Genotyping of Citrus Accessions with S9 And/or S10 Alleles for Self-incompatibility and Their Allelic Distribution

Gametophytic self-incompatibility, one of the key characteristics for breeding seedless Citrus cultivars, occurs in pummelo (Citrus maxima), mandarin (Citrus reticulata), and their hybrid cultivars. Allelic variation in Citrus was reported for the self-incompatibility gene (S); however, S allele frequencies and S genotypes of full- and semi-self-incompatible cultivars have been reported for a small number of alleles. To extend our knowledge of S alleles, we tested 146 Citrus accessions, including 82 pummelo accessions, for S-9 and S-10 alleles. Each accession was pollinated with homozygous S-1 seedlings of 'Hirado Buntan' pummelo (S9S9 and S10S10). The pollen tube growth arrest in the lower styles of their pollinated pistils indicated that four accessions, including 'Hirado Buntan [Citrus maxima (Burm.) Men.]', have the S-9 allele and five accessions, including '1-lirado Buntan', have the S-10 allele. The percentage of accessions with the S-9 allele was 3.2% (4 of 126 accessions examined), and the S-9 allele frequency was 1.8% (4 of 217 alleles, excluding the S f allele). The percentage of accessions with the S-10 allele was 3.9% (5 of 127 accessions examined), and the S-10 allele frequency was 2.3% (5 of 217 alleles, excluding the S-f allele). Japanese mandarin (another sources of S alleles) and its relatives had neither the S-9 nor the S-10 allele. Pummelo accessions had S9 and S-10 alleles at higher rates of 2.9% (2 of 70 accessions examined) and 7.0% (5 of 71 accessions examined), respectively. 'Kabusu' sour orange (a pummelo-mandarin hybrid; Citrus aurantium) and 'Kik udaidai' (a sour orange relative; Citrus canaliculata) had S-9 alleles. These results suggested that the two alleles originated from pummelo (the main sources of S alleles). The S genotypes with S-9 and/or S-10 alleles were fully determined in 'Hirado Buntan' (S9S10), 'Kabusu' sour orange (SfS9), the 'Kikudaidai' (S9S11) sour orange hybrid, and two local pummelo plants. The results of our study suggest that in comparison with the other S alleles reported, the pummelo plants with low frequencies of S(9 )and/or S-10 alleles contributed to very low rates of evolution and development of Citrus species and cultivars during the long history of citrus cultivation, except for those generating sour oranges (Citrus auratinum), which are used as root stocks, and for flesh and rind processing and ornamental plants. Finally, we examined the degree of self-incompatibility between S-9 and S-10 alleles in the lower part of styles of S-1 seedlings of '1-lirado Buntan' and Citrus accessions with S-9 and/or S-10 alleles. The result indicated no difference in the self-incompatibility reaction between the two alleles.