A parasitic transition from trees to herbs occurred at least twice in tribe Cystotheceae (Erysiphaceae): evidence from nuclear ribosomal DNA

Abstract

To understand the evolutionary history of Erysiphaceae tribe Cystotheceae, phylogentic trees were constructed from the nucleotide sequences of the ribosomal DNA internal transcribed spacer of 28 taxa of the fungal ingroup and two outgroup taxa. The first split of the ingroup taxa occurred between a clade composed of the genus Cystotheca and a clade composed of the genera Podosphaera and Sphaerotheca. Podosphaera and Sphaerotheca did not separate into clades. Instead, Podosphaera species parasitic to Prunus and Sphaerotheca section Magnicellulatae grouped together, with the remaining Podosphaera species and Sphaerotheca section Sphaerotheca forming another subclade. Since the first splits were shared by Podosphaera species in both subclades, it was assumed the ancestral features of both subclades were Podosphaera-like and the genus Sphaerotheca derived from a Podosphaera-like ancestral taxon on at least two independent occasions. The results of this study suggest that ancestral fungi of the tribe Cystotheceae were originally arbor-parasitic, and transition from arbor-parasitism to herb-parasitism may have occurred at least twice independently, accompanied by a morphological change of appendages. The mycelioid appendage of the genus Sphaerotheca and other herb-parasitic genera seems to have evolved convergently at multiple times as an adaptation to herb-parasitism.