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The Exobasidiales: An evolutionary hypothesis

Research paper by Dominik Begerow, Robert Bauer, Franz Oberwinkler

Indexed on: 01 May '02Published on: 01 May '02Published in: Mycological Progress



Abstract

To gain insight in the phylogenetic relationships within the Exobasidiales, septal pore apparatus, host-parasite interactions, sori, hymenia, basidia, and nucleotide sequences from the 5′ terminal domain of the nuclear large subunit rRNA gene were studied and compared.The results of our molecular phylogenetic analyses correlate well with the morphological data and both reflect the distribution of parasites on several host groups. Thus, the Exobasidiales seem to be divided into four groups, which are distinguishable by basidial morphology and host range as follows: (i) the Exobasidiaceae parasitizing mainly Ericanae are characterized by an abaxial orientation of the hilar appendices of the ballistosporic basidiospores on the elongate basidia, (ii) the Cryptobasidiaceae occurring mainly on Lauraceae sporulate inside the host tissue with elongate gastroid basidia, (iii) the Brachybasidiaceae living on monocots are characterized by elongate basidia bearing two ballistosporic basidiospores with adaxially oriented hilar appendices, and (iv) the Graphiolaceae occurring on palms produce chains of gastroid basidia in distinct basidiocarps. The arrangement of the four groups and the tree topology within Exobasidium derived from the molecular analyses essentially parallel phylogenetic host relationships, suggesting cospeciation. Based on our results, however, the radiation of Exobasidium on Vaccinioideae cannot be explained by cospeciation alone.