| Research Abstract |
1.Phylogenetic analyses based on the nucleotide sequences of the nuclear ribosomal DMA revealed that the powdery mildew fungi are divided into five major lineages. The respective lineage was well defined by the morphology of conidial stage, but not of ascomata. In this fungal group, tree-parasitic taxa are generally ancestral, and multiple times of host expansion from trees to herbs have occurred within the respective lineage. Accompanying with the host expansion to herbs, simplification of appendage morphology has occurred multiple times. The simple, mycelioid appendages are thus a result of convergence. Host expansion route was investigated in detail in the tribe Cystotheceae. Two sections of the genus Sphaerotheca were derived from the genus Podosphaera separately. The section Magnicellulatae was derived from a Podosphaera species parasitic to Prunus (Rosaceae), acquired parasitism to the Scrophulariaceae, and then expanded host range into the Asteraceae. After genetic radiation on
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the Asteraceae, they further expanded their host ranges into other plant families. According to the molecular clock (1.26%/100 myr) reported by Berbee and Taylor (2001), splitting of the Erysiphales and the Myxotrichaceae and first divergence within the Erysiphales was calculated to have occurred 100 and 76 myr ago, respectively.
2.Five major groups, each represented by isolates from a single tribe of the Asteraceae, were identified in Golovinomyces taxa analyzed in this study. Host plants of four groups were strictly restricted to the Asteraceae. The fifth group, the Lactuceae group, is a large group composed of isolates collected from the tribe Lactuceae of the Asteraceae and all other plant families, which suggests a close affinity between Golovinomyces and the Asteraceae in the early stages of their evolution. We suggest that there are two different phases in the evolutionary history of the host-parasite relationships of Golovinomyces. One phase is divergence in accord with the phytogeny of their hosts, which occurred within the Asteraceae. The another phase is host-jumping, which occurred from the Asteraceae to other families and within the Asteraceae. Less
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