Duplication of the S-locus F-box gene is associated with breakdown of pollen function in an S-haplotype identified in a natural population of self-incompatible Petunia axillaris

We previously identified both self-incompatible and self-compatible plants in a natural population of self-incompatible Petunia axillaris subsp. axillaris, and found that all the self-compatible plants studied carried either S _C1- or S _C2-haplotype. Genetic crosses showed that S _C2 was identical to S ₁₇ identified from another natural population of P. axillaris, except that its pollen function was defective, and that the pollen-part mutation in S _C2 was tightly linked to the S-locus. Recent identification of the S-locus F-box gene (SLF) as the gene that controls pollen specificity in S-RNase-based self-incompatibility has prompted us to examine the molecular basis of this pollen-part mutation. We cloned and sequenced the S ₁₇-allele of SLF of P.axillaris, named PaSLF ₁₇, and found that S _C2 S _C2 plants contained extra restriction fragments that hybridized to PaSLF ₁₇ in addition to all of those observed in S ₁₇ S ₁₇ plants. Moreover, these additional fragments co-segregated with S _C2. We used the S _C2-specific restriction fragments as templates to clone an allele of PaSLF by PCR. To determine the identity of this allele, named PaSLF _x , primers based on its sequence were used to amplify PaSLFalleles from genomic DNA of 40 S-homozygotes of P. axillaris, S ₁ S ₁ through S ₄₀ S ₄₀. Sequence comparison revealed that PaSLF _x was completely identical with PaSLF ₁₉ obtained from S ₁₉ S ₁₉. We conclude that the S-locus of S _C2 contained both S ₁₇-allele and the duplicated S ₁₉-allele of PaSLF. S _C2 is the first naturally occurring pollen-part mutation of a solanaceous species that was shown to be associated with duplication of the pollen S. This finding lends support to the proposal, based on studies of irradiation-generated pollen-part mutants of solanaceous species, that duplication, but not deletion, of the pollen S, causes breakdown of pollen function.