), it was found that Wild>lndica>Japonica (Table 2). Why such
different trends of gene diversity distribution occur remains unknown. It
may be inferred that the evolutionary change from wild to cultivated plants
has resulted in accumulation of allelic diversity at some kinds of loci, but
has led to reduction of allelic diversity in other gene systems. In the
former system, Japonicas are more diverse than Indicas, but Indicas are more
diverse than Japonicas in the latter system. It may be questioned whether
or not this suggests that Indicas are closer to the wild progenitor than
Japonicas evolutionarily. References
Oka, H. I., 1988. Origin of Cultivated Rice. Elsevier/Jpn. Sci. Soc. Press, Amsterdam/Tokyo, 254 pp.
Nei, M., 1975. Molecular Population Genetics and Evolution. North-Holland Publ., Amsterdam, 288 pp.
Takahashi, M., 1964. Linkage groups and gene schemes of some striking morphological characters in Japanese rice. In IRRI (ed.), Rice Genetics and Cytogenetics, p. 215-236. Elsevier, Amsterdam.