| In order to elucidate the mechanisms by which fungi are able to cause disease on a susceptible host, it is important to understand how the pathogen recognizes specific host-derived signals, penetrates physical barriers, overcomes host defenses, and proliferates on or in the invaded host tissues.; This dissertation focuses on two ascomycete fungi: a plant pathogenic fungus Nectria haematococca mating population (MP) VI (anamorph: Fusarium solani), and an entomopathogenic fungus Metarhizium anisopliae, isolates ARSEF 2575, 324 and 820. The plant-pathogen provides a model for the molecular dissection of signal transduction cascades involved in pathogenicity. In the first part of this work, I studied the modulation/regulation of the cAMP pathway in N. haematococca.; In the second half, I focused on the molecular characterization of the protease genes produced by various M. anisopliae strains, which have three-times more genes encoding subtilisin-like proteases than any other fungus so far characterized. I studied the role of gene duplication and divergence with reference to the subtilisin proteases in genetically and ecologically distinct strains of M. anisopliae var. anisopliae (ARSEF 2575 and 820) and M. anisopliae var. acridium (ARSEF 324). Comparisons were made between subtilisin sequences from the strains of M. anisopliae to elucidate the relationships among these genes and the divergence of orthologs through recent selection on different hosts. By comparing ratios of non-synonymous and synonymous substitutions I determined whether this diversification takes place by strong selective pressure or by genetic drift, i.e., fixation of neutral mutations. Consequently, I studied the diversification of introns by comparing the evolutionary patterns of exons and introns in the same genes in different strains of Metarhizium. I calculated mutation rates and assessed the consequences of functional constraints on purifying selection at non-synonymous exon sites. This has helped in the understanding of the gene duplication events leading to this multigene family and its effect on the evolution of this fungus and of ascomycetes as a group.; In summary, it appears that the divergence of subtilisin genes in M. anisopliae took place after the split between plant and insect-infecting pyrenomycetes, reflecting an adaptation to the parasitic mode of life. |
