A Thesis Submitted to the Faculty of the DIVISION OF PLANT PATHOLOGY SCHOOL OF PLANT SCIENCES In Partial Fulfillment of the Requirements For the Degree of MASTER OF SCIENCE In the Graduate College THE UNIVERSITY OF ARIZONA

Host specificity in plant pathogens is dictated by the presence of certain genes that enable the plant pathogen to circumvent host defenses leading to pathogenicity. Upon recognition of the pathogen, plants initiate defense responses that can include the production of antimicrobial compounds such as phytoalexins. Nectria haematococca mating population VI (MPVI) is a filamentous ascomycete that contains a cluster of genes known as the pea pathogenicity (PEP) cluster in which the pisatin demethylase (PDA) gene resides. PDA is responsible for the detoxification of the phytoalexin, pisatin, produced by the pea plant (Pisum sativum). Thus, PDA allows the fungus to colonize and become a pea pathogen. It has been proposed that the evolutionary origin of PDA and the PEP cluster is explained by a horizontal gene transfer (HGT) event. The observations supporting this hypothesis include the location of the PEP cluster on a conditionally dispensable (CD) chromosome, the phylogenetically discontinuous distribution of the cluster among closely related species, and bias in GC content and codon usage. This study used a three-pronged approach to test the hypothesis that PDA and the PEP cluster were inherited via HGT. Percent identities of PDA were measured and compared against values from nine putative orthologs that were highly conserved, and two beta-tubulin introns, across F. oxysporum f.sp pisi, Neocosmospora boniensis and N. haematococca MPVI. The GC content of PDA and the PEP cluster was analyzed and compared to those orthologs as well. Finally, phylogenetic analyses based on the orthologous