Distribution and evolution of citrus with S₄ and/or S₅ gene alleles for self-incompatibility with special focus on the origin of satsuma mandarin (Citrus unshiu Marc.; S_f S₄)

Gametophytic self-incompatibility, one of the key characters for seedless citrus production, is controlled by single codominant gene S known in pummelo, mandarin and their hybrid cultivars. However, little is known about S genotypes and S allele frequencies in Citrus cultivars. First, S genotypes and S allele frequencies of Citrus accessions were studied with S₄ and S₅ gene alleles of ‘Hassaku’ (C. hassaku hort. ex Tanaka) a putative hybrid between self-incompatible pummelo [C. maxima (Burm.) Merrill; syn. C. grandis Osbeck] and ‘Kunenbo’ (C. nobilis Lour. var. kunep Tanaka). About 200 Citrus accessions were pollinated with each of the homozygous S₁ seedlings (S₄S₄ and S₅S₅) of ‘Hassaku’. Pollen tube arrest in the styles indicated that 19 of 202 (9.4%) accessions have an S₄ allele and 12 of 191 (6.3%) accessions have an S₅ allele. In pummelo, the rate of accessions with S₄ allele was 1.4% (1/72), while that with S₅ allele was 5.9% (9/76). This suggests that the two alleles are originated from pummelo and the allele frequency is 0.7% (1/144) for S₄ and 5.9% (9/152) for S₅. Except ‘Kunenbo’, all accessions with S₄ alleles have their origin in Japan. These results suggest that self-incompatible ‘Kunenbo’ (S₄S_?) introduced from Southeast Asia to Japan about 600 years ago firstly generated satsuma mandarin (S_fS₄) and ‘Kabuchii’ (S₄S_?) about 400–500 years ago, followed by 16 S₄-carrying cultivars including ‘Hassaku’ (S₄S₅) appearing as a chance seedling in Hiroshima in the nineteenth century.