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| Chytrids |
Like all fungi, chytrids live in their food and have an absorptive mode of nutrition in which they secrete digestive enzymes and absorb the breakdown products.
Chytrids also have cell walls made of chitin, make the amino acid lysine via the amino adipic acid (AAA) pathway, and possess a ribosomal DNA (deoxyribonucleic acid) sequence that places them more closely with other fungi than with any other group of organisms. The feature that sets the mapart from other fungi is the possession of a motile zoospore. All other fungi produce spores without flagella.
Characteristics
Aposteriorly oriented, whiplash-type flagellum is the feature that unites all the organisms in the division Chytridiomycota within the kingdom Fungi. As absorptive heterotrophs, they live either as saprophytes, growing on dead organic matter, or as parasites in living plants, other fungi, insects, or algae.
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The vegetative organism may take the form of a spherical structure, with or without branching rhizoids, on the surface of substrate or host or may send mycelial threads through the material in which it is living.
Asexual reproduction occurs by a variety of means described below. Sexual reproduction is known to exist in several types of chytrids and in some species involves the alternation between a gamete-producing phase and a spore-producing phase.
The sporangia that produce the motile zoospores develop in a variety of ways. Two features are used to characterize development: the fate of the nucleus upon encystment of the zoospore and the number of zoosporangia produced from a single zoospore.
The three most common types of thallus development are endogenous-monocentric, exogenous-monocentric, and exogenous-polycentric. Endogenous-monocentric development occurswhen the zoospore nucleus stays within the encysted zoospore wall, undergoes mitosis, and produces a single zoosporangium.
Exogenous-monocentric development occurs when the zoospore nucleus migrates into the germ tube, undergoes mitosis, and produces a single zoosporangium. Exogenous-polycentric development occurs when the zoospore nucleus migrates into the germ tube, undergoes mitosis, and spreads to many locations for zoosporangium production.
The phylum-defining zoospore may be one of four basic morphological types. Though the types are determined by electron microscope, the morphological type can be recognized using light microscopy with experience. The four morphological types are the basis of classification at the ordinal level as described below.
Ecology and Habitats
Because chytrids are absorptive heterotrophs, they grow in their food, digesting complex food molecules and absorbing the simpler breakdown products. When growing in dead material, these fungi are saprophytes and are decomposing organisms in ecosystems. Because the zoospore requires water for dispersal, these fungi are found in aquatic environments.
However, they also can be found in soils that are wet with soil water. Chytrids also can live within living organisms as parasites, causing major declines in populations. The gut chytrids, Neocallimastigales, live in the rumina (stomachal cavities) of herbivorous mammals.
Taxonomy
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| Taxonomy |
Ultrastructure features used in taxonomy include the presence or absence of a connection between the nucleus and the kinetosome by microtubules; whether ribosomes are dispersed or collected into a mass surrounded by membranes; the degree of organization of the microbody-lipid complex (MLC); the location and number of lipid globules; and presence or absence of a rumposome—a honeycomblike organelle of unknown function. The main characteristics of the five orders are described below.
Chytridiales. During examination of the main features of the zoospore—lipid globule, microbody, mitochondria, and nucleus—the nucleus seems to occupy whatever space is left over within the zoospore.
Rootlet microtubules are located within the plasma membrane connecting the kinetosome to the rumposome. Ribosomes are gathered in the center of the cell, enclosed within membranes. In the MLC, the posteriorly located lipid globules are in close association with the microbody, mitochondrian, and rumposome.
Spizellomycetales. The nucleus of the zoospore is close to the kinetosome or, if separated, is connected to it viamicrotubules or a rhizoplast. Rootlet morphology is variable, and ribosomes are scattered throughout the cytoplasm.
The MLC has a loose association of the microbody and lipid at the anterior end of the zoospore with the mitochondria located toward the rear. There is no rumposome. Ribosomes are dispersed throughout the zoospore.
Neocallimastigales. Neocallimastix and other genera of the order are uniflagellate or multiflagellate and live in the rumen of herbivorous mammals. Because they live in this unique environment, rumen chytrids are obligate anaerobes.
The zoospores lack any of the MLC organelles and the rumposome. All these anaerobic fungi are cellulolytic and digest plant cell walls of the food upon which sheep and cattle feed.
Monoblepharidales. The zoospores have a centrally located nucleus that is not connected to the kinetosome. Microtubules extend randomly into the cytoplasm from the kinetosome.
The MLC has a rumposome in close association with a microbody and anteriorly located lipid globules. The ribosomes are centrally located, surrounding the nucleus. These fungi have amycelial growth form and reproduce sexually by producing amotilemale cell and a nonmotile egg cell.
Blastocladiales. A nuclear cap consisting of ribosomes encased within a membrane located anteriorly to a cone-shaped nucleus and a single largemitochondrian with a side body complex are the two most distinctive features of these fungi. Some of these fungi produce mycelial growth forms, whereas others produce the saclike zoosporangium with rhizoids.
Evolutionary History
Evolutionary history of the chytrids can be traced back to the Pennsylvanian period through fossil evidence. Sequential analysis of the small subunit ribosomal DNA gene from fifty-four chytrids indicates that the Chytridiomycota are related to other fungi and that there are natural groups within the division: Blastocladiales, Monoblepharidales, and Neocallimastigales.
Despite the diversity of the data, the monophyletic nature of the Chytridiales and Spizellomycetales is not rejected. The DNAgroupings closely resemble groupings based on zoospore ultrastructure.
Representative Organisms
Allomyces is a mycelial member of the Blastocladiales, which is interesting because it has an alternation of generations between a gamete-producing thallus (gametothallus) and a spore-producing thallus (sporothallus).
In all organisms with alternation of generations, the gametothallus produces gametes by mitosis in gametangia. The gametes are distinguished by size, the male being smaller than the female.
The motile male gamete is chemotactically attracted to the hormone sirenin, which is produced by the female gametes and enables fertilization. Upon fertilization, the zygote nucleus undergoes mitosis as the germ tube develops into mycelia without cross walls.
The dichotomously branched mycelia of the sporothallus produce two types of sporangia. The thin-walled sporangia produce diploid spores by mitosis. These diploid zoospores are responsible for increasing numbers of Allomyces in its habitat.
The sporothallus also can produce a thick-walled sporangium capable of with standing harsh environmental conditions. Zoospores in this sporangium are produced by meiosis. When these haploid zoospores geminate, the nucleus divides by mitosis and spreads throughout the dichotomously branched mycelia. The life cycle of the fungus now is completed.
Batrachochytrium is interesting because it parasitizes frogs. Within the last decade, declines in populations of frogs around the world have been described. Batrachochytrium dendrobatidis is responsible for this chytridiomycosis in amphibians, including salamanders.
Blastocladiella is a developmental biology tool. The thallus has the exogenous, monocentric developmental pathway resulting in a rhizoidal system with a single thin-walled, colorless sporangium or a single thick-walled, resistant sporangium.
The chemical environment of the developing thallus determines which sporangium is produced. High carbon dioxide levels favor the development of the thick-walled sporangium.
This shift from a thin-walled sporangium pathway to a thick-walled sporangium pathway has been traced to a disruption of the Krebs cycle. This organism is one of a few nongreen organisms in which light promotes the growth of the organism.
Coelomomyces is a mycelial member of the Blastocladiales that parasitizes invertebrate animals. Coelomomyces alternates between a haploid gametothallus and a diploid sporothallus.
The unique feature of Coelomomyces is that each phase is specific for a different host. The diploid sporothallus parasitizes mosquitoes and grows as wall-less mycelia within the hemocoel of the mosquito larvae. Coelomomyces has been studied as a possible mycoinsecticide against mosquitoes.
Difficulty in using Coelomomyces as a mycoinsecticide occurred until the discovery of the fact that an alternate host was required to achieve completion of the life cycle. The zoospores produced by the thick-walled sporangium within the mosquito are produced by the process of meiosis and are haploid.
The haploid zoopore must infect a microcrustacean copopod or ostracod in order for the gametes to be produced. The haploid zoospore develops into the gametothallus, which produces the motile gametes. The resulting zygote will infect mosquito larvae, completing the life cycle.
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