Distribution and evolution of Citrus accessions with S 3 and/or S 11 alleles for self-incompatibility with an emphasis on sweet orange [ Citrus sinensis (L.) Osbeck; S f S 3 or S f S 3sm ]

Gametophytic self-incompatibility, one of the key characters for production of seedless citrus , is known in pummelo [ Citrus maxima (Burm.) Merr.], mandarin ( C. reticulata Blanco) and their hybrid cultivars. The allelic variation in Citrus was reported for the self-incompatibility gene ( S ). However, S allele frequencies and S genotypes of self-incompatible and semi-self-incompatible Citrus cultivars were known only in a few S alleles. In the present study, S 3 and S 11 alleles were examined with homozygous S 1 seedlings ( S 3 S 3 and S 11 S 11 ) of self-incompatible Clementine mandarin ( C. clementina hort. ex Tanaka; S 3 S 11 ) a putative hybrid between self-compatible ‘Mediterranean’ mandarin ( C. deliciosa Ten.) and self-compatible sweet orange [ C. sinensis (L.) Osbeck]. One-hundred-forty-six Citrus accessions, including 43 pummelo accessions, were pollinated with each of the homozygous S 1 seedlings ( S 3 S 3 and S 11 S 11 ). Pollen tube arrest in the styles of their pollinated pistils indicated that 25 accessions, including Clementine, have an S 3 allele each and 18 accessions, including Clementine, have an S 11 allele each. Frequency of accessions with S 3 allele was 17.7% (25 of 141 accessions examined) and S 3 allele frequency was 11.8% (25 of 212 alleles excluding S f self-compatible allele). Frequency of accessions with S 11 allele was 13.2% (18 of 136 accessions examined) and S 11 allele frequency was 8.8% (18 of 205 alleles excluding S f allele). Pummelo accessions have S 3 and S 11 alleles with the frequencies of 12.2% and 33.3% respectively, suggesting that the two alleles originated from pummelo. Clementine shared S 3 allele with two navel oranges, ten common oranges and two tangors, but not with three blood oranges. In the three blood oranges, it was suggested that stylar part mutation ( S 3sm ) resulting in cross compatibility with S 3 pollen occurred in their common ancestor. Clementine shared S 11 allele with two tangors but did not share it with ‘Mediterranean’ mandarin. The present result for S 3 and S 11 alleles in Citrus accessions suggests that the two S alleles expanded their distribution in Citrus plants including sweet orange along with stylar part mutation for S 3 allele during long history of cultivation.