16. Distorted F2 ratios and changes in gene frequency observed for the wx and other three loci

It is well known that in hybrids between glutinous and non-glutinous rice varieties, glutinous homozygotes tend to be deficient in the F2, often being significantly less than 25%. This trend is particularly noticeable in distant crosses (Mori et al. 1974). It has been noticed that when an F1 plant is pollinated by a homozygous strain, the +: wx ratio in many cases fits the 1:1 ratio although it deviates from 1:1 in some crosses in which F1 sterility is involved. When an F1 plant is used as the pollen parent, the backcross ratio often deviates from 1:1 suggesting the effect of certation. However, when isogenic lines were used for crosses, the F2 ratio agreed well with the expected 3:1 or 1:2:1 (Tsai and Oka 1965). This indicates that the distorted ratios are caused by factors linked with the glutinous endosperm gene (wx, linkage group I).

When an heterozygous population is grown in bulk, the wx gene tends to decrease in frequency. The rate of decrease is generally high in early generations and becomes low as homozygosity increases. As an example, the frequencies of the wx gene observed in seven F2 lines, each grown in bulk, are shown in Fig. 1. In this experiment, the F4 to F6 populations were each raised with two replications. The analysis of variance of the data showed that the intraclass correlation

Fig. 1. Frequency of the glutinous gene in seven F2 lines derived from 563 (Kinoshita- mochi, Japonica)X 115 (U-kuh, indica) and grown in bulk. The gene frequency was estimated from the frequency of glutinous grains (x) in 5OO or more samples as

Different factors are known to be responsible for segregation distortion. A major factor is gametic selection caused by F1-sterility genes and certation genes. The F1-sterility genes are at least of two kinds, duplicate gametic lethals and genes causing one-locus sporogametophytic interaction (Oka 1988, p. 190-192). Usually, the sterility genes seem to affect both female and male gametes as is inferred from the parallelism between embryo sac and pollen sterilities (Chu et al. 1969), although exceptions are found. Certation seems to result from the ga genes which affect the male gametes only (Iwata et al. 1964).

In the 25 F2 populations shown in Fig. 2, the wx-gene frequency obtained from the numbers of + +, + wx and wx wx plants in the F2 ranged from 0.513 to 0.240. Its deviation from 0.5 was strongly correlated with the chi-square value for the deviation from 1:2:1 converted into logarithms (r=0.87), indicating that the chi-square value can represent the change in gene frequency. The distortion of F2 ratios as shown by chi-square values was found to be correlated with the F1 pollen sterility (Fig. 2; r=0.38, P=0.05). However, even when the F1 plants were fully fertile, some F2 Population showed significant chi-square values. This may be regarded as the result of certation. When seven such cases were excluded, the chi-square value converted into logarithm was correlated with F1 pollen sterility more clearly (r=0.65; Fig. 2). This suggests the significant role of F1-sterility genes in segregation distortion for the wx gene. Without conducting a specially

Fig. 2. Relationship between F2 segregation distortion for the glutinous gene (wx) as shown by chi-square values from 1:2:1 and F1 pollen fertility. Significant chi-square values if fertile hybrids (pollen fertility >0.88) show distortion due to certation. When they are excluded from the distribution, a significant correlation (r=0.65, P<0.01) was found between segregation distortion and F1 sterility. (I-Indica; J-Japonica).

designed experiment, it is not possible to specify the genes causing distortion. Yet, it may be admitted that the distortion results mainly from gametic selection caused by F1-sterility genes.

The C locus for apiculus coloration, which is linked with wx, also showed distorted F2 ratios frequently (26/66 crosses). The chi-square values for the deviation from 3:1 (9:7 in some crosses) ranged from nearly zero to about 30 among 103 F2 populations observed. The chi-square values converted into logarithms was correlated with F1 pollen sterility significantly (r=0.27, P<0.05). It may be conceded that segregation distortion for the C locus is mainly due to gametic selection in the same manner as for the wx locus. In Indica-Japonica hybrids, the c or C gene derived from the Indica parent tends to increase against the gene from the Japonica parent (Oka 1988, p. 172-173). The gametic selection causing segregation distortion would play an important role in this trend.

On the other hand, at the Ph locus for phenol reaction which belongs to linkage group II, significant chi-square values from the 3:1 ratio were found in only 7 out of 51 F2 populations observed, of which 41 were from Indica-Japonica crosses. Of the 7 populations showing distortion, 4 had an excess and three had a deficiency of recessive homozygotes (ph/ph). There was no tendency for segregation distortion to be associated with F1 sterility.

Similarly, at the Rc locus for red pericarp coloration belonging to linkage group IV, distorted F2 ratios were found in only four out of 24 crosses examined. Three of the four crosses showed an excess and one cross showed a deficiency of recessive homozygotes (rc/rc). Segregation distortion was not associated with F1 sterility. In a few other crosses, the F2 ratios appeared to be close to 3:13 although the inhibitory gene thereby assumed is left for studies in the future.

It may be concluded that segregation distortion is found frequently in linkage group I and sporadically in linkage groups II and IV.

Chu, Y. E., H. Morishima and H. I. Oka, 1969. Reproductive barriers distributed in cultivated rice species and their wild relatives. Jpn. J. Genet. 44: 207-223.

Iwata, N., T. Nagamatsu and T. Omura, 1964. Abnormal segregation of waxy and apiculus coloration by a gametophytic gene belonging to the first linkage group in rice. Jpn. J. Breed. 14: 33-39.

Mori, K., T. Kinoshita and M. Takahashi, 1974. Segregation distortion and its variability of endosperm character in crosses of distantly related rice varieties. Genetical studies on rice plant, 60. Rep. Exp. Farm, Hokkaido Univ. 19: 1-13. (Japanese/English)

Oka, H. I., 1988. Origin of Cultivated Rice. Elsevier/Jpn. Sci. Soc. Press, Amsterdam/Tokyo, 254 pp.

Tsai, K. H. and H. I. Oka, 1965. Characters of isogenic lines in rice. Bot. Bull. Acad. Sinica (Taipei) 6(l): 19-31.