In this project, population genetic and evolutionary aspects of interspecific hybridisation within the oomycetous genus Phytophthora will be linked with biological properties (i.e. pathogenicity and host range). The proposed research project will mainly focus on Phytophthora infestans and the closely related species P. mirabilis. Genetic marker studies on isolates of both species obtained from their primary centre of diversity in central Mexico will provide insight in the genetic architecture of both pathogen populations while indirect measures of gene flow estimate the amount of gene flow that occurs in nature. Interspecific hybrids between P. infestans and P. mirabilis will be generated in vitro. A genetic analysis of host specifity will be performed on gentically characterised F1, F2 and BC1 progeny on a range of potentiial host plants. Improving our understanding of the genetic basis of host range in plant parthogenic Phytophthora specis may be a valuable asset in risk assessment of plant pathogens. A molecular phylogeny of the genus Phytophthora will be constructed by sequence analysis of coding regions of both mitochondrial (mtDNA) and nuclear (nDNA) DNA. In total, five regions will be analyzed for DNA-sequence homology. Phylogenetic data will be used to compare evolution rates and pattersn of nDNA and mtDNA. They will be used to explore the possibility to link sexual, morphological and pathogenicity traits to clades, and so to study the emergence/evolution of niche preference, heterothallism/homothallism and specialization vs. broadening of host range. A fine grain phylogenetic study will be made for P. infestans and Phytophthora species closely related to the late blight pathogen. This study will provide insight in the possible primary and secondary centres of origin and diversity of P. infestans. for this closely related group of pathogens, radiation patterns and the possibility of relict species in the south American Andes will be inferred from sequence homology and marker diversity. The objectives of this project are: (i) quantification of gentic diversity in natural populations of the closely related (sister) species P. infestans and P. mirabilis and the occurrence of gene flow; (ii) a gentic analysis of host specificity in interspecific Phytophthora hybrids; (iii) reconstruction of the molecular evolution of the genus Phytophthora by means of a phylogentic approach using both nDNA coding sequences and (iv)to assess the tracks of evolution of P. infestans and several related plant pathogeneic species that are present in central and south America.