The Chytridiomycota is a phylum that basically derived lineages in the kingdom fungi. Informally they are known as Chytrids.
Most chytrids grow aerobically in soil, mud, or water and reproduce by zoospores with a single posterior flagellum of the whiplash type.
Classification and Evolution of Chytridiomycota
What we now know as the Chytridiomycota, along with zoospore-producing fungus-like organisms such as oomycetes, thraustochytrids, and hyphochytrids.
At one time were classified as Phycomycetes and later as the Mastigomycotina.
Until 2007, the Chytridiomycota included the Blastocladiomycetes and Neocallimastigomycetes.
But because of their ultrastructure, physiological, and molecular differences, most authors separate the latter two classes from Chytridiomycota as phylum Blastocladiomycota and Neocallimastigomycota.
Here, we include the classes Chytridiomycetes and Monoblepharidomycetes in the Chytridiomycota.
Their position within the fungi was postulated based on chitin in their cell wall composition, mitochondria with plate-like cristae, and synthesizing lysin by the α-amino adipic acid pathway.
Their identity as true fungi are supported by deoxyribonucleic acid (DNA) analysis.
Member of phylum Chytridiomycota are known as the most primitive fungi.
Because fossil of this fungal phylum has been reported from the 40o million-year-old Rhynie chert. A site is known for the discovery of the fossil remains of the earliest known vascular land plants.
Clusters of holocarpic, endobiotic thalli resembling the present-day Olpidium have been found inside cells of a coenobial alga.
Chytrid-like fossils have also been found in strata of the 340 million-year-old Pennsylvanian (Carboniferous) era and from the more recent Eocene strata.
Formally though to have an affinity for the Oomycota, Hyphochytriomycota, or Protists, the Chytridiomycota are now accepted as a member of the true fungi, the Eumycota.
They are probably ancestral to other group of true fungi, especially the zoosporic fungi.
The inclusion of the chytrids in the Eumycota is supported by several DNA- Based phylogenetic analyses, but the delimitation of orders within the Chytridiomycota is still problematic.
Habitat of Chytridiomycota
Member of the Chytridiomycota are mainly the water inhabiting fungi.
Zoospores require free water for dispersal, but due to this reason chytrids are not limited to lakes, streams, ponds, and other aquatic habitats.
Free water is resent in soils at certain times and many chytrids have short life cycles and drought-resistant resting stages that make them well adapted to live in terrestrial habitats.
Although some chytrids occur in marine habitats, most are inhabitants of freshwater and soils.
Scientists found many of the first- named chytrids growing on freshwater algae.
Other natural substrates for chytrids include pollen, cast exuviae of aquatic insects, protozoans and small invertebrates, other fungi, moribund pieces of plants, and for monoblephs, fruits, and waterlogged twigs.
Chytridiomycota Life Cycle:
Most chytrids have haploid zoospores and thalli but some show an alteration of haploid (gametothallic) and diploid (sporothallic) generation.
Apart from differences in the reproductive organs, the morphology of the two types of the thallus is very similar, a phenomenon known as isomorphic alteration of generations.
Sexual reproduction, that is a life cycle which includes nuclear fusion and meiosis, may occur in several different ways.
In some chytrids it is by gametogamy, the fusion of gametes which are posteriorly uniflagellate.
Isogamous conjugation occurs if there is no morphological distinction between the two fusion partners.
Oogamy, a fusion between an actively motile male gamete and a much larger, non- flagellate, immobile globose egg, is characteristic of members of class Monoblepharidomycetes.
The fusion of undifferentiated hyphae or rhizoids
Somatogamy, the fusion of undifferentiated hyphae or rhizoids, has been well documented in cultures of the freshwater fungus Chytriomyces hyalinus.
Chytriomyces hyalinus are released from the zoosporangium by the opening of lid -like operculum.
They germinate to form uninucleate rhizoidal thalli and the tips of the rhizoids from adjacent thalli, which are apparently not genetically distinct from each other, may fuse.
At the point of the fusion, an incipient resting body develops and swells while the cytoplasm and a nucleus migrate into it from each contributory thallus.
Nuclear fusion occurs in the resting body to form a diploid zygote nucleus. The resting body continues to enlarge and develops a thick wall.
This type of sexual reproduction by somatogamous conjugation probably occurs in several genera of inoperculate and operculate chytrids.
Fusion of gametangia
The fusion of gametangia (gametangio-gametangiogamy) has been reported in Zygorhizidium planktonicum, a parasite of the diatom Synedra.
This species reproduces asexually by epibiotic zoosporangia.
Germinating zoospores develop either new zoosporangial thalli or gametangial thalli of two sizes with globose uninucleate gametangia.
Conjugation occurs when a conjugation tube grows from the smaller donor to the larger recipient gametangium.
Following nuclear fusion, the larger gametangium develops a thick wall and functions as a diploid resting spore.
After a period of maturation, the resting spore act as a prosporangium, giving rise to a thin-walled meiosporangium.
Meiosis, as evidenced by the presence of synaptonemal complexes, occurs here, followed by mitosis and cytoplasmic cleavage to form zoospores.
A variant of this form of sexual differentiation has been reported in species of Rhizophydium; this involves copulation between the gametangium of the rhizoid- forming thallus and a motile gamete that encysts directly on the gametangium.
Generally, the product of sexual reproduction is a resting spore or resting sporangium with thick walls, but it is known that thick-walled sporangia may also develop asexually and in many chytrids, sexual reproduction has not been described and possibly does not occur.
Resting sporangia of some chytrids may remain viable for many years.
The thallus (body) of Chytridiomycota usually consists of one or more reproductive centers and nutrient- gathering structures.
The reproductive structure is usually a zoosporangium, within which zoospores form; however, it also can be a sexually or asexually formed resting spore.
The nutrient-gathering structures of many chytrids are rhizoids, which are anucleate root-like structures.
Some chytrids lack rhizoids and absorb nutrient through the developing sporangial wall.
Other species are mycelial, with coenocytic hyphae that look much like those of members of other fungi phyla.
Some species have a fine mycelium, sometime called rhizomycelium, in which nuclei occupy swellings.
A chytrid whose entire thallus comprises the zoosporangium (without rhizoid or any other nonreproductive structures) is holocarpic.
Whereas a thallus that has nonreproductive structure is described as being eucarpic.
A thallus with a single reproductive center is monocentric and one with multiple centers of reproduction is polycentric.
Other terms specific to the Chytridiomycota include apophysis, which is a descriptive term for a sub sporangial swelling of the major rhizoidal axis, and apiculus, which is the term for an unexplained portion of a zoospore cyst that remains on the zoosporangium of some species.
These terms are convenient descriptors, but relationship should not be inferred buy their use.
For example, an apophysis is simply a swollen rhizoid and such structures probably arose independently in many chytrid lineages.
Likewise, Chytridiomycota orders contain polycentric species, but this growth does not necessarily indicate a relationship.
The type of discharge apparatus through which zoospores release is important in the taxonomic description of members of the Chytridiomycetes.
Scientists divided taxa into two series based on whether their member discharged zoospores through an operculate or inoperculate opening.
An operculum a cap-like lid, and species were placed in the operculate series whether this lid was a distinct cap or simply a flap formed by a tear in the sporangial wall.
Some researchers considered species that formed a lid below hyaline material in the discharge tube as being operculate and others considered them inoperculate.
This debate no longer has scientific merit, although operculate can be a valuable character at the species or genus level, molecular analysis shows that it does not have the phylogenetic importance once accorded to it.
Examples of Chytridiomycota:
The Chytridiomycota are commonly called chytrids. Some examples of Chytridiomycetes are:
Synchytrium endobioticum, a pathogen of potato it causes black wart in potato.
Batrachochytrium dendrobatidis, which cause chytridiomycosis that is implicated in the global decline in amphibian population and extinction of multiple species
Physoderma sp., which cause the crown wart of alfalfa and the brown spot disease of corn.