Evolutionary analysis of subsection Magnicellulatae of Podosphaera section Sphaerotheca (Erysiphales) based on the rDNA internal transcribed spacer sequences with special reference to host plants

Abstract

To understand the evolutionary history of subsection Magnicellulatae of Podosphaera section Sphaerotheca, nucleotide sequences of the rDNA internal transcribed spacer region were determined for 79 isolates from 60 host species. With the exception of two species occurring on the Scrophulariaceae, all isolates formed a large, well-defined clade. Genetic diversity among species on the Scrophulariaceae was much larger than the diversity among all other taxa, and the first split of the Magnicellulatae clade was shared by these species. This suggests that the Scrophulariaceae is the earliest host of subsection Magnicellulatae. Isolates from the Asteraceae shared the largest sequence diversity and were represented in all major groups. Most members of the basal groups consisted of isolates from the Asteraceae. This suggests that an early radiation in the large clade occurred on the Asteraceae. Based on the small sequence diversity and placement at the terminal end of the phylogenetic tree of isolates from the Cucurbitaceae and the Fabaceae, it is suggested that the ability of subsection Magnicellulatae to infect these families was acquired more recently. The groupings of fungal taxa by molecular phylogeny showed general agreement with groupings by infectivity, which suggests niche separation caused by host specialization triggers genetic divergence in these fungi.Key words: Ascomycetes, Erysiphaceae, host range, phylogeny, Podosphaera, Sphaerotheca.