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The classification and physiology of the zoosporic plant-pathogen Olpidium brassicae and its relationships with the closely-related species are often confusing. This review focuses on these species and intends to differentiate them based on the literatures published since the discovery and establishment of the species by Woronin in 1878 under the name of Chytridium brassicae to current molecular era. The goal of this review is to help researchers better understand the taxonomy, the host range, and the potential role in plant health of O. brassicae-related species. To reach the goal, we reviewed the rationales behind the creation or reduction in synonymy of the different names for O. brassicae and its allied species in order to elucidate the evolution of the species concept on them based on the traditional morphological studies. Furthermore, the studies by molecular biology methods improve our knowledge and perspectives on O. brassicae and its host specificity. In particular, we clarify the differences between O. brassicae and Olpidium virulentus, and propose potential new research avenues. We therefore hope that this review will give a better perspective on Olpidium spp. and their potential role in the root microbiome of plants in natural environments and in agricultural settings.

Chytridium olla A. Braun, the first described chytrid and an obligate algal parasite, is the type for the genus and thus the foundation of family Chytridiaceae, order Chytridiales, class Chytridiomycetes and phylum Chytridiomycota. Chytridium olla was isolated in coculture with its host, Oedogonium capilliforme. DNA was extracted from the coculture, and 18S, 28S and ITS1-5.8S-ITS2 rDNA were amplified with universal fungal primers. Free swimming zoospores and zoospores in mature sporangia were examined with electron microscopy. Molecular analyses placed C. olla in a clade in Chytridiales with isolates of Chytridium lagenaria and Phlyctochytrium planicorne. Ultrastructural analysis revealed C. olla to have a Group II-type zoospore, previously described for Chytridium lagenaria and Phlyctochytrium planicorne. On the basis of zoospore ultrastructure, family Chytridiaceae is emended to include the type of Chytridium and other species with a Group II-type zoospore, and the new family Chytriomycetaceae is delineated to include members of Chytridiales with a Group I-type zoospore.

Podochytrium dentatum occurred during spring and fall on chitin bait placed in a Maine lake. Zoospore ultrastructure is of the chytridialean type and is a variant of the Chytridium subtype. The zoosporangium forms from an apical expansion of the encysted zoospore. At maturity the dentate apical part of the zoosporangium, with orange contents, is separated from the basal part of the zoosporangium by a wall. Zoospores discharge in a mass surrounded by papillar material. Rhizoids branch from a knob-like rhizoidal base and are either fine or coarse.

Entophlyctis luteolus, a new species of sap- rophytic Chytridiales from aquatic habitats, develops endogenously or exogenously on nutrient agar and usually has two rhizoidal axes, which are slightly swol- len at the base. Zoospores emerge en masse from the sporangium after deliquescence of the tip of the dis- charge tube and are surrounded by gelatinous mate- rial. Resistant sporangia form on onion skin. This is the first report of a species with monocentric, exog- enous development that has a Chytridium subtype of zoospore. Other monocentric, exogenously develop? ing species that have been studied have a Nowakows- kiella subtype of zoospore. Exogenous, monocentric development cannot be considered a homologous character state in all species of the Chytridiales.

The operculate genus Chytridium Braun is emended and restricted to approximately 25 known species and several incompletely known members which lack an apophysis and develop much like species of Rhizophydium, insofar as the zoosporangium and absorbing or rhizoidal system are concerned, and form endobiotic or intramatrical resting spores. This interpretation corresponds fairly closely to Braun's diagnosis of the type species. A new genus, Diplochytridium, is established to segregate the former Chytridium species with an endobiotic or intramatrical apophysis or prosporangium and resting spores, which may develop sexually or asexually. Some of these species have a well-defined endo-exogenous alternation of growth and development in which the apophysis appears to function as a prosporangium. As interpreted here, Diplochytridium includes approximately 20 of the known species. Another new genus, Canteria, is created for a parasite of Mougeotia which Canter first described as a species of Phlyctidium but later found to develop endobiotic resting spores or zygospores by the fusion of conjugation tubes.

Chytridium aggregatum occurs as a saprophyte on dead and decaying filaments of Spirogyra crassa, Cladophora sp., and Oedogonium sp. in New Jersey and New York and has been cultured to a limited ext...

Water, soil, and alga samples collected on Ross Island and the dry valleys of Victoria Land during January and February, 1971, were found to contain monocentric chytridiaceous fungi, a blastocladiaceous fungus, and filamentous Oomycetes. Of the alga-infesting chytrids, only Scherffelliomyces appendiculatus and Chytridium versatile could be identified with certainty. Saprophytic chytrids which grew on baits were represented by Rhizophlyctis rosea, and two new species, Rhizophydium proliferum and Phlyctochytrium recurvastomum. Catenophlyctis variabilis occurred as a saprophyte of keratinous substrata. The Oomycetes were represented by Pythium tenue and an undetermined sterile species of Pythium. Aside from unverified observations of Aphanomyces, members of the Saprolegniaceae were lacking. The low diversity of aquatic Phycomycetes with frequent reoccurrence of a few species is noted and commented upon.

DURING RECENT YEARS there has been increasing interest in the use of nutritional characters in tracing phylogenetic relationships among the aquatic Phycomycetes. In 1950, Cantino gathered together all the important nutritional data on the water molds and proposed a tentative phylogenetic arrangement of orders based partly on nutritional characters interpreted in terms of Lwoff's (cf. 1932, 1944) hypothesis of regressive physiological evolution. Although Cantino considered speculation along these lines to be promising, he pointed out that the nutritional information available was not sufficient to warrant *any far-reaching generalizations. Because the order Chytridiales holds a key position in the phylogeny of the uniflagellate water molds, and because the physiology of relatively few of its species had been studied, an investigation was begun on the nutrition of two chytrids, Chytridiwn sp. and Macrochytrium botrydioides Minden. No member of either genus had been studied previously from a nutritional point of view. The principal accounts of nutritional studies in the Chytridiales are those of Whiffen (1941), Stanier (1942), Quantz (1943), Ajello (1948), and Haskins and Weston (1950). These reports present valuable information on species belonging to ten genera, but Rhizophlyctis rosea is the only species that has been comprehensively investigated thus far (cf. particularly Stanier, 1942). ORGANISMS AND METHODS.-In the spring of 1947, D. F. V. Ranzoni made collections of skeletonized leaves and debris from a stream near Pope Valley, California. Water cultures inoculated with this material were baited with autoclaved grass blades and placed in a window box where the temperature ranged between 10? .and 15?C. It was on the grass blades in one of these cultures that Dr. Ralph Emerson, almost a year later, discovered the Chytridium sp. that was used in the study reported here. Macrachytrium was also found on grass blades, in a water culture that had been inoculated with plant debris which Dr. Ranzoni had collected in the Middle River, seven miles west of Stockton, California, in the spring of 1950. Since the original discovery of Macrochytrium by Minden in 1902, it has been reported by only two other investigators, Lund (1934) and Kanouse (in Sparrow, 1943, p. 368). The first pure cultures of these organisms on

In a recent note (1937) I pointed out that the spelling of Gobi's (1879) algal genus Asterocytis differs slightly from de Wildeman's (1893) chytrid genus Asterocystis and that there were no orthographic grounds for substituting Olpidiaster for the latter, as Pascher (1917) had maintained. Shortly thereafter Arwidsson (1938), however, showed that Berkeley and Broome (1875) had used the name Astrocystis for a group of ascomycetes, and since Asterocystis is but an orthographic variation of the latter, Berkeley and Broome's genus has priority over de Wildeman's. Pascher's viewpoint has thus been confirmed, but from other evidence than that which he presented. In a study of the early mycological literature I have found another case of priority among the chytrid genera. In 1855 Braun established Phlyctidium as a sub-genus of Chytridium to include non-operculate species whose intramatrical absorbing system consists of an unbranched peg, knob, or short filament. A few years later Rabenhorst (1868) raised it to generic rank, and since that time it has been recognized as such by most students of the chytrids. A survey of the literature, however, shows that Wallroth used this name for a group of ascomycetes as early as 1833, and since Phlyctidium was thus established subsequent to Fries (1821-1832) Systema mycologicum, it has status and validity under the present rules of nomenclature. Braun was apparently unaware of Wallroth's genus, for in none of his writings have I been able to find any reference to it. The thirteen ascomycetous species included by Wallroth in Phlyctidium have since been gradually transferred to other genera as far as I am aware, and the genus is no longer recognized among the ascomycetes except as a tautonymy. As has been pointed out by numerous investigators, it is very doubtful whether the chytrids included at present in Phlyctidium are sufficiently different from those of Rhizophidium to justify a 286

Catenochytridium hemicysti is a monocentric chytrid with exogenous-endogenous development. A prosporangium develops inside the germ tube and the incipient sporangium develops from the outflow of protoplasm through the zoospore cyst. The zoospore cyst ruptures longitudinally and remains appended to the base of the sporangium. Sporangia are operculate, the single operculum is flexuous and folds up or collapses. Zoospores are released as a globose mass, and almost at once drift apart before swimming away. C. hemicysti utilizes glucose, cellobiose, sucrose, lactose, galactose, starch, cellophane (cellulose), and xylan (hemicellulose). It utilizes organic, ammonium and nitrate nitrogen, and it requires thiamine. It has a zoospore similar to Nowakowskiella, Cladochytrium and Allochytridium. The systematic relationship of Catenochytridium to Chytridium and Diplochytridium is discussed.

SUMMARY Single-sporangium isolates of Chytridium ottariensis, Chytriomyces hyalinus, Phlyctochytrium powhatanensis, and Rhizophydium sphaerocarpum have a stable type of thallus development regardless of the growth medium or substrate used. However, medium is highly influential on the size range of sporangia and resting spores. Thallus growth on standard chytrid baits is epi-endobiotic and also interbiotic; resting spores produced on these baits occur on or in the substrate. Sporangium and resting spore size ranges for 17 isolates of C. hyalinus on a given medium were not significantly different. Apophysis shape within any one strain on any medium was highly variable and ranged from spherical, clavate, elongate, to catenulate. Both lid and flap opercula occurred in five of the 17 strains but in the remaining 12 the opercula were either flap or lid but not both. Location of thallus, position of resting spore, and presence of catenulate apophyses were not constant within any strain for any medium or substrate. In an exploration of species concepts in the chytrids, it becomes immediately apparent that the constancy of the characters used to delineate families, genera, and species needs verification. The stability of many characters used for separation of genera has not been established since very few species of chytrids have been studied in axenic culture or tested for variability on natural substrates. Some indications of the morphologic plasticity of these organisms are found in recent

In the course of an examination of conjugating filaments of Spirogyra Weberi Czurda, collected in a roadside ditch near Seymour, Wisconsin, in the early summer of 1938, a rhizidiaceous fungus was found parasitizing the zygospores but not the vegetative cells. Because of its characteristic aculeated zoosporangia and its general habit of growth the organism was recognized as one described by Blytt in 1882 as Chytridium spinulosum. Study of the method of zoospore discharge and the appearance of empty zo6sporangia revealed that the apical structure, taken by Blytt to be an operculum of the Chytridium type, is actually an apical ornamentation similar in certain respects to the apical spine of Obelidium. It plays no part in zo6spore discharge since the spores escape through a sub-apical exit pore. On the basis of this difference it seems necessary to separate this species from Chytridium as the type of a new genus. The name Blyttiomyces is thus proposed in commemoration of the man who first collected this interesting fungus. Since Blytt (1882) was unable to see zoospore discharge in his material, he concluded from the general appearance and location of the sporangial apiculus that its function was that of an operculum. This, in addition to the intramatricality of the resting spores, led him to conclude that he was dealing with an undescribed species of Chytridium, and he accordingly described the chytrid under the binomial of Chytridium spinulosum. The present observations are not the first to show the inoperculate nature of the sporangium; it was recognized by Petersen (1910) since he described its dehiscence by a lateral orifice. It was also recognized later by Scherffel (1926) since he stated clearly that the apical 558