IV. Genetics of Disease Resistance and Other Physiological Traits

16. Some considerations on linkage relationships between Pii and Piz in the blast resistance of rice


1 ) Hokkaido University, Sapporo, 060 Japan
2) Sakura-ga-oka, 31-8, Kukizaki-machi, Inashiki-gun, 300-12 Japan

In accordance with the agreement on naming and symbolization of blast resistance genes, it was decided to redesignate the genes with Pi to be followed by a numeral, such as Pi1. However, the gene symbols assigned by Kiyosawa have been retained because of their priority and common usage among rice geneticists (Kinoshita 1993). Therefore, the genes, Pii, Piz and Pia derived from Ishikari Shiroke, Zenith and Aichi Asahi, respectively. retain these designations.

In this note, the genetic relations between Pii and Piz are reconsidered, in the light of recent information. Kiyosawa (1967) first reported linkage between Pii and Piz, with a recombination value of 30.9%. Fukuyama et al. (1970) quoted the linkage relation and a part of the linkage map of chromosome 6 was constructed as shown in Fig. 1. Subsequently, this linkage information was included in the map of chromosome 6 by Shinoda et al. ( 1971 ) and Kinoshita ( 1984).

Goto (1979) also showed a weak linkage between Pii and Piz with a recombination value of about 35%, and an independent segregation with fs- (fine stripe), which belongs to chromosome 6, was also indicated. Following this, Goto et al. (1981) suggested independence between the two resistance genes based on the fact that Pii is independent of fs and C (Chromogen). Ise (1992, 1993) also indicated a digenic ratio of 15:1 for the segregation of resistance in F2 populations from the crosses between near-isogenic lines of Nipponbare having Pii and Piz. He also reported a new linkage between Pii and pcs for susceptibility to pentachlorobenzyl alchohol. Furthermore, Pii segregated independently of Se1-u for photoperiod-sensitivity. Therefore, Kinoshita (1993) deleted the inclusion of Pii from chromosome 6.

On the other hand, it is possible to explain the discrepancy in the linkage relations between Pii and Piz, in unified system. It is known that the gene Pii was found originally in the following six varieties: Taichung Yu-qui 26 (Chinese variety), Sekiyama, Fujisaka 5, Hokuriku 11, Hokuriku 12 and Miyazaki 7. Except for Taichung Yu-qui 26, the phylo-genetic relations of the other five varieties are shown in Fig. 2. In this figure, Pii and Pia are responsible for the resistance of Ishikari Shiroke and Aichi Asahi types, respectively, but Shin 2 type is susceptible to the disease and lacks both Pii and Pia. It should be noted that Pii is not involved in the parentage of varieties which produced Fujisaka 5, Hokuriku 11, Hokuriku 12 and Miyazaki 7 showing 'Ishikari Shiroke' type.

Therefore, there is a possibility that the resistance of 'Ishikari Shiroke' type is due to a complementary action of two different genes such as Pii1 and Pii2 each of which are present in different parents. If the susceptible lines usually contain one of the complementary genes, then a monogenic inheritance due to one of the Pii genes can be expected in F2 populations when crossed to a resistant line of 'Ishikari Shiroke' type. Further, it is supposed that Pii1 is linked to Piz, while Pii2 is independent of Piz.

Thus, the genotypes of parental varieties are expected to be as shown in Fig. 3, depending on the experimental results. In the construction of a partial linkage map containing Pii, the three point tests were carried out. Therefore, there is another possibility that Pii1 is located on an opposite side of Piz as shown in Fig. 4. In this case, nearly independent relation should exist between Pii1 and fs. This hypothesis may be applicable to explain the results showing a discrepancy on the Pii locus. However, it is necessary to prove this hypothesis through the crossing experiments.


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