Neocallimastigomycota: history, habitat, and LifeCycle

Neocallimastigomycota is a phylum containing anaerobic fungi, which are symbionts found in the digestive tracts of herbivores.

History of Neocallimastigomycota:

These ruminal anaerobic fungi, reported as early as in 1910, were first recognized as a flagellated Protozoa and placed in the genera Callimastix, Sphaeromonas, and Oikomonas.

This flagellated Protozoa first recognized as fungi for the first time in the 1970s with the first-named species Neocallimastix frontalis.

The flagellate zoospores encyst and germinate on ingested forge with reading rhizoids that produce a single zoosporangium.

In terms of lifecycle and morphology, N. frontalis is similar to members of Chytridiomycota and its fungal affinities are conformed by chitin in the cell wall.

Though uniquely among the fungi it is an obligate anaerobe.

About twenty different species of rumen fungi have been reported in various ruminant and hindgut-fermenting mammals.

It is established that removal of these fungi from the rumen, results in a significant diminution in in-vitro gas production and degradation of fibrous feed. Significantly these fungi play a vital role, such as in fiber degradation.

The enzyme profile of various fungi studies indicated secretion of a wide range of lignocellulolytic enzymes.

Scanning electron microscopic studies ascertained that these fungi preferably attach to lignified tissues of plant feed.

Hence, fiber-based diets stimulate their proliferation in the rumen compared to diets rich in easily fermentable carbohydrates.

Pelleted diets generally have a shorter transit time through the gastrointestinal tract and therefore do not support good anaerobic growth of rumen fungi in-situ.

Habitat of phylum Neocallimastigomycota:

Fungi include in the phylum Neocallimastigomycota have been reported in all geographic regions of the world. Being ubiquitous among foregut fermenters and ruminants such as cattle, buffalo, goat, red deer, impala, and wild Blue bull (Boselaphus tragocamelus).

These fungi have also been isolated from a faecal sample of the Horse, Zebra, Rhinoceros, and Indian Elephant. All being hindgut fermenters.

Therefore these fungi appear to be a standard constituent of the gut microflora in many herbivores fed on a highly fibrous diet.

Taxonomic classification of Neocallimastigomycota:

Anaerobic zoospore producing fungi are recently assigned to Chytridiomycota, a basal group within kingdom Fungi.

Based on the ultrastructural characteristics of zoospores, anaerobic fungi were originally placed in Order Spizellomycetales. But later transferred to a separate order Neocallimastigales by Li et al in 1993.

The precise relatedness of the Neocallimastigales to other chytrid fungi is at present unclear since they possess several features not common with other chytrid taxa.

Here is some feature which are not common with the other chytrid taxa.

Hydrogenosomes, Polyflagellate zoospores, Distinctive flagellar attachment and are distinctive in other respects too. For example very high AT-rich DNA base ratio approx.70%.

The recent international collaborative effort to establish multiple gene genealogies for kingdom Fungi. Found Chytridiomycota to be polyphyletic but confirmed anaerobic fungi basal to the ‘core’ chytrid clade.

Based on many distinctive features of anaerobic fungi relative to other chytrids, these may be assigned to a new phylum Neocallimastigomycota containing a single class Neocallimastigomycetes.

 In 1988 Gold et al recommended subdivision of anaerobic fungi into three genera. However, at this time there are a total of 18 genera with more complex growth morphology have subsequently been discovered i.e. 

1) Agriosomyces
(Hanafy et al., 2020)
7) Cyllamyces
(Ozkose et al. 2001)
13) Oontomyces
(Dagar et al. 2015)
2) Aklioshbomyces
(Hanafy et al. 2020)
8) Feramyces
(Hanafy RA et al. 2018)
14) Orpinomyces
(Barr et al. 1989)
3) Anaeromyces
(Breton et al. 1990)
9) Ghazallomyces
(Hanafy et al. 2020)
15) Pecoramyces
(Hanafy et al. 2017)
4) Buwchfawromyces
(Callaghan & G.W. Griff. 2015)
10) Joblinomyces
(Hanafy et al. 2020)
16) Piromyces
(Gold et al. 1988)
5) Caecomyces
(J.J. Gold. 1988)
11) Khoyolomyces
(Hanafy et al. 2020)
17) Tahromyces
(Hanafy et al. 2020)
6) Capellomyces
(Hanafy et al. 2020)
12) Nocallimastix
(Vavara & joyon 1983)
18) Liebetanzomyces
(Dagar et al 2018)
Reference: Taxonomy browser NCBI

Presently 37 species of anaerobic fungi have been identified and described. No anaerobic fungi have hitherto been reported to have a sexual stage.

The Life Cycle:

The life cycle of this fungi is asexual and shifts between a motile, zoosporic, and a vegetative, zoosporangial stage.

The flagellate zoospores move by chemotaxis to colonize the fiber material. N. patriciarum zoospores show chemotaxis towards several carbohydrates as receptors.

For example: Glucose, Mannose, sorbitol and Sucrose

Then it moves across the plant surface, presumably to stumble on the right location for encystment.

After release, these get encysted and germinate exogenously to form a germ tube from which the rhizoids emerge.

The cell mass develops into a sporangium and cytokinesis results in unnucleated zoospores to be released to complete the cycle.

From studies on Neocallimastix, it is established that the life cycle lasts about 23-32 hours.

Whilst in Cyllamyces aberensis, it is slightly shorter to 18-24 hrs.

Zoospores development from young sporangia may occur within 8 hours after encystment under appropriate conditions.

Unlike monocentric and bulbous taxa, polycentric fungi have fewer determinate lifecycles and can differentiate multiple sporangia over several days.

Nuclei are visible within rhizoids but it is unclear to what extent these resemble hyphae of higher fungi.

Zoosporogenesis is asynchronous as in other taxa and in culture, zoospores are after very rare.

Metabolism and Physiology:

These anaerobic fungi derive energy from anaerobic fermentation of carbohydrates.

A large number of poly-, oligo- and monosaccharides including glucose, cellobiose, fructose, maltose, sucrose, and xylose support their growth.

Anaerobic fungi follow a mixed acid fermentation profile similar to Enterobacteria such as E. Coli with the conversation of hexose to acetate, formate, lactate, succinate, ethanol, CO2 and H2.

These products may fluctuate among different genera that are high malate and lactate by Anaeromyces spp. Compared to Orpinomyces.

Anaerobic fungi lack mitochondria, cytochrome, and other biochemical features of the oxidative phosphorylation pathway.

In the cytosol, all major enzymes required for glycolysis through the Embden-Meyerhof-Parnas pathway are present. While glucose-6-phosphate dehydrogenase and the other enzyme of Entner- Dodouroff pathway are absent.

The group possesses organelles for a major part of anaerobic energy metabolism.

Hydrogenosomes are spherical double-membrane bound redox organelles and have been reported in phylogenetically distant amitochondriate eukaryotes. That inhabit anaerobic or microaerophilic environments.

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