Molecular Characterization of the S Locus in Two Self-Incompatible Brassica napus Lines

Abstract

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.

Comparative Mapping of the Brassica S Locus Region and Its Homeolog in Arabidopsis: Implications for the Evolution of Mating Systems in the Brassicaceae

“…The placement of SLG 8 , SRK 8 , s298, s299 (designated as SLL2 in Yu et al [1996]), s40.2-1.7, s6B-1.2, and sbBH2.0 has been described previously (Boyes et al, 1997). Markers sp14 and sp400 are single-copy sequences recently isolated by walking in the S 8 haplotype.…”

“…In addition to the stigmatic SLG and SRK genes, the S locus complex is thought to encode at least one gene required for the SI phenotype in pollen. Indeed, S locus-encoded sequences that are expressed specifically in anthers have been isolated (Boyes and Nasrallah, 1995;Yu et al, 1996), although a role for these genes in the SI response has not been established.…”

“…In addition to the stigmatic SLG and SRK genes, the S locus complex is thought to encode at least one gene required for the SI phenotype in pollen. Indeed, S locus-encoded sequences that are expressed specifically in anthers have been isolated (Boyes and Nasrallah, 1995;Yu et al, 1996), although a role for these genes in the SI response has not been established.Current evidence supports the view that SI evolved multiple times during the diversification of flowering plants and that SI has a single origin within any given plant family (Weller et al, 1993;Uyenoyama, 1995), with its recognition genes having been recruited early in the family lineage by duplication and modification of preexisting genes that perform other functions in the plant. These conclusions are based on the lack of relatedness of S locus genes from different plant families and on the observation that these genes typically belong to large gene families that include members expressed in nonreproductive tissues (Walker, 1993;Dwyer et al, 1994;Green, 1994).…”

Flowering plant self-incompatibility: the molecular population genetics of Brassica S-loci

“…The directions of transcription are for B. napus (Yu et al 1996). In B. oleracea, transcripts in both directions are generated from SRK (reviewed in Pastuglia et al, 1997b).…”

Inheritance and dominance of self-incompatibility alleles in polyploid Arabidopsis lyrata

“…Study of the breakdown of SI can provide valuable insights into the mechanisms controlling functional SI systems, as has been described for self-compatible Brassica napus (Goring et al, 1993;Robert et al, 1994;Yu et al, 1996;Ciu et al, 1999), and may provide clues to how SI evolved in these groups. Study of dominance interactions among alleles in tetraploids with functional SI systems has the added benefit of combining multiple allelic classes in the same individual to test hypotheses about intransitiveness (ie nonlinearity) of allelic interactions.…”