Estimation of Self-incompatibility S Genotypes of Citrus Cultivars and Plants Based on Controlled Pollination with Restricted Number of Pollen Grains

Abstract

Seedlessness in Citrus cultivars is one of the desirable characters for consumers. Many mandarin and mandarin relative cultivars released recently were seedless ones with male sterile Satsuma mandarins in their pedigrees. Breeding seedless cultivars with self–incompatibility is another method to expand and strengthen the breeding possibility. There are two systems for self–incompatibility in angiosperms, i.e., sporophytic and gametophytic. In a gametophytic self–incompatibility system, rejection of pollen tube occurs when an S allele carried by the haploid pollen matches either of two S alleles existing in the diploid tissue of the pistil. Successful pollen tube growth occurs when the S allele carried by the haploid pollen is different from both S alleles expressing in the diploid pistil. The interaction between pollen tubes and the pistil in a gametophytic system controlled by single gene is divided into three types, i.e., full compatible, semi compatible (or semi incompatible) and incompatible crosses. By pollination tests, these incompatibility reactions were defined in cherries (Cane and Brown, 1938). In apple, Manganaris and Alston (1987) observed the difference in pollen tube behaviors in incompatible, semi compatible and full compatible crosses. In Citrus, Nagai and Tanikawa (1928) firstly reported several self–incompatible cultivars, and then many authors did (e.g., Miwa, 1951; Nuriyal, 1952; Soost, 1956 and 1964; Iwamasa and Oba, 1980). Soost (1969) proposed a self–incompatibility S gene system in Citrus on the basis of the segregation of hybrid seedlings with self– incompatibility. There was one report (Khan and De Mason, 1986) on the pollen tube behavior in semi compatible crosses. In the previous study (Ngo et al., 2001), not only normal but also abnormal pollen tube growth was detected in the stigmas and styles of self–compatible cultivars after self–pollination. These results suggest the possibility that S genotypes predicted for the cultivars (Wakana et al., 1998) can be certified directly by the reaction between pistil and pollen tubes even in semi compatible pollinations. In this study, therefore, pollen tube behaviors were analyzed to certify the cultivar S genotypes in probably semi compatible and full compatible crosses that were estimated on the basis of predicted S genotypes of several cultivars (Wakana et al., 1998).