Rhizophydium Clade

The Rhizophydium clade (James et al. 2000) is the largest and most diverse clade in the Order Chytridiales. Members of the clade are predominantly from the genus Rhizophydium, with more than 200 described species, which are characterized (sensu Sparrow 1960):

Thallus morphology among clade members is relatively conserved. They occur world-wide in aquatic systems primarily as parasites of algae (Powell 1993), and in soil primarily as saprobes of pollen, and to a lesser extent, chitin and keratin. The highly destructive parasite of frogs, Batrachochytrium dendrobatidis, evolved within this clade (Letcher et al. 2004).

Taxa:

Based on molecular analyses (Letcher et al., 2004) and ultrastructural analyses (Barr and Hadland-Hartmann 1978, Letcher and Powell 2005, Longcore 1999, McNitt 1973, Powell and Roychoudhury 1992, Roychoudhury and Powell 1992), taxa that group within this clade include:

 

Molecular Analyses:

Not all isolates identified as Rhizophydium on the basis of thallus morphology fall into the clade when analyzed with 18S rDNA sequence data (James et al. 2000). More recent studies utilizing 28S rRNA gene sequences (Chambers 2003, Letcher et al. 2004, Letcher and Powell 2005) have revealed remarkable diversity among isolates included in the Rhizophydium clade. Distinct molecular groupings represent new and undescribed genera. Letcher and Powell (2005) circumscribed three isolates in the clade that exhibited significant differences in molecular constitution from all other isolates, thus delineating the genus Kappamyces. Clearly this clade is in need of revision to understand generic and species delineation.

 

Zoospore ultrastructure:

Compared with morphological features, the ultrastructural arrangement of organelles in the chytrid zoospore is conserved, but variation also exists at that level. The Group III-type chytridialean zoospore (Chytridiales sensu Barr) was described for members of the genus Rhizophydium (Barr and Hadland-Hartmann 1978, Barr 1980), and is 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. The Group III-type zoospore exhibits variable ultrastructural morphological character states (Letcher 2003, Letcher et al. 2005, Letcher and Powell 2005) including presence or absence of a :

additional variations in character states are found in the:

In the Rhizophydium lineage, zoospore ultrastructural character states are applicable at the generic level (Letcher and Powell 2005a), but analyses of molecular sequences will be necessary to resolve species.

 

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 Rhizophydium 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).

 

Indicative Morphological and Ecological Features:

Morphological and ecological features may indicate an organism’s affinity with the Rhizophydium 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 Rhizophydium clade, a primary nucleus is unknown, vesicular discharge of zoospores has not been reported, and swimming zoospores tend to be spherical. Members of the Rhizophydium clade are common saprobes of pollen grain and rare on chitinous substata. They also are common in soil, but less so in aquatic habitats.

 

References:


This research is supported through NSF REVSYS: Phylogenetics and Taxonomic Revision of the Rhizophydium Clade (Chytridiomycota) DEB-0516173.

Home