A molecular phylogeny of the flagellated fungi (Chytridiomycota) and description of a new phylum (Blastocladiomycota)

Abstract

Chytridiomycota (chytrids) is the only phylum of true Fungi that reproduces with motile spores (zoospores). Chytrids currently are classified into five orders based on habitat, zoospore characters and life cycles. In this paper we estimate the phylogeny of the chytrids with DNA sequences from the ribosomal RNA operon (18S+ 5.8S+ 28S subunits). To our surprise the morphologically reduced parasites Olpidium and Rozella comprise two entirely new, and separate, lineages on the fungal tree. Olpidium brassicae groups among the Zygomycota, and Rozella spp. are the earliest branch to diverge in the fungal kingdom. The phylogeny also suggests that Chytridiomycota is not monophyletic and there are four major lineages of chytrids: Rozella spp., Olpidium brassicae, the Blastocladiales and a “core chytrid clade” containing the remaining orders and families and the majority of flagellated fungi. Within the core chytrid group 11 subclades can be identified, each of which correlates well with zoospore ultrastructure or morphology. We provide a synopsis of each clade and its morphological circumscription. The Blastocladiales appears to be the sister taxon of most nonflagellated fungi. Based on molecular phylogenetic and ultrastructural characters this order is elevated to a phylum, the Blastocladiomycota.

• Blastocladiomycota
• Chytridiales
• holocarpic
• kinetosome
• phylogeny
• zoospore ultrastructure

We thank Alison Mao for diligent technical assistance with DNA sequencing. Material in the form of cultures and herbarium material used for these studies was kindly provided by Robert Campbell, John W. Taylor and Howard Whisler. We thank Will Blackwell for nomenclatural assistance and Gary Easton, Marilyn R.N. Mollicone and D’Ann Rochon for providing photos. We thank Carol Nichols for her help preparing the ultrastructural schematics. Financial support was provided by the National Science Foundation through the Assembling the Fungal Tree of Life grant (AFTOL, award DEB-0228668) and the Partnership for Enhancing Expertise in Taxonomy grants (PEET, awards DEB-9978094 and 0529694). This collaborative project would not have been possible without additional support from the Research Coordination Network: A Phylogeny for Kingdom Fungi grant (Deep Hypha) by NSF 0090301 to M. Blackwell, J.W. Spatafora and J.W. Taylor.