Morphological Monograph-

Illustrated Keys to Species of Rhizophydium

Updated Taxonomic
Revision of the Genus Rhizophydium


Contemporary Monograph -
Illustrated Keys to Taxa of the Order Rhizophydiales

Use of Ultrastructural Characters


Biology -
Life History

Habitats and Collection




Other Links





What are the Rhizophydiales?

The Rhizophydiales are zoosporic fungi in the Phylum Chytridiomycota. They reproduce by releasing posteriorly uniflagellate zoospores from sporangia that are attached to substrates or hosts by rhizoids.

Rhizophydiales, formerly designated as the Rhizophydium clade in Chytridiales, is a molecularly monophyletic group and is the largest and most diverse order in Chytridiomycetes.

Thallus morphology among clade members is relatively conserved. The classical genus Rhizophydium Schenk has more than 200 described species characterized as:

Classically described species of Rhizophydium were based on features of the thallus and the nature of the substrate or host the chytrid used for growth.The beginning steps for the identification of a Rhizophydium observed or isolated are: determining the shape of the sporangium, observing characteristics of the zoospore discharge apparatus, and noting what the chytrid was growing on.

Identification of classically defined species, thus, uses keys based on sporangial shape. To explore these keys click on: thallus shape is the primary character used to identify these chytrids with light microscopy. To find the reference for the original description of a Rhizophydium species, click on: literature.

However, we now know that morphology can be deceptive of relationships and more techniques are needed to adequately describe these fungi.

This site summarizes the classical understanding of these chytrids and our contemporary synthesis of relationships discovered with the additional resolving power of molecular and ultrastructural characters. For the updated "Taxonomic Summary and Revision of Rhizophydium" click here.

Why are they important to study?

Members of Rhizophydiales occur world-wide in aquatic systems primarily as parasites of algae and on a range of organisms including invertebrates and other chytrids. They may have a role in the natural control of algal populations and a transformational role in aquatic food webs as parasites of planktonic desmids and diatoms. They are also common in soil, primarily as saprobes of pollen and may have a role in nutrient recycling.

The highly destructive and pathogenic parasite of frogs, Batrachochytrium dendrobatidis, evolved within this clade. This chytrid has been reported as the infective agent of 100 species of amphibians.

What is the NSF Rhizophydiales REVSYS?

To understand the evolution of B. dendrobatidis, as well as that of many algal parasites, it is necessary to understand their radiation within the Rhizophydiales. The NSF REVSYS grant focuses on phylogenetics of all members in Rhizophydiales and on generating data for comparative analyses of both zoospore ultrastructure and molecular sequences.

Once considered a single genus (Rhizophydium) of more than 220 described species represented by a single zoospore morphology, research has resulted in the circumscription of the order Rhizophydiales, which currently encompasses 10 families, 18 genera, and several described species that are family incertae sedis. Transmission electron microscopic observations of more than 300 isolates in Rhizophydiales have revealed 18 distinct zoospore morphologies in the order, each of which contains a unique suite of ultrastructural features.

The end product will be an updated monographic treatment of the group that includes consideration of classically described species and newly discovered species within a phylogenetic framework based on multi-gene sequence analyses and comparative zoospore ultrastructure.

Who are the collaborators on this project?

Martha Powell and Peter Letcher at The University of Alabama have been exploring the range of diversity within the "Rhizophydium clade" for the past decade. For organisms with relatively simple thalli, the group has remarkable genetic diversity. Unexpectantly, zoospore ultrastructural organization and thallus variation have been found to be diverse.

Undertaking such an effort requires collaboration with scientists from around the world. Primary collaborators are Drs. Carlos Velez (Universidad de Buenos Aires), Peter McGee (University of Sydney), Tim James (University of Michigan), Will Blackwell (emeritus, Miami University) and Joyce Longcore (University of Maine). Much of our focus in the past has been on members of Rhizophydiales from terrestrial habitats, and we are now beginning to investigate the biodiversity of the order in aquatic habitats.

In Argentina, for example, we are training students in identification, isolation and culturing of Rhizophydiales that are parasites of algae, other chytrids, and aquatic invertebrates, through collection of chytrids while on field trips to a variety of aquatic habitats,and subsequent culturing and isolation in the laboratory. We are interested in interactions with others excited about discovering more about these fascinating fungi. Rather than being rare organisms, we have found that they are essentially everywhere.

FOR COMMENTS: contact Martha Powell

last update September 26, 2012