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, S3 and S11 alleles were examined with homozygous S1 seedlings (S3S3 and S11S11) of self-incompatible Clementine mandarin (C. clementina hort. ex Tanaka; S3S11) 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 S1 seedlings (S3S3 and S11S11). Pollen tube arrest in the styles of their pollinated pistils indicated that 25 accessions, including Clementine, have an S3 allele each and 18 accessions, including Clementine, have an S11 allele each. Frequency of accessions with S3 allele was 17.7% (25 of 141 accessions examined) and S3 allele frequency was 11.8% (25 of 212 alleles excluding Sf self-compatible allele). Frequency of accessions with S11 allele was 13.2% (18 of 136 accessions examined) and S11 allele frequency was 8.8% (18 of 205 alleles excluding Sf allele). Pummelo accessions have S3 and S11 alleles with the frequencies of 12.2% and 33.3% respectively, suggesting that the two alleles originated from pummelo. Clementine shared S3 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 (S3sm) resulting in cross compatibility with S3 pollen occurred in their common ancestor. Clementine shared S11 allele with two tangors but did not share it with ‘Mediterranean’ mandarin. The present result for S3 and S11 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 S3 allele during long history of cultivation.