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 PaSLF17, and found that S-C2 S-C2 plants contained extra restriction fragments that hybridized to PaSLF17 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 PaSLFx, 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 PaSLFx was completely identical with PaSLF19 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.