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Shared ITS DNA substitutions in isolates of opposite mating type reveal a recombining history for three presumed asexual species in the filamentous ascomycete genus Alternaria

Published online by Cambridge University Press:  13 May 2003

Mary L. BERBEE
Affiliation:
Department of Botany, 6270 University Boulevard, University of British Columbia, Vancouver, BC, Canada V6T 1Z4. E-mail: berbee@interchange.ubc.ca
Brendan P. PAYNE
Affiliation:
Department of Botany, 6270 University Boulevard, University of British Columbia, Vancouver, BC, Canada V6T 1Z4. E-mail: berbee@interchange.ubc.ca
Guojuan ZHANG
Affiliation:
Department of Botany, 6270 University Boulevard, University of British Columbia, Vancouver, BC, Canada V6T 1Z4. E-mail: berbee@interchange.ubc.ca
Rodney G. ROBERTS
Affiliation:
USDA, ARS, Tree Fruit Research Laboratory, 1104 North Western Ave. Wenatchee, WA 98801, USA.
B. Gillian TURGEON
Affiliation:
Department of Plant Pathology, 334 Plant Science Building, Cornell University, Ithaca, NY 14853, USA.
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Abstract

About 15000 species of ascomycete fungi lack a known sexual state. For fungi with asexual states in the anamorph genera Embellisia, Ulocladium, and Alternaria, six species have known sexual states but more than 50 species do not. In sexual filamentous ascomycetes, opposite mating type information at the MAT1 locus regulates mating and the opposite mating type genes each have a clonal, non-recombining phylogenetic history. We used PCR to amplify and sequence fragments of the opposite mating type genes from three supposedly asexual species, A. brassicae, A. brassicicola and A. tenuissima. Each haploid fungal isolate had just one mating type, but both mating types were present in all the three species. We sequenced the ribosomal ITS regions for isolates of opposite mating type, for the three asexual species and four known related sexual species. In a phylogenetic analysis including other ITS sequences from GenBank®, the three asexual species were not closely related to any of the known sexual species. Isolates of opposite mating type but the same species had identical ITS sequences. During any period of asexual evolutionary history, lineages of each mating type would have had a separate evolutionary history and any ITS substitutions shared between isolates of opposite mating type would have had to accumulate by convergence. Allowing for varying substitution rates and assuming a Poisson distribution of substitutions, the probability that isolates of opposite mating type shared an ITS substitution through convergence was low. This suggests that isolates of opposite mating type of A. brassicae, A. brassicicola and A. tenuissima were exchanging substitutions through sexual or parasexual reproduction while the ITS was evolving. If sexuality was lost, it was lost after the period of evolutionary history represented by the shared substitutions.

Type
Research Article
Copyright
© The British Mycological Society 2003

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