Molecular characterization of the S locus in two self-incompatible Brassica napus lines

In Brassica species, self-incompatibility has been mapped genetically to a single chromosomal location. In this region, there are two closely linked genes coding for the S locus glycoprotein (SLG) and S locus receptor kinase (SRK). They appear to comprise the pistil component of the self-incompatibility reaction. SLG and SRK are thought to recognize an unknown pollen component on the incompatible pollen, and the gene encoding this pollen component must also be linked to the SLG and SRK genes. To further our understanding of self-incompatibility, the chromosomal region carrying the SLG and SRK genes has been studied. The physical region between the SLG-910 and the SRK-910 genes in the Brassica napus W1 line was cloned, and a search for genes expressed in the anther revealed two additional S locus genes located downstream of the SLG-910 gene. Because these two genes are novel and are conserved at other S alleles, we designated them as SLL1 and SLL2 (for S locus-linked genes 1 and 2, respectively). The SLL1 gene is S locus specific, whereas the SLL2 gene is not only present at the S locus but is also present in other parts of the genomes in both self-incompatible and self-compatible Brassica ssp lines. Expression of the SLL1 gene is only detectable in anthers of self-incompatible plants and is developmentally regulated during anther development, whereas the SLL2 gene is expressed in anthers and stigmas in both self-incompatible and self-compatible plants, with the highest levels of expression occurring in the stigmas. Although SLL1 and SLL2 are linked to the S locus region, it is not clear whether these genes function in self-incompatibility or serve some other cellular roles in pollen-pistil functions.