CHYTRIDIOMICOTA: CHARACTERISTICS, NUTRITION, HABITAT AND REPRODUCTION - BIOLOGY - 2025

Chytridiomicota is a phylum that groups microscopic flagellate spore fungi. They are possibly the ancestral group of the higher fungi. Its taxonomy is complicated due to the plasticity of its forms under different growing conditions.

They are nourished by absorption. There are saprophytes and parasites, some of which are pathogens on crops such as potatoes and others wreak havoc on amphibian populations. They inhabit the ground and also in both fresh and salty waters. Some obligate anaerobic species inhabit the digestive tracts of herbivores.

Electron microscope micrograph of zoospore and sporangia of chytrido Batrachochytrium dendrobatidis causing chytridiomycosis in amphibians. Author: Dr. Alex Hyatt, CSIRO. Reused from hscienceimage.csiro.au

They reproduce asexually by zoospores, giving rise to a haploid phase in which ciliated gametes are produced. Then by plasmogamy and karyogamy a diploid phase is generated that gives rise to the sporothal.

General characteristics

They are mostly filamentous fungi with multinucleated hyphae without septa (cenocytic). Although there are also unicellular or unicellular species with rhizoids (false roots with absorption function formed by short filaments without a nucleus).

Their cell walls are composed of chitin, although cellulose also exists in some species. They do not form colonies of branched hyphae (mycelia) as it does in non-flagellated fungi. They produce multinucleated spheroidal bodies called thalli.

The thalli are the feeding structures that later transform into sporrangia. The sporangia is a thin-walled, sac-like structure in the protoplasm of which zoospores are formed.

The Chytridiomicotas present very varied forms of structure of the thallus and of the reproductive organs. Some species have holocarpic thallus (it transforms completely into sporrangia).

Other forms show eukarpic thalli (it maintains vegetative and reproductive functions), and can be monocentric (a sporrangia with rhizoids) or polycentric (several sporrangia and rhizoids). The simplest forms are endoparasites.

Zoospores

The most relevant of these fungi is the production of mobile cells: flagellate spores and ciliated gametes. Zoospores are cells without a cell wall, usually 2 to 10 μm in diameter that contain a single nucleus.

With the exception of some genera of the Neocallimastigales, zoospores are propelled by a single smooth posteriorly oriented flagellum.

The zoospore provides important characters for the classification of flagellate fungi. Thanks to the use of the electron microscope, several peculiar structures were detected. Among these are: the rumposome and the nuclear cap.

Release of spores generally occurs through uncapped (uncapped) pores or discharge tubes and rarely through capped openings.

In zoospores there is a set of fenestrated or non-fenestrated cistern membranes called a rumposome, whose function is not known. The nuclear cap is an aggregation of ribosomes attached to the nucleus and covered by a prolongation of the nuclear membrane.

Parasites in animals

Chytridiomycosis is a disease that affects amphibians, especially toads and frogs. It is caused by the fungus Batrachochytrium dendrobatidis, the only Chytridiomicota known to be a parasite of a vertebrate.

The skin of animals becomes infected when they come into contact with waters where zoospores of the fungus are found. In some cases this fungus exterminated 100% of the frog population and is even the cause of the extinction of four species of frogs in Australia.

It is thought that the enzymes secreted by the fungus affect the epidermal structure, preventing the various functions of this tissue in the frog (osmotic regulation, water absorption, respiration), which ends up being fatal in most cases.

Species of the genus Coelomyces parasitize mosquito larvae, which is why they are important for the biological control of these insects.

Parasites in plants

There are also several species of obligate parasitic Chytridiomycotes of a wide variety of vascular plants. Species of the genus Synchytrium attack potato crops.

Synchytrium endobioticum is an endobiotic fungus that produces the so-called “black potato wart”, an important disease in this crop. The genus Olpidium includes species that affect various cucurbitaceae.

This disease is caused when zoospores penetrate the tuber tissue, invading the cells and using them as reproductive receptors.

The reproductive cycle repeats itself over and over again in potato tissue causing cell proliferation. The tubers acquire a warty appearance, similar to the proliferation of flower buds.

Observation and study

Due to their microscopic size, Chytridiomycotes cannot be observed directly in nature. They can only be detected by microscopic analysis of tissues or plant remains, the skin of amphibians affected by some species of these fungi, water or soil samples.

Specialists in these fungi have developed special techniques to collect them and later cultivate them in the laboratory on artificial media.

Phylogeny and taxonomy

The Chytridiomicota phylum is a monophyletic group considered one of the basal lineages of the evolutionary tree of the Fungi kingdom. It comprises a single class (Chytridiomycetes), traditionally divided into five orders: Chytridiales, Spizellomycetales, Blastocladiales, Monoblepharidales and Neocallimastigales.

The criteria for the separation of these orders are mainly the mode of reproduction and the ultrastructure of the zoospore. It includes about 1000 species.

Monoblofaridales are oogamic (immobile female gamete fertilized by a mobile male gamete). The Blastocladiales present sporic meiosis and alternation of sporophytic and gametophytic generations.

Chytridiales are characterized by their zygotic meiosis (haploid individuals). Spizellomycetales are defined by peculiar ultrastructural characters.

Neocallimastigales exclusively include anaerobic symbionts from the rumen of herbivores and with multiflagellate zoospores.

Taxonomic changes

Recently it was proposed to segregate Blastocladiales and Neocallimastigales from this group, elevating them to the category of edge. While Monoblepharidales is reduced to the category of class.

At the same time a new order of Chytridiomicota was postulated: Lobulomycetales. This new order is based mainly on genetic analysis, to a lesser extent on ultrastructural data and morphology.

Nutrition

They feed by absorbing the substrate through their rhizoids. These structures secrete enzymes that digest the substrate and the substances migrate towards the absorption hyphae. There are saprophytes and parasites.

Habitat

Chytridiomycotics are classified according to the position they occupy with respect to the substrate: on the substrate (epibiotics) or within the substrate (endobiotics).

They live in the soil in decomposing organic matter, on the surface of plants or animals and also in water. They are located from tropical areas to the Arctic.

Some species are parasitic on other fungi, such as algae, plankton, and vascular plants, as well as animals.

In fresh water they are found in streams, ponds, and estuaries. In marine ecosystems mainly as parasites of algae and components of plankton. Chytridiomycot species are found from coastal sediments to depths of 10,000 m.

Perhaps most Chytridiomicota species are found in terrestrial habitats such as forests, agricultural and desert soils, and in acid swamps as saprotrophs of refractory substrates such as pollen, chitin, keratin, and cellulose in the soil.

Some species inhabit the interior of plant tissues as obligate parasites; both in leaves, stems and roots.

The species of the order Neocallimastigales live in the rumen of herbivorous animals. Due to their ability to degrade cellulose, they play an important role in rumen metabolism.

Reproduction

The life cycle of Chytridiomycos has an asexual and a sexual phase.

Asexual phase

In the asexual phase, reproduction can be planogametic. From a sporangium of resistance or dormancy, zoospores are produced, that is, haploid, mobile flagellate spores.

Zoospores swim free for a time and become cysts that germinate under favorable conditions give rise to a gametotalus.

Here the male and female gametangia are formed, which will give rise to the respective planogametes, starting the sexual phase. These gametes having cilia are mobile.

Sexual phase

In the sexual phase, by fusion of cytoplasms (plasmogamy) and subsequent fusion of the nuclei (karyogamy) of the planogametes, the zygote is formed. This is where the diploid phase of the cycle begins.

The zygote may take the form of a resistance cyst or transform into a diploid cenocytic thallus (sporothal).

The sporotheliums give rise to resistance sporrangia that form haploid zoospores, thus closing the cycle. From the sporothelium, zoosporangia can also form that produce diploid zoospores that encyst and, when germinating, form new sporotheliums.

Sexual reproduction can also occur by oogamy, as is the case in the order Monoblefaridales. Here it is an immobile female gamete located in the oogonium that receives the mobile flagellated male gamete.

Life cycles in parasitic Chytridiomycotics

Parasitic Chytriomycotes usually have a simpler cycle.

In parasitic plant fungi, such as Olpidium viciae, the zygote is mobile and infects the host plant. Resistance sporrangia develop inside the plant cells. Within these sporrangia occurs karyogamy.

Finally germination occurs and zoospores are released. Zoospores can also directly infect the host plant.

In the fungus Batrachochytrium dendrobatidis, a parasite on the skin of frogs, after a mobility period of less than 24 hours, the zoospores reabsorb their flagella and encyst. They then germinate into rhizoids that cling to the frog's skin.

The thallus develops into a sporangia that gives rise to a new generation of zoospores. The sexual phase is still unknown.

References

1. Australian Government. Department of Sustainability, Environment, Water, Population and Communities (2013) Chytridiomycosis (Amphibian chytrid fungus disease)
2. Berger, L, A. Hyatt, R Speare, and J. Longcore (2005) Life cycle stages of the amphibian chytrid Batrachochytrium dendrobatidis. Diseases of aquatic organisms Vol. 68: 51–63.
3. James TY, P Letcher, JE Longcore, SE Mozley-Standridge, D Porter, MJ Powell, GW Griffith and R Vilgalys (2006) A molecular phylogeny of the flagellated fungi (Chytridiomycota) and description of a new phylum (Blastocladiomycota). Mycologia 98: 860–871.
4. Manohar C, Sumathi and C Raghukuma (2013) Fungal diversity from various marine habitats deduced through culture-independent studies. FEMS Microbiol Lett 341: 69-78.
5. Money, N (2016). Fungi Diversity. In: Watkinson, S; Boddy, L. and Money, N (ed.) The fungi. Third edition. Academic Press, Elsiever. Oxford, UK.
6. Simmons, D, T Rabern, Y James, AF Meyer, and JE Longcore (2009) Lobulomycetales, a new order in the Chytridiomycota. Mycological Research 113: 450-460.