Evolution of marine mushrooms

Fungi make up one of the most diverse, ecologically important groups of eukaryotes. The vast majority of fungi are terrestrial, but the chytridiomycetes, a basal group of fungi, includes flagellated, unicellular, aquatic forms, and it is likely that this was the ancestral condition of the group of (1). The more derived groups of fungi-zygomycetes, ascomycetes, and basidiomycetes-are all predominantly filamentous and terrestrial, and lack flagellated cells at any stage of the life cycle. Within the basidiomycetes, the most conspicuous group is the homobasidiomycetes, which includes about 13,000 described species of mushrooms and related forms. Eleven species of homobasidiomycetes (in eight genera) occur in marine or freshwater habitats. To resolve the relationships among terrestrial and aquatic homobasidiomycetes, we assembled a data set of ribosomal DNA (rDNA) sequences that includes 5 aquatic species and 40 terrestrial species. Phylogenetic trees obtained using parsimony and maximum likelihood (AIL) methods suggest that there have been three or four independent transitions from terrestrial to aquatic habitats within the homobasidiomycetes. Three of the marine taxa in our data set are associated with mangroves, suggesting that these ecosystems provide a common evolutionary stepping-stone by which homobasidiomycetes have reinvaded aquatic habitats.