The selfing syndrome and beyond: diverse evolutionary consequences of mating system transitions in plants

The shift from outcrossing to self-fertilization (selfing) is considered one of the most prevalent evolutionary transitions in flowering plants. Selfing species tend to share similar reproductive traits in morphology and function, and such a set of traits is called the 'selfing syndrome'. Although the genetic basis of the selfing syndrome has been of great interest to evolutionary biologists, knowledge of the causative genes or mutations was limited until recently. Thanks to advances in population genomic methodologies combined with high-throughput sequencing technologies, several studies have successfully unravelled the molecular and genetic basis for evolution of the selfing syndrome in Capsella, Arabidopsis, Solanum and other genera. Here we first introduce recent research examples that have explored the loci, genes and mutations responsible for the selfing syndrome traits, such as reductions in petal size or in pollen production, that are mainly relevant to pre-pollination processes. Second, we review the relationship between the evolution of selfing and interspecific pollen transfer, highlighting the findings of post-pollination reproductive barriers at the molecular level. We then discuss the emerging view of patterns in evolution of the selfing syndrome, such as the pervasive involvement of loss-of-function mutations and the relative importance of selection versus neutral degradation. This article is part of the theme issue 'Genetic basis of adaptation and speciation: from loci to causative mutations'.