The switch from out-crossing to self-fertilizing (inbreeding) mating system is one of the most prevalent evolutionary trends in plant reproduction. A major goal of the proposed study is to identify and characterize the genes underlying evolutionary switches to self-fertility, and thus understand a major process in the reproductive isolation of plant species. In crucifers, two genes, which encode the S-locus receptor kinase SRK and its ligand the S-locus cysteine-rich peptide SCR, function together to prevent self-pollination and are thus responsible for the self-incompatible (out-crossing) mode of mating. The model plant Arabidopsis thaliana is a highly self-fertile species, but it can be rendered self-incompatible by transfer of these two genes from its self-incompatible relative A. lyrata. Natural populations of A. thaliana were found to exhibit polymorphisms in their SRK/SCR sequences, and to harbor hidden variation for expression of self-incompatibility, which can only be uncovered by transfer of A. lyrata SRK and SCR genes. Genetic and molecular analysis of this variability will allow, for the first time, a study of the genetics of inbreeding in natural populations of a genetically tractable species. The study is expected to provide insight into the changes in mating system that occur during or subsequent to speciation, further our understanding of the molecular mechanism of self-incompatibility, and possibly reveal the evolutionary origins of self-incompatibility and its genes. In the long term, the project should also enhance the ability of breeders to manipulate plant reproduction for crop improvement.
