Mating system and the evolutionary dynamics of hybrid zones

The genetic consequences of hybridization and introgression (gene exchange among species) have traditionally considered the genome a single entity. However, the concept of a porous genome suggests differential rates of introgression across the genome as a result of different selective pressures; loci under balancing selection (frequency-dependent selection) are likely to show high rates of introgression, while loci under divergent selection (selection in contrasting directions) will resist introgression. The action of these different selective pressures results in variation in both the level of genetic diversity and divergence across the genome. In this project I will focus on the effect of balancing selection at the self-incompatibility locus and compare this to neutral processes (genomic background) and flower colour genes subject to divergent selection. The Antirrhinum system provides a unique opportunity to test these ideas because it has both well-characterized self-incompatibility and flower colour variation. Interestingly, to date, research on hybridization and mating systems have remained largely separate fields, despite the potential importance of mating system for both the rates and outcomes of hybridization and patterns of genetic variation across the genome. In this project I aim to bring together these distinct fields to assess the role of self-incompatibility in the evolutionary dynamics of hybridization in Antirrhinum by comparing how genetic signatures of selection vary across the genome. Examining the outcomes of selection and how this interacts with hybridization requires an assessment of selection over different evolutionary timescales. I aim to compare the genetic signatures at loci under balancing and divergent selection to background levels (neutral loci) over small (one to several generations), medium (hundreds of generations) and large (thousands of generations) evolutionary timescales. To consider the relative importance of selection vs. drift I will consider this for specific demographic and ecological contexts. This data can provide important insights into two long-standing and fundamental questions in evolutionary biology; what is the role of different forms of selection in structuring genetic variation and what are the factors that contribute to reproductive isolation? To address these questions I will use a combination of molecular genetics, genomics, crossing experiments and ecological approaches. Together, these data will enable a comprehensive examination of the genetic consequences of selection and gene flow across the genome, and whether there is adaptive introgression among species.

Ever since Charles Darwin pondered why flowers show such amazing diversity in colour and structures, biologists have been fascinated by the mechanisms by which flowering plants promote mating among genetically different individuals. One means by which flowers prevent self-pollination is genetically based self-incompatibility. These systems are controlled by two linked genes which show high diversity (to ensure maximum mating types) and populations often share similar mating types (low population differentiation). This can result in the specific genetic signatures of high diversity and low differentiation at the parts of the genome that control self-incompatibility. In contrast, flower colour genes are expected to show different genetic signals, as selection can result in divergence among different species in flower colour genes. This results in low genetic diversity at flower colour loci within a species, but large genetic differences when comparing between species. Here we examine how signals of selection vary at the self-incompatibility locus and flower colour genes in natural populations of two sub-species of Snapdragons (Antirrhinum). This research shows that in line with expectations, these two parts of the genome show contrasting genetic signatures; self- incompatibility loci are highly diverse and there are few differences among populations and species. While flower colour loci have low genetic diversity within species, but show large genetic differences when compared between species. This project provides important insight into how selection can act differently across the genome depending on the form of selection involved. From this we can develop a better understanding of how selection shapes genetic variation in the evolution of new species.