Ever since Darwin's pioneering research, the evolution of self-fertilisation (selfing) has been regarded as one of the most prevalent evolutionary transitions in flowering plants. A major mechanism to prevent selfing is the self-incompatibility (SI) recognition system, which consists of male and female specificity genes at the S-locus and SI modifier genes. Under conditions that favour selfing, mutations disabling the male recognition component are predicted to enjoy a relative advantage over those disabling the female component, because male mutations would increase through both pollen and seeds whereas female mutations would increase only through seeds. Despite many studies on the genetic basis of loss of SI in the predominantly selfing plant Arabidopsis thaliana, it remains unknown whether selfing arose through mutations in the female specificity gene (S-receptor kinase, SRK), male specificity gene (S-locus cysteine-rich protein, SCR; also known as S-locus protein 11, SP11) or modifier genes, and whether any of them rose to high frequency across large geographic regions. Here we report that a disruptive 213-base-pair (bp) inversion in the SCR gene (or its derivative haplotypes with deletions encompassing the entire SCR-A and a large portion of SRK-A) is found in 95% of European accessions, which contrasts with the genome-wide pattern of polymorphism in European A. thaliana. Importantly, interspecific crossings using Arabidopsis halleri as a pollen donor reveal that some A. thaliana accessions, including Wei-1, retain the female SI reaction, suggesting that all female components including SRK are still functional. Moreover, when the 213-bp inversion in SCR was inverted and expressed in transgenic Wei-1 plants, the functional SCR restored the SI reaction. The inversion within SCR is the first mutation disrupting SI shown to be nearly fixed in geographically wide samples, and its prevalence is consistent with theoretical predictions regarding the evolutionary advantage of mutations in male components.

Self-incompatibility (SI) in Brassicaceae is sporophytically controlled by a single Slocus with multi allelic variety. The male S determinant, SP11/SCR (S-locus protein 11/S-locus cysteine-rich protein), is a small cysteine-rich protein, and the female S determinant, SRK (S-locus receptor kinase), functions as a receptor for SP11 at the surface of stigma papilla cells. Although a few of the following downstream factors in the SP11-SRK signaling cascade have been identified, a comprehensive understanding of the SI mechanism still remains unexplained in Brassicaceae. Analysis of self-compatible (SC) mutants is significant for understanding the molecular mechanism in SI reactions, thus we screened SC lines from a variety of Japanese bulk-populations of B. rapa vegetables. Two lines, TSC4 and TSC28, seem to have disruptions in the SI signaling cascade, while the other line, TSC2, seems to have a deficiency in a female S determinant, SRK. In TSC4 and TSC28, known SI-related factors, i.e. SRK, SP11, MLPK (M-locus protein kinase), THL (thioredoxin-h-like), and ARC1 (arm repeat containing 1), were expressed normally, and their expression levels were comparable with those in SI lines. On a B. rapa genetic linkage map, potential SC genes in TSC4 and TSC28 were mapped on linkage groups A3 and A1, respectively, whereas MLPK, ARC1, and THL were mapped on A3, A4, and A6, respectively. Although potential SC genes of TSC4 and MLPK were on the same linkage group, their positions were apparently independent. These results indicate that the SC genes of TSC4 and TSC28 are independent from the S-locus or known SI-related genes. Thus, the SC lines selected here have mutations in novel factors of the SI signaling cascade, and they will contribute to fill pieces in a signal transduction pathway of the SI system in Brassicaceae.