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The switch from an out-crossing to a self-fertilizing mating system is one of the most prevalent evolutionary trends in plant reproduction and is thought to have occurred repeatedly in flowering plants However, little is known about the evolution of self-fertility and the genetic architecture of selfing. Here, we establish Arabidopsis thaliana as a model for genetic analysis of the switch to self-fertility in the crucifer family, where the ancestral out-crossing mode of mating is determined by self-incompatibility (SI), a genetic system controlled by the S locus. We show that A. thaliana ecotypes exhibit S-locus polymorphisms and differ in their ability to express the SI trait upon transformation with S-locus genes derived from the obligate out-crosser Arabidopsis lyrata. Remark- ably, at least one ecotype was reverted to a stable, self-incompatible phenotype identical to that of naturally self-incompatible species. These ecotype differences are heritable and reflect the fixation in different A. thaliana populations of independent mutations that caused or enforced the switch to self-fertility. Their continued analysis promises to identify the loci that were the targets of natural selection for selfing and to contribute to a mechanistic understanding of the SI response. [ABSTRACT FROM AUTHOR]