To analyse the natural variation at a disease resistance gene locus in Arabidopsis. The chromosome 4 locus specifying resistance to the Peronospora parasitica isolate Cala2 will be examined in a large number of host accessions. Resistance genes from two accessions will be sequenced and compared to determine the degree of sequence divergence that occurs between functional alleles. Additionalyy, the corresponding susceptibility loci will be sequenced and compared. Current evidence is that susceptibility is caused by loss of the gene, but non-functional structural homologues could exist in other accessions. The universality of this observation will be tested and the loci sequenced. Analysis of the deleted regions could reveal the cause of this genomic instability. Sequence analysis of non-functional homologues will reveal the structures that are essential for recognition of the Cala2 pathogen isolate. Finally, the project will reveal the current evolutionary state of this gene locus in 100 UK accessions of Arabidopsis and give clues as to its stability in natural plant populations. To become familiar with the techniques involved in the molecular and classical analysis of disease resistance genes. This will envable me to assess strategies for manipulating disease resistance in crop plants. All these skills be useful to me in my future research career in Greece.