Molecular taxonomy, origins and evolution of freshwater ascomycetes

Abstract

Fungi are the most diverse and ecologically important group of eukaryotes with the majority occurring in terrestrial habitats. Even though fewer numbers have been isolated from freshwater habitats, fungi growing on submerged substrates exhibit great diversity, belonging to widely differing lineages. Fungal biodiversity surveys in the tropics have resulted in a marked increase in the numbers of fungi known from aquatic habitats. Furthermore, dominant fungi from aquatic habitats have been isolated only from this milieu. This paper reviews research that has been carried out on tropical lignicolous freshwater ascomycetes over the past decade. It illustrates their diversity and discusses their role in freshwater habitats. This review also questions, why certain ascomycetes are better adapted to freshwater habitats. Their ability to degrade waterlogged wood and superior dispersal/ attachment strategies give freshwater ascomycetes a competitive advantage in freshwater environments over their terrestrial counterparts. Theories regarding the origin of freshwater ascomycetes have largely been based on ecological findings. In this study, phylogenetic analysis is used to establish their evolutionary origins. Phylogenetic analysis of the small subunit ribosomal DNA (18S rDNA) sequences coupled with bayesian relaxed-clock methods are used to date the origin of freshwater fungi and also test their relationships with their terrestrial counterparts. Phylogenies indicate that freshwater ascomycetes have evolved from terrestrial fungi and appear to occur in only three classes. The adaptation to populate freshwater substrates has occurred in several lineages. The earliest possible date, when fungi became adapted to freshwater habitation is estimated at 390 million years ago (MYA).