Chytridiales is defined on the basis of molecular sequence data and zoospore subtypes (Vélez et al. 2011, Barr 1980). Following the scheme that Dr. D.J.S. Barr proposed based on the ultrastructural architecture of the zoospore (Barr 1980, Barr and Hartmann 1976), the members of this clade have a "Chytridium" type zoospore, either a group I zoospore subtype (Chytriomycetaceae) or a group II zoospore subtype (Chytridiaceae) (Letcher et al. 2005, 2012; Vélez et al. 2011). These zoospore subtypes are characterized by having a cord of microtubules making up the primary root. Recent exploration of zoospore ultrastructure within the Chytridiales has revealed that there are additional subtypes of zoospores in subclades, which are still undersampled (Powell et al. 2013).
In a recent parsimony-based ancestral character state reconstruction, character histories for six zoospore ultrastructural characters were traced. Results indicated that zoospores of the last common ancestor of Chytridiales had: the nucleus located within the ribosomal aggregation, a fenestrated MLC cisterna, a bundle of microtubules composing the microtubular root, a globule as part of the kinetosome-associated structure, and a thick flagellar plug (Letcher and Powell 2014). Polarity of characters states for the paracrystalline inclusion was not resolved. The evouationary trend in each of the lineages within the Chytridiales is from a more complex zoospore ultrastructurally to a less complex zoospore where features are reduced or absent (Letcher and Powell 2014).
In reference to morphology, both monocentric and polycentric thallus forms are represented. The group also contains a mix of operculate and inoperculate members as well as different types of development (exogenous, endogenous) (Letcher et al. 2005, Vélez et al. 2011). Asexual reproduction is by the release of uniflagellate zoospores from sporangia. Sexual reproduction is common in members such as Chytriomyces hyalinus and Siphonaria sp. but overall is rarely reported. Resting spores are thick walled and store abundant lipid and may arise either vegetatively or sexually. (Glossary of Terms)
Chytridales are found in both soil and water, but aquatic members are more numerous in the clade than are soil-inhabiting members to date (Vélez et al. 2011). Certain taxa live exclusively in bodies of water, such as Asterophlyctis and Podochytrium (Longcore 1992a). Others including, Chytriomyces hyalinus, are found in both soil and aquatic habitats. Many members of this order are obligate parasites on algae, including the type species, Chytridium olla (Vélez et al. 2011). Rhizidium phycophilum grows in culture only in the company of a coccoid green alga, suggesting a symbiotic partnership (Picard et al 2009, 2013). Chytridiales phylotypes are commonly retrieved in molecular analyses of lake environmental samples (Lefevre et al. 2012).
Molecular sequence analysis has revealed that a chytrid once classified as Chytriomyces angularis (Longcore 1992b) is not in the Chytridiales clade but is actually a member of a newly described order, Lobulomycetales.
Genera/species placing in the Order Chytridiales based on zoospore ultrastructure and molecular sequence analyses:
Last updated: 1/14/2015