Population genomic studies of mating system transitions

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Our lab investigates the evolution and genetics of reproductive systems in plants. This project proposes to extend our knowledge of the evolutionary biology of self-incompatibility systems. Self-incompatibility is the major mechanism by which plants avoid inbreeding. The loss of self-incompatibility is hypothesized to be the first step in the evolution of self-pollination, an evolutionary transition that is among the most common in plants. The proposed research focuses on the genus Leavenworthia, where there have been multiple mating system transitions. It centers on the S-locus, which consists of two tightly linked genes, SRK (which functions in the recognition of self-pollen) and SCR (which codes for a ligand recognized by the SRK product). The locus has recently been cloned in several mustard species, and our lab has extended these results to Leavenworthia. The genome of one species of Leavenworthia (L. alabamica) has recently been sequenced. We propose to build upon this progress to address the following questions: (1) Do S-locus mutations occur at low frequencies in self-incompatible populations? (2) When self-compatibility evolves, is it generally due to mutations in SCR or to mutations SRK? (3) What is the nature and strength of selection on mutations at the S-locus that have given rise to populations with self-compatibility? (4) Why do selfing taxa appear to be short-lived in evolutionary time? We will conduct high throughput re-sequencing of samples from populations, detailed molecular analysis of S-locus mutations, expression analyses of the S-locus, and field experiments. Studies of the genetic basis of selfing have so far been restricted to a few taxa. It is not known to what extent mutations at single loci lead to selfing and what types of mutations are selected. The proposed work will be among the first studies to assess the strength of selection acting on mutations that cause selfing, and to adopt a whole genome level approach to investigating how the evolution of selfing influences genome evolution. The issues addressed are important to understanding the evolution and long-term prospects of selfing taxa, which represent a significant proportion of flowering plants and a disproportionate number of crop species.