A new class of S sequences defined by a pollen recessive self-incompatibility allele of Brassica oleracea

Abstract

Self-incompatibility in Brassica oleracea is controlled by a single genetic locus (the S locus) with nearly 50 different alleles. In this paper, we report the characterization of the S2 allele, a pollen recessive self-incompatibility allele that exhibits weak DNA homology to the other previously sequenced S locus glycoprotein genes (SLG-6, -13, -14, -22 from alleles S6, S13, S14 and S22, respectively). Stigma cDNA clones with sequence homology to SLG-13 were isolated from two different S2 homozygous strains belonging to two different B. oleracea cultivars, var. alboglabra (Chinese kale) and var. italica (broccoli). The two S2 cDNA sequences are 90% homologous to each other, but only 70% homologous to SLG-13. Using the Chinese kale S2 genetic background, we demonstrate that the isolated alboglabra cDNA sequence is a transcript from a gene, designated SLG-2A that resides at the S locus, and propose that it is a putative determinant of S2 allelic specificity. Among the estimated 10-15 genomic copies of SLG-related genes detected in the S2 genome, we cloned and characterized the SLG-2A gene and another closely related and genetically linked gene copy, SLG-2B. A complete open reading frame that is 94% homologous to SLG-2A is located within SLG-2B. The existence of this intact duplicated S gene raises the possibility that SLG-2B may also be involved in the functioning of self-incompatibility in Brassica oleracea.