What are these chytrids?
Rhizophydiales (Letcher et al. 2006), formerly designated as the Rhizophydium clade in Chytridiales (James et al. 2000, 2006), is a molecularly monophyletic group and is the largest and most diverse order in Chytridiomycetes (Letcher et al. 2006, 2008b). Thallus morphology among clade members is relatively conserved. The classical genus Rhizophydium Schenk has more than 200 described species characterized (sensu Sparrow 1960) as:
- eucarpic and monocentric;
- with an epibiotic, inoperculate and operculate, uniporous or multiporous sporangium;
- with an endobiotic rhizoidal system originating from a single site on the sporangium;
- with an epibiotic resting spore.
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. Thus 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 notation of 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. For the most recently updated monograph of the genus Rhizophydium, click here.
Members of Rhizophydiales occur world-wide in aquatic systems primarily as parasites of algae and on a rage of organisms including invertebrates and other chytrids (Powell 1993). 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. The highly destructive parasite of frogs, Batrachochytrium dendrobatidis (Longcore 1999), evolved within this clade (Letcher et al. 2004).
In terrestrial habitats, members of Rhizophydiales are common saprobes of pollen grains, but are more rare on keratin, chitin, and cellulose substrates (Sparrow 1943, 1960; Powell 1993). Members of Rhizophydiales have been recovered from acid bogs, nitrogen-deficient soils (Letcher et al. 2004b, c), submersed mud from an oligotrophic lake (Letcher et al. 2008a), and tree canopy detritus (Longcore 2005). The highly destructive and pathogenic parasite of frogs, Batrachochytrium dendrobatidis, evolved within this clade (James et al. 2006). This chytrid has been reported as the infective agent of 100 species of amphibians. 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. An NSF – REVSYS grant supported research focused on phylogenetics of all members in Rhizophydiales and on generating data for comparative analyses of both zoospore ultrastructure and molecular sequences. This research has resulted in the discovery of tremendous genetic diversity within this order.
Recent molecular phylogenetic analyses have revealed remarkable diversity among the Rhizophydiales (Letcher et al. 2008a,b). The Rhizophydiales are terminal representatives in the evolutionary radiation of chytrids (James et al. 2006, Letcher et al. 2004a). Molecular analyses (James et al. 2006, Letcher et al. 2006) include taxa in Rhizophydiales with character states such as operculation, multiple rhizoids, and endobiotic thallus development, features not clasically associated with Rhizophydium and which indicate morphological character convergence among different lineages in Chytridiomycetes. Molecular environmental studies have identified clades that occur in Rhizophydiales with no known described species, signifying unrealized biodiversity (Lefevre et al. 2007). A member of the Rhizophydiales, Kappamyces was the first chytrid genus to be delineated based on combined molecular phylogeny and distinct zoospore architecture (Letcher and Powell 2005).
Current systematic synthesis of the order Rhizophydiales:
The Order Rhizophydiales is comprised of 10 families (F) and 18 genera (G) (Letcher et al. 2004a, 2006, 2008a, b, 2012; Letcher and Powell 2005; Longcore et al. 2011; Powell et al. 2011) listed at the left below. Each family forms a monophyletic group, and each genus has a unique zoospore morphology. Our research substantiates the hypothesis that in chytrid systematics, molecular phylogeny predicts zoospore ultrastructure, and ultrastructure predicts phylogeny (Letcher et al. 2008b). Our current phylogenetic hypothesis based on gene sequence and zoospore ultrastructure analyses is below.
*indicates taxa previously in Rhizophydium
Several taxa that phylogenetically group within the Order Rhizophydiales based on zoospore type or molecular sequences are in need of generic revision, including Rhizophlyctis harderi (Powell and Roychoudhury 1992, Roychoudhury and Powell 1992), Entophlyctis helioformis JEL 326, Phlyctochytrium irregulare (McNitt 1974), Rhizophydium littoreum (Amon 1984), and Rhizophydium aestuarii (Ulken 1972). Although traditionally this group was considered an inoperculate clade, we have now discovered that operculate members also occur in the Rhizophydiales (Letcher et al. 2008b, Powell et al. 2011)
Thirty years ago, Barr recognized one type of zoospore (Group III-type) for Rhizophydium (Barr 1980, Barr and Hadland-Hartmann 1978). It was characterized by a microtubular root composed of two to six microtubules aligned one above the other and the absence of an electron-opaque plug in the base of the flagellum.
Our research has discovered 18 distinct zoospore forms in Rhizophydiales, each of which corresponds to a molecularly-based clade. Each zoospore form in Rhizophydiales is defined on the basis of having a unique suite of ultrastructural characters and character states.
Mitosis and septal structure:
Although examples of mitosis among the chytrid fungi are limited, the structure of centric, intracellular mitosis and of organelles associated with nuclei have been described in developing sporangia of Rhizophydium sphaerotheca (Powell 1980) and Phlyctochytrium irregulare (McNitt 1974). Mitosis in these two chytrids of the Rhizophydiales clade is similar, and is distinct from mitosis in Powellomyces variabilis (Powell 1975), a member of the Spizellomycetales. Thus, this feature may have phylogenetic significance.
Septal structure ranges from thin-walled discs with plasmodesmata to a solid, thickened diaphragm with a thin plate which contains plasmodesmata (Powell and Gillette 1987).
Morphological and ecological features may indicate an organism’s affinity with the Rhizophydiales clade. Sexual reproduction has been reported (reviewed by Sparrow 1960) but not confirmed, leaving in question the occurrence of sexuality in this clade. In members of the Rhizophydiales clade, a primary nucleus is unknown, vesicular discharge of zoospores has not been reported, and swimming zoospores tend to be spherical. These characteristics are in contrast with characters typical of the Chytridiales.
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- James, TY, PM Letcher, JE Longcore, SE Mozley-Standridge, D Porter, MJ Powell, GW Griffith, and R Vilgalys. 2006. A molecular phylogeny of the flagellated Fungi (Chytridiomycota) and description of a new phylum (Blastocladiomycota). Mycologia 98: 860-871.
- Lefevre, E, Bardot, C, Noel, C, Carrias, J, Viscogliosi, E, Amblard, C, Sime-Ngando, T. 2007. Unveiling fungal zooflagellates as members of freshwater picoeukaryotes: evidence from a molecular diversity study in a deep meromictic lake. Environmental Microbiology9: 61–71.
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- Powell MJ. 2011. Operculomyces is a new genus in the order Rhizophydiales. Mycologia 103:854-862. http://www.mycologia.org/content/103/4/854.abstract
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This research is supported through NSF REVSYS: Phylogenetics and Taxonomic Revision of the Rhizophydium Clade (Chytridiomycota) DEB-0516173.