AN ANTHER-SPECIFIC GENE ENCODED BY AN S-LOCUS HAPLOTYPE OF BRASSICA PRODUCES COMPLEMENTARY AND DIFFERENTIALLY REGULATED TRANSCRIPTS

The self-incompatibility locus of Brassica consists of a coadapted gen e complex that contains at least two genes required for the recognitio n and inhibition of pollen by the stigma when self-pollinated. Here, w e report the identification of a third S locus-linked gene from the S- 2 haplotype of Brassica oleracea. This gene, which we designated SLA ( for S Locus Anther), is a novel gene with an unusual structure. SLA is transcribed from two promoters to produce two complementary anther-sp ecific transcripts, one spliced and the other unspliced, that accumula te in an antiparallel manner in developing microspores and anthem. The sequence of the spliced transcript showed the presence of two open re ading frames that predict proteins of 10 and 7.5 kD. Neither transcrip t was produced in a self-compatible a napus strain carrying an S-2-lik e haplotype, indicating that the SLA gene in this strain is nonfunctio nal. Interestingly, sequences related to SLA were not detected in DNA or RNA from plants carrying S haplotypes other than S-2. The haplotype specificity of SLA, its anther-specific expression, and its physical linkage to the S locus are properties expected for a gene that encodes a determinant of S-2 specificity in pollen.