S locus genes and the evolution of self-fertility in Arabidopsis thaliana

Loss of self-incompatibility (SI) in Arabidopsis thaliana was accompanied by inactivation of genea required for SI, Including S-LOCUS RECEPTOR KINASE (SRK) and S-LOCUS CYSTEINE-RICH PROTEIN (SCR), coadopted genea that constitute the SI specificity-determining S haplotype. Arabidopsis accessions are polymorphic for ΨSRK and ΨSCR, but it is unknown if the species harbors structurally different S haplotypes, either representing relics of ancestral functional and structurally heteromorphic S haplotypes or resulting from decay concomitant with or subsequent to the switch to self-fertility. We cloned and sequenced the S haplotype from C24, in which self-fertility is due solely to S locus inactivation, and show that this heplotype was produced by interhaplotypic recombination. The highly divergent organization and sequence of the C24 and Columbia-0 (Col-0) S haplotypes demonstrate thst the A. thaliana S locus underwent extensive structural remodeling in conjunction with a relaxation of selective pressures that once preserved the integrity and linkage of coadopted SRK and SCR allelee. Additional evidence for this process was obtained by assaying 70 accessions for the presence of C24- or Col-0-specific sequences. Furthermore, analysis of SRK and SCR polymorphisms in these accesalona argues against the occurrence of a selective sweep of a particular allele of SCR, as previoualy proposed.