Population genomic processes in Brassicaceae in the light of mating system variation

Tiina Maria Mattila, Tanja Slotte

Mating system in plants is a highly variable trait ranging from obligatory outcrossing to full self-fertilization, i.e. selfing. In nature, there are various mechanisms to prevent self-fertilization, indicating evolutionary advantages of outcrossing. Still, self-fertilization is very common in plants. In addition, selfing is a useful property in plant breeding and research since it allows replication and maintenance of the same genotype over generations. Hence, it is of great interest to understand the evolutionary drivers of mating system shifts, as well as population genomic consequences across different time scales. In recent years, advances in population genomics methodologies, as well as large-scale population sequencing projects, have provided multiple insights into plant mating system variation, particularly in Brassicaceae model systems. Utilizing information from Brassicaceae systems, we review the effects of self-fertilization and genetically controlled single locus self-incompatibility (SI) on population genetic and genomic variation. The empirical results are focused on model systems from the Brassicaceae family, where in the ancestral form, high levels of outcrossing are ensured by genetically controlled sporophytic SI. The standard form of SI in Brassicaceae is controlled by a single tightly linked genetic locus, the S-locus, containing two genes, the male specificity SCR/SP11 and the female specificity SRK gene, involved in the self-recognition reaction. Loss-of-function at these S-locus genes or in the downstream cascade leading to the SI reaction leads to self-compatibility (SC) allowing self-fertility. Breakdown of SI and associated selfing can be favorable under specific circumstances, such as if mate availability is low, for example, in colonizing or range edge populations. Population investigations have characterized multiple independent losses of SI across the Brassicaceae, which has provided a framework to investigate the genome-wide diversity patterns associated with different mating systems. By summarizing the current knowledge on the population genomic properties of selfing and SI populations in Brassicaceae model systems, we aim to improve our understanding of the evolutionary forces shaping mating system variation.