Double-Locking Mechanism of Self-Compatibility inArabidopsis thaliana: The Synergistic Effect of Transcriptional Depression and Disruption of Coding Region in the Male Specificity Gene

Self-compatibility inArabidopsis thalianarepresents the relatively recent disruption of ancestral obligate cross pollination, recognized as one of the prevalent evolutionary pathways in flowering plants, as noted by Darwin. Our previous study found that inversion of the male specificity gene (SP11/SCR) disrupted self-incompatibility, which was restored by overexpressing theSCRwith the reversed inversion. However,SCRinA. thalianahas other mutations aside from the pivotal inversion, in both promoter and coding regions, with probable effects on transcriptional regulation. To examine the functional consequences of these mutations, we conducted reciprocal introductions of native promoters and downstream sequences from orthologous loci of self-compatibleA. thalianaand self-incompatibleA. halleri. Use of this inter-species pair enabled us to expand the scope of the analysis to transcriptional regulation and deletion in the intron, in addition to inversion in the native genomic background. Initial analysis revealed thatA. thalianahas a significantly lower basal expression level ofSCRtranscripts in the critical reproductive stage compared to that ofA. halleri, suggesting that the promoter was attenuated in inducing transcription inA. thaliana. However, in reciprocal transgenic experiments, thisA. thalianapromoter was able to restore partial function if coupled with the functionalA. hallericoding sequence, despite extensive alterations due to the self-compatible mode of reproduction inA. thaliana. This represents a synergistic effect of the promoter and the inversion resulting in fixation of self-compatibility, primarily enforced by disruption ofSCR. Our findings elucidate the functional and evolutionary context of the historical transition inA. thalianathus contributing to the understanding of the molecular events leading to development of self-compatibility.