Transformation studies provided direct evidence that SLF/SFB determines S-specificity. In one study, PiSLF2 was expressed in SiSi and S2S3 Petunia inflata. This transformation was not expected to introduce new recognition specificity into pollen; rather, SI breakdown was predicted, because of the het-eroallelic pollen (HAP) effect, which describes the loss of SI observed when two different S-haplotypes are expressed in pollen. This is exactly what was observed; P. inflata S1S1 plants expressing PiSLF2 became SC. Similar results were observed when the Antirrhinum AhSLF2 gene was expressed in self-incompatible Petunia hybrida S3S3 (Qiao et al. 2004). These experiments implicated SLF genes in SI, but did not directly demonstrate S-specificity. To address this issue, it had to be shown that a putative pollen-S gene affects S-haplotypes differently. An S-specific effect was demonstrated in P. inflata (Sijacic et al. 2004). Critically, PiSLF2 did not cause breakdown of SI when it was expressed in conjunction with a native PiSLF2 gene. S2 S3 plants expressing PiSLF2 were SC. However, only S3-pollen expressing the transgene was compatible; S2-pollen was rejected regardless of the transgene's presence.
To date, only the PiSLF2 gene has been unequivocally shown to determine S-specificity. Given the multiplicity of these genes and the ambiguity of identifying orthologues across wide phylogenetic distances, a complete set of results in each family must be obtained to establish their roles. S-specificity experiments should be repeated in other species. Since multiple S-linked F-box genes are expressed in pollen, they could have other roles and should be tested for this.
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