An S receptor kinase gene in self-compatible Brassica napus has a 1-bp deletion

S locus glycoprotein (SLG) and S locus receptor kinase (SRK) cDNAs were isolated from an S allele present in a number of self-compatible Brassica napus lines. This A10 allele did not segregate with self-incompatibility in crosses involving other self-incompatible B. napus lines. The SLG-A10 cDNA was found to contain an intact open reading frame and was predicted to encode an SLG protein with sequence similarities to those previously associated with phenotypically strong self-incompatibility reactions. SLG-A10 transcripts were detected in the developing stigma at steady state levels even higher than those detected for SLG alleles linked with self-incompatibility. Analysis of the corresponding SRK-A10 cDNA showed that it was very similar to other S locus receptor kinase genes and was expressed predominantly in the stigma. However, a 1-bp deletion was detected in the SRK gene toward the 3' end of the SLG homology domain. This deletion would lead to premature termination of translation and the production of a truncated SRK protein. The A10 allele was determined to represent a B. oleracea S allele based on its segregation pattern with the B. oleracea S24 allele when both these alleles were present in the same B. napus background. These results suggest that a functional SRK gene is required for Brassica self-incompatibility.