While the discovery of hybrid vigor was one of the most important breakthroughs in modern agriculture, failure of many wide crosses limits the beneficial traits that can be introduced into crops. My goal is to understand the molecular and biochemical basis of hybrid incompatibility within, and also between, species. My first focus will be to characterize a particularly interesting incompatibility (identified in the laboratory of Prof. Weigel), in which offspring develop ectopic outgrowths (tumoring) on leaf petioles. The model will be a cross between two Arabidopsis thaliana accessions (strains) from Spain and Tajikistan. Preliminary results suggest that this example of severe negative epistasis is caused by an interaction between two alleles of a disease resistance (R) protein. Once confirmed, I will characterize the interaction at the protein level by domain-swapping and mutagenesis to determine the biochemical mechanisms of the deleterious interaction. A second, more speculative focus will be to assess the effect of intra- and inter-specific hybrid formation on genomic regulation through RNA silencing. I will do this by deep sequencing of the sRNA and transcriptome complements of intra- and inter-specific hybrids in the Arabidopsis genus. The resultant data will be analysed for occurrence of novel sRNAs in hybrid progeny, correlating with down regulation of transcription from the cognate genes or vice versa. Further experiments will be decided on the basis of these initial results and are anticipated to include profiling of DNA methylation at genomic intervals of interest, and analysis of the effects of knock down of RNA silencing pathways on hybrid formation and gene regulation. The two aspects of the proposed work are linked in that they aim to further understanding of hybrid genetic interactions that have been proposed to affect diverse biological phenomena from disease to speciation.