Pendulichytrium sphaericum gen. et sp. nov. (Chytridiales,Chytriomycetaceae), a new chytrid parasitic on the diatom,Aulacoseira granulata

During the last decade, our concept of chytrid systematics has dramatically changed based on molecular phylogeny and zoospore ultrastructure. In contrast with well-studied saprotrophic chytrids, only a few obligate parasitic chytrids have been investigated with modern methods. Here, we investigate the novel chytrid culture KS93 that is parasitic on the diatom Aulacoseira granulata. Thallus morphology of KS93 was characterized by a spherical, stalked zoosporangium with a single, apical inoperculate discharge pore and zoospore discharge as a mass in a vesicle. A cross-inoculation experiment revealed that the infection of KS93 was specific to A. granulata. Zoospores of KS93 possessed the characters of the Group I type zoospore of the Chytriomycetaceae in the Chytridiales, but does not appear to have a paracrystalline inclusion. Additionally, KS93 also possessed a globule-type KAS (kinetosome-associated structure), first reported here for members of Chytriomycetaceae. In our molecular phylogeny, KS93 was placed in the basal position of the Chytriomycetaceae and was distinguished from any known species in the family. Morphological features of KS93 were distinct from those of any other taxa in the Chytriomycetaceae and from any described chytrids. Based on these results, we describe this chytrid as Pendulichytrium sphaericum gen. et sp. nov. in the family Chytriomycetaceae.     

HOST PARASITE INTERACTIONS BETWEEN FRESHWATER PHYTOPLANKTON AND CHYTRID FUNGI (CHYTRIDIOMYCOTA)1

Some chytrids are host‐specific parasiticfungithat may have a considerable impact on phytoplankton dynamics. The phylum Chytridiomycota contains one class, the Chytridiomycetes, and is composed of five different orders. Molecular studies now firmly place the Chytridiomycota within the fungal kingdom. Chytrids are characterized by having zoospores, a motile stage in their life cycle. Zoospores are attracted to the host cell by specific signals. No single physical–chemical factor has been found that fully explains the dynamics of chytrid epidemics in the field. Fungal periodicity was primarily related to host cell density. The absence of aggregated distributions of chytrids on their hosts suggested that their hosts did not vary in their susceptibility to infection. A parasite can only become epidemic when it grows faster than the host. Therefore, it has been suggested that epidemics in phytoplankton populations arise when growth conditions for the host are unfavorable. No support for such a generalization was found, however. Growth of the parasitic fungus Rhizophydium planktonicum Canter emend, parasitic on the diatom Asterionella formosa Hassal, was reduced under stringent nutrient limitation,because production and infectivity of zoospores were affected negatively. A moderate phosphorous or light limitation favored epidemic development, however. Chytrid infections have been shown to affect competition between their algal hosts and in this way altered phytoplankton succession. There is potential for coevolution between Asterionella and the chytrid Zygorhizidium planktonicum Canter based on clear reciprocal fitness costs, absence of overall infective parasite strains, and possibly a genetic basis for host susceptibility and parasite infectivity.     

Effects of phosphorus limitation on the epidemiology of a chytrid phytoplankton parasite

SUMMARY 1. The effect of phosphorus limitation of the diatom Asterionella formosa Hass. on growth, survival and epidemic development of its fungal parasite Rhizophydium planktonicum Canter emend. was estimated, using measurements of production and infectivity of the zoospores of the chytrid grown on host cultures with different phosphorus-limited growth rates.
2. Phosphorus-limited host cells were less susceptible to infection with zoospores of the parasite than non-limited host cells.
3. The sporangia on phosphorus-limited algae produced substantially less zoospores, but the development time of these sporangia was only slightly reduced.
4. As a result of these effects, Rhizophydium will reach lower growth rates at a given host density, and survival of the parasite will require higher host densities when Asterionella is phosphorus-limited.
5. The zoospore production remained high enough to enable the parasite to grow faster than the alga at sufficiently high host densities, both at limiting and non-limiting phosphorus levels.
6. In spite of the reduced growth rate of the parasite, phosphorus limitation of Asterionella was found to facilitate the development of a Rhizophydium epidemic. This was a consequence of the reduced algal growth rate at phosphorus limitation, which makes the host population more easily outgrown by the parasite.
7. Phosphorus limitation of the host could reduce the threshold host density required for the development of an epidemic by a factor of 2.5.     

PARASITISM OF ASTERIONELLA FORMOSA HASS. BY A CHYTRID IN TWO LAKES OF THE RAWAH WILD AREA OF COLORADO1

The populations of Asterionella formosa Hass. in two subalpine lakes in the Rawah Wild Area, Colorado, were composed of colonies of five distinct size classes based on frustule length. The spring bloom in these two lakes was dominated by cells of the Beta or Gamma population, whereas the fall pulse was dominated by cells of the Alpha and Beta, or Alpha populations. The Beta population was physiologically unique in that it was parasitized by the chytrid Rhizophidium planktonicum Canter. The development of the parasite population could not be correlated with the concentration or the growth phase of the host population. The occurrrence of parasitism did not noticeiceably affect the population growth patterns of the Asterionella populations.     

FUNGI FROM THE LOWER DEVONIAN RHYNIE CHERT: CHYTRIDIOMYCETES

Several different chytridiomycetes are described from the Lower Devonian (Siegenian) Rhynie chert. Included are both eucarpic and apparently holocarpic forms that occur in Palaeonitella, Aglaophyton, Lyonophyton, Horneophyton, and clusters of algal cells, as well as in the surrounding chert matrix. Holocarpic types consist of endobiotic sporangia, each characterized by one discharge tube. Sporangia can be traced from the thallus stage to the discharge of zoospores. Monocentric and polycentric eucarpic chytrids are associated with the miospores of Aglaophyton and various thick-walled fungal spores. In these forms the sporangia are variable in size and shape ranging up to 30 μm. Most appear to be inoperculate and there is evidence that the sporangium ruptured on the distal surface. Some contain zoospores with flagella. One operculate eucarpic form had parasitized the cellular gametophyte emerging from the proximal surface of an Aglaophyton spore. Several of the Rhynie chert chytrids are comparable with a number of extant forms (e.g., Olpidiaceae and Spizellomycetaceae), while others possess features that encompass several groups. These fossil fungi are discussed in the context of their interactions with other organisms in this Lower Devonian freshwater paleoecosystem.     

PARASITISM OF ASTERIONELLA FORMOSA HASS. BY A CHYTRID IN TWO LAKES OF THE RAWAH WILD AREA OF COLORADO1

The populations of Asterionella formosa Hass. in two subalpine lakes in the Rawah Wild Area, Colorado, were composed of colonies of five distinct size classes Based on frustule length. The spring bloom in these two lakes was dominated by cells of the Beta or Gamma population, whereas the fall pulse was dominated by cells of the Alpha and Beta, or Alpha populations. The Beta population was physiologically unique in that it was parasitized by the chytrid Rhizophidium planktonicum Canter. The development of the parasite population could not be correlated with the concentration or the growth phase of the host population. The occurrence of parasitism did not noticeably affect the population growth patterns of the Asterionella populations.     

Phylogeny of Powellomycetaceae fam. nov. and description of Geranomyces variabilis gen. et comb. nov

The genus Powellomyces was described to accommodate two monocentric chytrid species from soil that develop exogenously and possess zoosporic ultrastructure similar to other members of the order Spizellomycetales. Despite Powellomyces-like chytrids being commonly observed in gross culture, the genus contained only two species. To determine diversity in this group I amassed 49 isolates of Powellomyces-like chytrids, including the cultures upon which species types were based and new isolates from pollen-baited water cultures of soils, plant detritus and manure. I sequenced portions of nucSSU and nucLSU rDNA regions and the EF-1α-like gene from each isolate to produce a molecular phylogeny. This phylogeny supports monophyly of spizellomycetalean chytrids with exogenous development and suggests that multiple distinct lineages exist within this group. This phylogeny, along with a reevaluation of the ultra-structural features of the two described species, supports the recognition of a new family, Powellomycetaceae, and genus, Geranomyces, which contains 31 isolates of G. variabilis comb. nov.     

The Isolation, Maintenance and Host Range Studies of a Chytrid Rhizophydium planktonicum Canter emend., Parasitic on Asterionella formosa Hassall

The methods of isolation and maintenance in dual clone cultureof the chytrid Rhizophydium planktonicum Canter emend, parasiticon Asterionella formosa Hass. from the plankton are described.The ability of a single fungal isolate to infect other clonesof Asterionella, Fragilaria, Tabellaria, Synedra and Cyclotella,as well as dead material was also tested. All the clones of Asterionella proved to be highly susceptibleto infection whereas only in rare instances could a sporangiumbe found which had grown and dehisced on Fragilaria and Synedra.Very few zoospores encysted on Tabellaria and Cyclotella andthose which did died without further growth. There was no evidenceto suggest that the chytrid could complete its life historyon dead material. freshwater phytoplankton, culture, parasitism, host–parasite relationships, chytridiomycetes, Rhizophydium, Zygorhizidium, diatoms, Asterionella, Fragilaria     

A Further Study on Parasitism of the Diatom Fragilaria crotonensis Kitton by Chytridiaceous Fungi in Culture

Further detailed experiments under laboratory conditions havebeen carried out using clones of Fragilaria crotonensis Kitton.Two morphologically distinct types (rod- and flared-cell forms)were investigated in relation to their infection by a chytridreferred to as species 3. The pathogenicity of the chytrid Rhizophydiumfragilariae Canter towards four newly isolated clones of F.crotonensis was also studied. The results in general confirmedthat clones of the diatom composed of flared-type cells werecompatible with the development and increase of species 3 butremained more or less resistant to R. fragilariae. In contrast,rod-celled type clones showed exactly the opposite host parasiterelationships. Further study has shown that slight differencesin susceptibility towards a particular chytrid can occur withindiatom clones of similar morphological type. The presence ofhypersensitive algal cells was noted in relation to parasitismby both fungi. Some observations are recorded on the developmentof mucilage around filaments of the Fragilaria clones. Fragilaria crotonensis Kitton, Rhizophydium fragilariae Canter, chytrid, diatom, culture, parasitism, hypersensitivity, mucilage envelope     

Fungal Parasitism: Life Cycle,Dynamics and Impact on Cyanobacterial Blooms

Many species of phytoplankton are susceptible to parasitism by fungi from the phylum Chytridiomycota (i.e. chytrids). However, few studies have reported the effects of fungal parasites on filamentous cyanobacterial blooms. To investigate the missing components of bloom ecosystems, we examined an entire field bloom of the cyanobacterium Anabaena macrospora for evidence of chytrid infection in a productive freshwater lake, using a high resolution sampling strategy. A. macrospora was infected by two species of the genus Rhizosiphon which have similar life cycles but differed in their infective regimes depending on the cellular niches offered by their host. R. crassum infected both vegetative cells and akinetes while R. akinetum infected only akinetes. A tentative reconstruction of the developmental stages suggested that the life cycle of R. crassum was completed in about 3 days. The infection affected 6% of total cells (and 4% of akinètes), spread over a maximum of 17% of the filaments of cyanobacteria, in which 60% of the cells could be parasitized. Furthermore, chytrids may reduce the length of filaments of Anabaena macrospora significantly by “mechanistic fragmentation” following infection. All these results suggest that chytrid parasitism is one of the driving factors involved in the decline of a cyanobacteria blooms, by direct mortality of parasitized cells and indirectly by the mechanistic fragmentation, which could weaken the resistance of A. macrospora to grazing.     

Comparison of sterol and fatty acid profiles of chytrids and their hosts reveals trophic upgrading of nutritionally inadequate phytoplankton by fungal parasites

Chytrids are ubiquitous fungal parasites in aquatic ecosystems, infecting representatives of all major phytoplankton groups. They repack carbon from inedible phytoplankton hosts into easily ingested chytrid propagules (zoospores), rendering this carbon accessible to zooplankton. Grazing on zoospores may circumvent bottlenecks in carbon transfer imposed by the dominance of inedible or poorly nutritious phytoplankton (mycoloop). We explored qualitative aspects of the mycoloop by analysing lipid profiles (fatty acids, sterols) of two chytrids infecting two major bloom-forming phytoplankton taxa of contrasting nutritional value: the diatom Asterionella formosa and the filamentous cyanobacterium Planktothrix agardhii. The polyunsaturated fatty acid composition of chytrids largely reflected that of their hosts, highlighting their role as conveyors of otherwise inaccessible essential lipids to higher trophic levels. We also showed that chytrids are capable of synthesizing sterols, thus providing a source of these essential nutrients for grazers even when sterols are absent in their phytoplankton hosts. Our findings reveal novel qualitative facets of the mycoloop, showing that parasitic chytrids, in addition to making carbon and essential lipids available from inedible sources, also upgrade their host's biochemical composition by producing sterols de novo, thereby enhancing carbon and energy fluxes in aquatic food webs.     

Some Observation on the Alga Fragilaria crotonensis Kitton and its Parasitism by Two Chytridiaceous Fungi

It has been shown by experimentation that two morphologicallydistinct clones of Fragilaria crotonensis Kitton, isolated fromthe plankton of Windermere at approximately the same time, wereaffected quite differently by the chytrid Rhizophydium fragilariaeCanter. In one case the diatom filaments became heavily parasitizedand could be used for the maintenance of the fungus. In theother case infection was negligible. Additional clones, representative of these two morphologicalforms, from other bodies of water in the Windermere drainagebasin were tested. The same pattern of infection or relativenon-infection again ensued in direct accordance with the particulartype of filament used. Some preliminary experiments undertaken with a second, as yetlittle known chytrid parasite of F. crotonensis are reported.In this instance the fungus appeared to find its natural hoston cells belonging to the morphological form which failed tosupport the continued increase of R. fragilariae. The parasitism of morphological variants of F. crotonensis bythese fungi in the wild from Britain and else where is discussed. Fragilaria crotonensis Kitton, Rhizophydium fragilariae Canter, chytrid, diatom, parasitism, culture     

Phylogenetic Position of Parasitic Chytrids on Diatoms: Characterization of a Novel Clade in Chytridiomycota

Chytrids are true fungi that reproduce with posteriorly uniflagellate zoospores. In the last decade, environmental DNA surveys revealed a large number of uncultured chytrids as well as undescribed order‐level novel clades in Chytridiomycota. Although many species have been morphologically described, only some DNA sequence data of parasitic chytrids are available from the database. We herein discuss five cultures of parasitic chytrids on diatoms Aulacoseira spp. and Asterionella formosa. In order to identify the chytrids examined, thallus morphologies were observed using light microscopy. We also conducted a phylogenetic analysis using 18S, 5.8S, and 28S rDNA sequences to obtain their phylogenetic positions. Based on their morphological characteristics, two cultures parasitic on As. formosa were identified as Rhizophydium planktonicum and Zygorhizidium planktonicum. The other three cultures infecting Aulacoseira spp. (two on Aulacoseira ambigua and the other on Aulacoseira granulata) were regarded as Zygorhizidium aff. melosirae. The results of the molecular phylogenetic analysis revealed that R. planktonicum belonged to the known order Chytridiales, while the two species of Zygorhizidium were placed in a novel clade that was previously reported as an undescribed clade composed of only the environmental sequences of uncultured chytrids.     

Experimental transmission of cutaneous chytridiomycosis in dendrobatid frogs

In a series of three experiments during March-October, 1998, two species of captive-bred poison dart frogs (Dendrobates tinctorius and D. auratus) were exposed to Batrachochytrium dendrobatidis, a recently-described chytridiomycete fungus (chytrid) that was originally isolated from a blue poison dart frog (D. azureus). All frogs exposed to the chytrids developed a fatal skin disease, whereas none of the control frogs developed skin lesions. The most consistent clinical sign in chytrid-exposed frogs was excessive shedding of skin. Gross lesions were subtle, usually affected the legs and ventrum, and consisted of mild skin thickening and discoloration. Microscopic examination of shed skin pieces and/or skin imprints demonstrated the presence of chytrids and was used for ante mortem and post mortem confirmation of chytrid infection. Histologically, there was epidermal hyperkeratosis, hyperplasia, and hypertrophy associated with low to moderate numbers of chytrids in the keratinized layers. These experiments demonstrated that Batrachochytrium dendrobatidis can be a fatal pathogen in poison dart frogs. The experimentally-induced disease in these frogs resembled cases of cutaneous chytridiomycosis that have recently been described in several other species of captive and wild amphibians.     

STUDIES OF THE MOTILE CELLS OF CHYTRIDS. III. MAJOR TYPES

Koch , William J. (U. North Carolina, Chapel Hill.) Studies of the motile cells of chytrids, III. Major types. Amer. Jour. Bot. 48(9): 786–788. Illus. 1961.—Six major types of chytrid motile cells are recognized on the basis of the internal parts present and their appearances and arrangements. Eighteen species are placed in these 6 types.     

Adaptation of the fungal parasite Zygorhizidium planktonicum during 200 generations of growth on homogeneous and heterogeneous populations of its host, the diatom Asterionella formosa

We followed adaptation of the chytrid parasite Zygorhizidium planktonicum during 200 generations of growth on its host, the freshwater diatom Asterionella formosa, in a serial passage experiment. Evolution of parasite fitness was assessed both on a homogenous and heterogeneous host population, consisting of respectively a single new and ten different new host strains. These 10 host strains were genetically different and also varied in their initial susceptibility to the parasite. Parasite fitness increased significantly and rapidly on the new, genetically homogenous host population, but remained unaltered during 200 generations of growth on the heterogeneous host population. Enhanced parasite fitness was the result of faster and more efficient transmission, resulting in higher values of R0 (number of secondary infections). Consequently, parasites that evolved within the uniclonal host population infected significantly more of these hosts than did their ancestors. We thus provide experimental evidence for the widely held view that host genetic diversity restricts evolution of parasites and moderates their harmful effects. Genetically uniform host populations are not only at increased risk from fungal epidemics because they all share the same susceptibility, but also because new parasite strains are able to adapt quickly to new host environments and to improve their fitness.     

The Effect of Light and Darkness Upon Infection of Asterionella formosa Hassall by the Chytrid Rhizophydium planktonicum Canter emend

Observations, both experimental and microscopic, indicate thatzoospores of Rhizophydium planktonicum Canter emend., can remainalive, but rarely become adherent upon cells of Asterionellaformosa Hassall under conditions of very low light or in completedarkness. The behaviour patterns of zoospores towards host cells underdarkened conditions were compared with those which took placeunder well illuminated conditions leading to the settlementof zoospores on host cells. The differences noted may help toexplain the lack of zoospores found upon inadequately illuminateddiatom cells. Some evidence suggests that young zoospores lack the abilityto adhere to host cells. After the encystment of zoospores uponAsterionella cells in the light, their further growth and developmentcan continue in darkness. Rhizophydium planktonicum Canter emend., Asterionella formosa Hassall, chytrid, diatom, infra-red illumination, zoospore     

On Chytridium braun,Diplochytridium N. G., and Canteria N. G. (Chytridiales)

Summary 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.