Molecular characterization of SLG and S-related genes in a self-compatible Brassica campestris L. var. yellow sarson

From a stigma cDNA library of a self-compatible strain, designated C636, of Brassica campestris var. yellow sarson, we isolated three cDNA clones that showed a high degree of sequence similarity with cDNAs encoding either SLG or SLR's. These three cDNA clones were designated SLG (C636), SLR1 (C636), and SLR2 (C636), respectively. Restriction fragment length polymorphism (RFLP) linkage analyses of a segregating F₂ progeny of a hybrid between C636 and the self-incompatible S⁸-homozygote revealed that SLG (C636) was linked to the S locus, whereas SLR1 (C636) and SLR2 (C636) were not. The latter two genes were not linked to each other. The transcripts of SLG (C636), SLR1 (C636), and SLR2 (C636) were detected in stigmas, but not in anthers, of C636. However, the steady-state level of the SLG (C636) transcript was significantly lower than that of the SLG transcript in the self-incompatible S⁹-homozygote. No SRK transcripts were detected in the stigma tissue of C636, whereas an RNA band of the expected size of the SRK transcript was detected in the self-incompatible S⁹-homozygote. The SLG protein was detected in C636 and in three other self-compatible yellow sarson strains by immunoblot analysis; however, the amounts were lower than those of SLGs in self-incompatible strains. We conclude that one reason for the breakdown of self-compatibility in C636 yellow sarson is the down-regulation of the SLG gene and/or failure of the expression of the SRK gene.