Molecular Genetics of Self-incompatibility in Brassica napus

Abstract

Self-incompatibility (SI) is one of the mechanisms evolved by higher plants to promote outbreeding. In Brassica species, SI is controlled sporophytically by a single Mendelian genetic locus, the S-locus. Pollen rejection or acceptance is determined by the parental genotype, and pollen grains carrying the same allele as that present in the stigma are rejected. Among the genes identified so far from different S-haplotypes, the S-locus glycoprotein (SLG) and the S-locus receptor kinase (SRK) have been extensively studied. The current model of SI in Brassica postulates that a highly polymorphic pollen-borne ligand would be specifically recognized by SLG and/or SRK. This would activate a signal transduction pathway leading to rejection of self-pollen. We used a genomic approach in an attempt to identify a putative ligand gene. The S-locus regions of two functional Brassica napus S-haplotypes (910 and A14) were subcloned in Fosmid vectors and sequenced. New transcription units were identified by homology searches in the databases, gene prediction programs and polymerase chain reaction (PCR) amplification. The current study reports the structural features of both S-locus regions and a functional analysis using a transformation-based assay.