The transition from outcrossing to self-fertilization is one of the most common evolutionary changes in plants, yet only about 10-15% of flowering plants are predominantly selfing. To explain this phenomenon, Stebbins proposed that selfing may be an 'evolutionary dead end'. According to this hypothesis, transitions from outcrossing to selfing are irreversible, and selfing lineages suffer from an increased risk of extinction owing to a reduced potential for adaptation. Thus, although selfing can be advantageous in the short term, selfing lineages may be mostly short-lived owing to higher extinction rates. Here, we review recent results relevant to the 'dead-end hypothesis' of selfing and the maintenance of outcrossing over longer evolutionary time periods. In particular, we highlight recent results regarding diversification rates in self-incompatible and self-compatible taxa, and review evidence regarding the accumulation of deleterious mutations in selfing lineages. We conclude that while some aspects of the hypothesis of selfing as a dead end are supported by theory and empirical results, the evolutionary and ecological mechanisms remain unclear. We highlight the need for more studies on the effects of quantitative changes in outcrossing rates and on the potential for adaptation, particularly in selfing plants. In addition, there is growing evidence that transitions to selfing may themselves be drivers of speciation, and future studies of diversification and speciation should investigate this further.

3 1 l e t t e r sThe shift from outcrossing to selfing is common in flowering plants 1,2 , but the genomic consequences and the speed at which they emerge remain poorly understood. An excellent model for understanding the evolution of self fertilization is provided by Capsella rubella, which became self compatible <200,000 years ago. We report a C. rubella reference genome sequence and compare RNA expression and polymorphism patterns between C. rubella and its outcrossing progenitor Capsella grandiflora. We found a clear shift in the expression of genes associated with flowering phenotypes, similar to that seen in Arabidopsis, in which self fertilization evolved about 1 million years ago. Comparisons of the two Capsella species showed evidence of rapid genome-wide relaxation of purifying selection in C. rubella without a concomitant change in transposable element abundance. Overall we document that the transition to selfing may be typified by parallel shifts in gene expression, along with a measurable reduction of purifying selection.

The evolution from outcrossing to predominant self-fertilization represents one of the most common transitions in flowering plant evolution. This shift in mating system is almost universally associated with the ''selfing syndrome,'' characterized by marked reduction in flower size and a breakdown of the morphological and genetic mechanisms that prevent self-fertilization. In general, the timescale in which these transitions occur, and the evolutionary dynamics associated with the evolution of the selfing syndrome are poorly known. We investigated the origin and evolution of selfing in the annual plant Capsella rubella from its self-incompatible, outcrossing progenitor Capsella grandiflora by characterizing multilocus patterns of DNA sequence variation at nuclear genes. We estimate that the transition to selfing and subsequent geographic expansion have taken place during the past 20,000 years. This transition was probably associated with a shift from stable equilibrium toward a nearcomplete population bottleneck causing a major reduction in effective population size. The timing and severe founder event support the hypothesis that selfing was favored during colonization as new habitats emerged after the last glaciation and the expansion of agriculture. These results suggest that natural selection for reproductive assurance can lead to major morphological evolution and speciation on relatively short evolutionary timescales.divergence population genetics ͉ reproductive assurance ͉ bottleneck ͉ selfing syndrome ͉ mating system evolution S elfing plants benefit from 2 distinct advantages over their outcrossing competitors (1-3). First, because selfers are 100% related to their progeny and can also act as outcross pollen donors for seed produced by other individuals, they have an inherent transmission advantage over outcrossers (4). A second major advantage conferred by selfing, first discussed by Darwin (5), is the ability to reproduce when pollinators or potential mates are limited (reproductive assurance). One important aspect of reproductive assurance is the ability of selfing lineages to colonize new habitats from a very small founding population. Evolutionary theory predicts that selfing will evolve when these advantages outweigh the costs associated with inbreeding depression, reduced fitness caused by increased homozygosity of deleterious recessive alleles (6).In most cases the timescale over which the selfing syndrome has evolved remains difficult to ascertain. In the model genus Arabidopsis, for example, the selfing Arabidopsis thaliana diverged from its closest outcrossing relatives Ϸ5 million years ago (7), and patterns of diversity have suggested a possibly complicated picture of the breakdown of outcrossing over a period of more than a million years (7,8). Thus, it remains unclear how rapidly the selfing syndrome can arise, and what historical conditions have favored mating system evolution. Capturing and characterizing a recent transition is important for inferring this timescale, and the relative import...