Abstract
Many plants harbor complex mechanisms that promote outcrossing and efficient pollen transfer. These include floral adaptations as well as genetic mechanisms, such as molecular self-incompatibility (SI) systems. The maintenance of such systems over long evolutionary timescales suggests that outcrossing is favorable over a broad range of conditions. Conversely, SI has repeatedly been lost, often in association with transitions to self-fertilization (selfing). This transition is favored when the short-term advantages of selfing outweigh the costs, primarily inbreeding depression. The transition to selfing is expected to have major effects on population genetic variation and adaptive potential, as well as on genome evolution. In the Brassicaceae, many studies on the population genetic, gene regulatory, and genomic effects of selfing have centered on the model plant Arabidopsis thaliana and the crucifer genus Capsella. The accumulation of population genomics datasets have allowed detailed investigation of where, when and how the transition to selfing occurred. Future studies will take advantage of the development of population genetics theory on the impact of selfing, especially regarding positive selection. Furthermore, investigation of systems including recent transitions to selfing, mixed mating populations and/or multiple independent replicates of the same transition will facilitate dissecting the effects of mating system variation from processes driven by demography.
Highlights
Flowering plants harbor a great variety of mating systems and associated floral and reproductive adaptations [1], and there is a rich empirical and theoretical literature on the causes of this diversity [2–6]
The effects of the transition to self-fertilization on population genomic variation and molecular evolution have been extensively studied in two systems from the Brassicaceae family, Arabidopsis and Capsella
Population Genomics of Shifts to Selfing 275 species, increased homozygosity renders recessive alleles visible to selection, and as a result, fixation probabilities of recessive advantageous alleles are expected to be higher in selfers than in outcrossers [53]
Summary
The effects of the transition to self-fertilization on population genomic variation and molecular evolution have been extensively studied in two systems from the Brassicaceae family, Arabidopsis and Capsella. Both of these genera have outcrossing SI as well as SC species with high selfing rates, and serve as good models to study this evolutionary transition [16–18]. The gene SRK encodes an S-locus receptor kinase that is located on the stigma surface and acts as the female specificity determinant, whereas the gene SCR encodes a pollen ligand that is deposited on the pollen surface and acts as the male specificity determinant [27]. With recent progress in long-read sequencing, which facilitates assembly of the S-locus, this area is ripe for further investigation
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