Myxozoa

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Myxozoa
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Triactinomyxon stage of Myxobolus cerebralis
Scientific classification OOjs UI icon edit-ltr.svg
Domain: Eukaryota
Kingdom: Animalia
Phylum: Cnidaria
Subphylum: Myxozoa
Grassé, 2021
Classes

Myxozoa (etymology: Greek: μύξα myxa "slime" or "mucus" [1] + thematic vowel o + ζῷον zoon "animal" [2] ) is a subphylum of aquatic cnidarian animals – all obligate parasites. It contains the smallest animals ever known to have lived. Over 2,180 species have been described and some estimates have suggested at least 30,000 undiscovered species. [3] Many have a two-host lifecycle, involving a fish and an annelid worm or a bryozoan. The average size of a myxosporean spore usually ranges from 10 μm to 20 μm, [4] whereas that of a malacosporean (a subclade of the Myxozoa) spore can be up to 2 mm. Myxozoans can live in both freshwater and marine habitats.

Contents

Myxozoans are highly derived cnidarians that have undergone dramatic evolution from a free swimming, self-sufficient jellyfish-like creature into their current form of obligate parasites composed of very few cells – sometimes only a single cell [ citation needed ]. As myxozoans evolved into microscopic parasites, they lost many genes responsible for multicellular development, coordination, cell–cell communication, and even, in some cases, aerobic respiration. The genomes of some myxozoans are now among the smallest genomes of any known animal species. [5] [6]

Life cycle and pathology

Myxozoans are endoparasitic animals exhibiting complex life cycles that, in most of the documented cases, involve an intermediate host, usually a fish, but in rare cases amphibians, [7] reptiles, [7] birds, [8] and mammals; [9] [10] and a definitive host, usually an annelid or an ectoproct.

Myxozoan life cycle Myxozoans Life Cycle.jpg
Myxozoan life cycle

Only about 100 life cycles have been resolved and it is suspected that there may be some exclusively terrestrial. [11] The mechanism of infection occurs through valve spores [ clarification needed ] that have many forms, but their main morphology is the same: one or two sporoplasts, which are the real infectious agent, surrounded by a layer of flattened cells called valve cells, which can secrete a layer protective coating and form float appendages. Integrated into the layer of valve cells are two to four specialized capsulogenic cells (in a few cases, one or even 15), each carrying a polar capsule containing coiled polar filaments, an extrudable organelle used for recognition, contact and infiltration. [12] Myxospores are ingested by annelids, in which the polar filaments extrude to anchor the spore to the gut epithelium. Opening of the shell valves allows the sporoplasms to penetrate into the epithelium. Subsequently, the parasite undergoes reproduction and development in the gut tissue, and finally produces usually eight actinosporean spore stages (actinospores) within a pansporocyst. After mature actinospores are released from their hosts they float in the water column. [13] Upon contact with skin or gills of fish, sporoplasms penetrate through the epithelium, followed by development of the myxosporean stage. Myxosporean trophozoites are characterized by cell-in-cell state, where the secondary (daughter) cells develop in the mother (primary) cells. The presporogonic stages multiply, migrate via nervous or circulatory systems, and develop into sporogonic stages. At the final site of infection, they produce mature spores within mono- or di-sporic pseudoplasmodia, or poly-sporic plasmodia. [14]

Relationships between myxosporeans and their hosts are often highly evolved and do not usually result in severe diseases of the natural host. Infection in fish hosts can be extremely long-lasting, potentially persisting for the lifetime of the host. However, an increasing number of myxosporeans have become[ when? ] pathogens with significant impact to the commercial fish industry, largely as a result of aquaculture bringing new species into contact with myxosporeans to which they had not been previously exposed, and to which they are highly susceptible. The economic impact of such parasites can be severe, especially where prevalence rates are high; they may also have a severe impact on wild fish stocks.

The diseases caused by myxosporeas in cultured fish with the most significant economic impact worldwide are proliferative kidney disease (PKD) caused by the malacosporean T. bryosalmonae, and whirling disease, caused by a myxosporean M. cerebralis; both diseases affect salmon. Enteromyxosis is caused by E. leei in cultured marine sparids, while proliferative gill disease (or “hamburger disease”) is caused by H. ictaluri in catfish and S. renicola infections occur in common carp.

Anatomy

Myxozoans are very small animals, typically 10–300  μm in length. [15]

Like other cnidarians they possess cnidocysts, which were referred to as "polar capsules" before the discovery that myxozoans are cnidarians. These cnidocysts fire tubules as in other cnidarians; some inject substances into the host. However, the tubules lack hooks or barbs, and in some species are more elastic than in other cnidarians.

Myxozoans have secondarily lost epithelial structures, a nervous system, gut, and cilia. Most lack muscles, though these are retained in some members of malacosporea. Those who have lost their muscles move around inside the host using other forms of locomotion, such as the use of filopodia, spore valve contractions, amoeboid movements, and rapidly creating and reabsorbing folds on the cell membrane. [16] Myxozoans do not undergo embryogenesis during development and have lost true gametes. [3] Instead, they reproduce via multicellular spores. These spores contain the polar capsules, which are not typically present in somatic cells. Centrioles are not involved in the nuclear division of myxozoans. Cell division by binary fission is rare, and cells divide instead via endogeny. [15]

In 2020, the myxozoan Henneguya salminicola was found to lack a mitochondrial genome, and thus be incapable of aerobic respiration; it was the first animal to be positively identified as such. Its actual metabolism is currently unknown. [17]

Phylogenetics

Myxozoans were originally considered to be protozoans, [18] and were included among other non-motile forms in the group Sporozoa. [19] As their distinct nature became clear through 18S ribosomal DNA (rDNA) sequencing, they were relocated in the metazoa. Detailed classification within the metazoa was however long hindered by conflicting rDNA evidence: although 18S rDNA suggested an affinity with Cnidaria, [20] other rDNA sampled, [21] [22] and the HOX genes of two species, [23] were more similar to those of the Bilateria.

The discovery that Buddenbrockia plumatellae , a worm-like parasite of bryozoans up to 2 mm in length, is a myxozoan [21] initially appeared to strengthen the case for a bilaterian origin, as the body plan is superficially similar. Nevertheless, closer examination reveals that Buddenbrockia's longitudinal symmetry is not twofold, but fourfold, casting doubt on this hypothesis.

Further testing resolved the genetic conundrum by sourcing the first three previously identified discrepant HOX genes (Myx1-3) to the bryozoan Cristatella mucedo and the fourth (Myx4) to northern pike, the respective hosts of the two corresponding Myxozoa samples. [24] This explained the confusion: the original experiments had used samples contaminated by tissue from host organisms, leading to false positives for a position among the Bilateria. More careful cloning of 50 coding genes from Buddenbrockia firmly established the clade as severely modified members of the phylum Cnidaria, with medusozoans as their closest relatives. [24] Similarities between myxozoan polar capsules and cnidarian nematocysts had been drawn for a long time, but were generally assumed to be the result of convergent evolution.

Taxonomists now recognize the outdated subgroup Actinosporea as a life-cycle phase of Myxosporea. [25]

Molecular clocks suggest that myxozoans and their closest relatives, the polypodiozoa, shared their last common ancestor with medusazoans about 600 million years ago, during the Ediacaran period. [3]

Taxonomy

Myxozoan taxonomy has undergone great and important changes in its levels of generic, family and suborder classification. Fiala et al. (2015) proposed a new classification based on spores. [26]

Phylum: Cnidaria
Subphylum: Myxozoa [26]
Class: Malacosporea Class: Myxosporea(sequel)
Order: Malacovalvulida Order: Bivalvulida (sequel)
   Family: Saccosporidae    Family: Myxobilatidae
     Genus: Buddenbrockia, Tetracapsuloides      Genus: Myxobilatus, Acauda, Hoferellus
Class: Myxosporea    Family: Chloromyxidae
Order: Bivalvulida      Genus: Chloromyxum, Caudomyxum, Agarella
  Suborder: Variisporina    Family: Coccomyxidae
   Family: Sphaeromyxidae      Genus: Coccomyxa, Auerbachia, Globospora
     Genus: Sphaeromyxa    Family: Alatosporidae
   Family: Myxidiidae      Genus: Alatospora, Pseudalatospora, Renispora
     Genus: Myxidium, Zschokkella, Enteromyxum, Sigmomyxa, Soricimyxum, Cystodiscus    Family: Parvicapsulidae
   Family: Ortholineidae      Genus: Parvicapsula, Neoparvicapsula, Gadimyxa
     Genus: Ortholinea, Neomyxobolus, Cardimyxobolus, Triangula, Kentmoseria   Suborder: Platysporina
   Family: Sinuolineidae    Family: Myxobolidae
     Genus: Sinuolinea, Myxodavisia, Myxoproteus, Bipteria, Paramyxoproteus, Neobipteria, Schulmania, Noblea, Latyspora      Genus: Myxobolus, Spirosuturia, Unicauda, Dicauda, Phlogospora, Laterocaudata, Henneguya, Hennegoides, Tetrauronema, Thelohanellus, Neothelohanellus, Neohenneguya, Trigonosporus
   Family: Fabesporidae Order: Multivalvulida
     Genus: Fabespora    Family: Trilosporidae
   Family: Ceratomyxidae      Genus: Trilospora, Unicapsula
     Genus: Ceratomyxa, Meglitschia, Ellipsomyxa, Ceratonova    Family: Kudoidae
   Family: Sphaerosporidae      Genus: Kudoa
     Genus: Sphaerospora, Wardia, Palliatus    Family: Spinavaculidae
     Genus: Octospina
Incertae sedis in Multivalvulida: Trilosporoides
 
  Phylum:  
 Cnidaria 
  Subphylum:  
 Myxozoa  [26]
  Class:  
 Malacosporea 
Order:
   Malacovalvulida  
Family:
   Saccosporidae  

Genus:Buddenbrockia, Tetracapsuloides

  Class:  
 Myxosporea 
Order:
   Bivalvulida  
Suborder:
   Variisporina  
Family:
   Sphaeromyxidae  

Genus:Sphaeromyxa

Family:
   Myxidiidae  

Genus:
Myxidium, Zschokkella, Enteromyxum, Sigmomyxa, Soricimyxum, Cystodiscus

Family:
   Ortholineidae  

Genus:
Ortholinea, Neomyxobolus, Cardimyxobolus, Triangula, Kentmoseria

Family:
   Sinuolineidae  

Genus:
Sinuolinea, Myxodavisia, Myxoproteus, Bipteria, Paramyxoproteus, Neobipteria, Schulmania, Noblea, Latyspora

Family:
   Fabesporidae  

Genus:Fabespora

Family:
   Ceratomyxidae  

Genus:
Ceratomyxa, Meglitschia, Ellipsomyxa, Ceratonova

Family:
   Sphaerosporidae  

Genus:
Sphaerospora, Wardia, Palliatus

Family:
   Myxobilatidae  

Genus:
Myxobilatus, Acauda, Hoferellus

Family:
   Chloromyxidae  

Genus:
Chloromyxum, Caudomyxum, Agarella

Family:
   Coccomyxidae  

Genus:
Coccomyxa, Auerbachia, Globospora

Family:
   Alatosporidae  

Genus:
Alatospora, Pseudalatospora, Renispora

Family:
   Parvicapsulidae  

Genus:
Parvicapsula, Neoparvicapsula, Gadimyxa

Suborder:
   Platysporina  
Family:
   Myxobolidae  

Genus:
Myxobolus, Spirosuturia, Unicauda, Dicauda, Phlogospora, Laterocaudata, Henneguya, Hennegoides, Tetrauronema, Thelohanellus, Neothelohanellus, Neohenneguya, Trigonosporus

Order:
   Multivalvulida  
Family:
   Trilosporidae  

Genus:Trilospora, Unicapsula

Family:
   Kudoidae  

Genus:Kudoa

Family:
   Spinavaculidae  

Genus:Octospina

 incertae sedis:    Trilosporoides 

     other Cnidarians     

   . . .  
 

See also

Related Research Articles

<span class="mw-page-title-main">Cnidaria</span> Aquatic animal phylum having cnydocytes

Cnidaria is a phylum under kingdom Animalia containing over 11,000 species of aquatic animals found both in fresh water and marine environments, including jellyfish, hydroids, sea anemones, corals and some of the smallest marine parasites. Their distinguishing features are a decentralized nervous system distributed throughout a gelatinous body and the presence of cnidocytes or cnidoblasts, specialized cells with ejectable flagella used mainly for envenomation and capturing prey. Their bodies consist of mesoglea, a non-living, jelly-like substance, sandwiched between two layers of epithelium that are mostly one cell thick. Cnidarians are also some of the only animals that can reproduce both sexually and asexually.

<span class="mw-page-title-main">Myxosporea</span> Class of cnidarians comprising microscopic parasites

Myxosporea is a class of microscopic animals, all of whom are parasites. They belong to the Myxozoa clade within Cnidaria. They have a complex life cycle that comprises vegetative forms in two hosts—one an aquatic invertebrate and the other an ectothermic vertebrate, usually a fish. Each parasitized host releases a different type of spore. The two forms of spore are so different that until relatively recently they were treated as belonging to different classes within the Myxozoa.

<i>Myxobolus cerebralis</i> Species of parasite

Myxobolus cerebralis is a myxosporean parasite of salmonids that causes whirling disease in farmed salmon and trout and also in wild fish populations. It was first described in rainbow trout in Germany in 1893, but its range has spread and it has appeared in most of Europe, the United States, South Africa, Canada and other countries from shipments of cultured and wild fish. In the 1980s, M. cerebralis was found to require a tubificid oligochaete to complete its life cycle. The parasite infects its hosts with its cells after piercing them with polar filaments ejected from nematocyst-like capsules. This infects the cartilage and possibly the nervous tissue of salmonids, causing a potentially lethal infection in which the host develops a black tail, spinal deformities, and possibly more deformities in the anterior part of the fish.

<span class="mw-page-title-main">Myxobolidae</span> Family of marine parasites

Myxobolidae is a family of myxosporean parasites which typically infect freshwater fishes, and includes the economically significant species, Myxobolus cerebralis. They have been shown to have a complex life cycle, involving an alternate stage in an invertebrate, typically an annelid or polychaete worm.

The term polar filament may refer to either of two analogous structures used for host invasion by different groups of parasites: Myxozoa (Metazoa) and Microsporidia (Fungi), respectively.

Tetracapsuloides bryosalmonae is a myxozoan parasite of salmonid fish. It is the only species currently recognized in the monotypic genus Tetracapsuloides. It is the cause of proliferative kidney disease (PKD), one of the most serious parasitic diseases of salmonid populations in Europe and North America that can result in losses of up to 90% in infected populations.

Kudoa thyrsites is a myxosporean parasite of marine fishes. It has a worldwide distribution, and infects a wide range of host species. This parasite is responsible for causing economic losses to the fisheries sector, by causing post-mortem "myoliquefaction", a softening of the flesh to such an extent that the fish becomes unmarketable. It is not infective to humans.

<span class="mw-page-title-main">Xenoma</span> Growth caused by various species of protists and fungi

A xenoma is a growth caused by various protists and fungi, most notably microsporidia. It can occur on numerous organisms; however is predominantly found on fish.

Buddenbrockia plumatellae is a worm-like parasite of bryozoans whose taxonomic placement long puzzled biologists. It is now classified as one of only three myxozoans in the Malacosporea subclass and its only family, Saccosporidae, on the basis of both genetic and ultrastructural studies. It was the first multicellular myxozoan identified and its vermiform shape initially gave strong support to the theory that the enigmatic group belongs among the Bilateria. Five years later, this was refuted by a study of fifty genes from this same "worm", which had rarely even been seen since its discovery in 1851. These 50 phylogenetic markers reveal that Buddenbrockia is closely related to jellyfish and sea anemones, typical members of the animalian phylum Cnidaria. Because of the highly divergent nuclear protein sequences of Buddenbrockia, relative to those of the other animals compared in this study, only the use of a sophisticated tree-building approach allowed for recovery of its cnidarian evolutionary affinities. One of the researchers talked about the problems encountered studying its morphology: “It has no mouth, no gut, no brain and no nerve cord. It doesn’t have a left or right side or a top or bottom – we can’t even tell which end is the front!” As the myxozoans are now demonstrably non-bilaterian in origin, he concluded that “the worm-like body shape evolved at least twice from two completely different kinds of animal.”

<i>Henneguya zschokkei</i> Species of Myxosporea

Henneguya zschokkei or Henneguya salminicola is a species of a myxosporean endoparasite. It afflicts several salmon in the genera Oncorhynchus and Salmo,where it causes milky flesh or tapioca disease. H. zschokkei is notable for its reliance on an exclusively anaerobic metabolism as well as its lack of mitochondria and mitochondrial DNA. It is the only known multicellular animal that does not require oxygen to survive.

<span class="mw-page-title-main">Diseases and parasites in salmon</span>

Diseases and parasites in salmon, trout and other salmon-like fishes of the family Salmonidae are also found in other fish species. The life cycle of many salmonids is anadromous, so such fish are exposed to parasites in fresh water, brackish water and saline water.

<i>Kudoa</i> Genus of marine parasites

Kudoa is a genus of Myxozoa and the only genus recognized within the monotypic family Kudoidae. There are approximately 100 species of Kudoa all of which parasitize on marine and estuarine fish. Kudoa are most commonly known and studied for the negative effects the genus has on commercial fishing and aquaculture industries.

Gadimyxa sphaerica is a species of parasitic myxozoan. Together with G. arctica and G. atlantica, they infect Gadus morhua and Arctogadus glacialis by developing coelozoically in bisporic plasmodia in their urinary systems. These 3 species' spores exhibit two morphological forms: wide and subspherical, being both types bilaterally symmetrical along the suture line. The wide spores have a mean width ranging from 7.5 to 10μm, respectively, while the subspherical ones range from 5.3-8μm in mean width. The subspherical forms of Gadimyxa are similar to Ortholinea, differing in the development of the spores and in the arrangement of the polar capsules.

Gadimyxa arctica is a species of parasitic myxozoan. Together with G. atlantica and G. sphaerica, they infect Gadus morhua and Arctogadus glacialis by developing coelozoically in bisporic plasmodia in their urinary systems. These 3 species' spores exhibit two morphological forms: wide and subspherical, being both types bilaterally symmetrical along the suture line. The wide spores have a mean width ranging from 7.5-10μm, respectively, while the subspherical ones range from 5.3-8μm in mean width. The subspherical forms of Gadimyxa are similar to Ortholinea, differing in the development of the spores and in the arrangement of the polar capsules.

Saccosporidae is a family of myxozoans. It is the only family within the class Malacosporea and has only three species, while the other class of Myxozoa, Myxosporea, includes more than a thousand.

<i>Polypodium hydriforme</i> Species of marine parasites

Polypodium is a genus of cnidarians that parasitizes in the eggs of sturgeon and similar fishes. It is one of few animals that lives inside the cells of other animals.

<i>Enteromyxum leei</i> Species of marine parasite

Enteromyxum leei is a species of myxozoan, histozoic parasite that infects the intestinal tract and sometimes associated organs, like gall bladder and liver, of several teleostean fish species. Myxozoans are microscopic metazoans, with an obligate parasitic life-style. The parasite stages of this species live in the paracelullar space between fish enterocytes. It is the causative agent of enteromyxosis, or emaciative disease, also known as "razor blade syndrome" in sparid fish. E. leei has a wide host and geographical range within marine fish, and even freshwater fish have been infected experimentally. E. leei initially emerged in the Mediterranean in the late 1980s and it is believed to have been unintentionally introduced into the Red Sea. Its pathogenicity and economic impact depend on the host species. In the gilt-head seabream, it is manifested as a chronic disease that provokes anorexia, delayed growth with weight loss, cachexia, reduced marketability and increased mortality. In other species, it has no clinical signs. In sharpsnout seabream, infection results in very high mortality rates, which have pushed fish farmers to abandon the culture of this fish species.

<i>Sphaerospora molnari</i> Species of marine parasite

Sphaerospora molnari is a microscopic endoparasite of carp in pond cultures and natural freshwater habitats in Central and Eastern Europe. In natural infections, S. molnari invades the epithelia of gills and surrounding skin regions. It then forms spores in between epithelial cells, causing sphaerosporosis, a pathological condition of the skin and gill tissues. Affected tissues show marked dystrophic changes and necrosis, causing secondary bacterial infections and resulting in osmoregulatory and respiratory failure. Mortalities can reach 100% but little is known about the overall distribution of the parasite species in European carp ponds or its economic impact on carp aquaculture.

Ellipsomyxa is a genus of cnidarian that is part of the family Ceratomyxidae.

Zschokkella are identified as parasitic organisms from the family Myxidiidae of the suborder Variisporina. This genus of Myxozoa tend to inhabit common areas of their host and can be found around the globe in both marine to freshwater habitats. Zschokkella also have spore shapes and host specific parasitism that differentiates it from other Myxidiidae.

References

  1. ↑ μύξα . Liddell, Henry George ; Scott, Robert ; A Greek–English Lexicon at the Perseus Project
  2. ↑ ζῷον . Liddell, Henry George ; Scott, Robert ; A Greek–English Lexicon at the Perseus Project
  3. 1 2 3 Atkinson, Stephen D.; Bartholomew, Jerri L.; Lotan, Tamar (1 August 2018). "Myxozoans: Ancient metazoan parasites find a home in phylum Cnidaria". Zoology . 129: 66–68. Bibcode:2018Zool..129...66A. doi:10.1016/j.zool.2018.06.005. ISSN   0944-2006. PMID   30170750. S2CID   52141614.
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  5. ↑ Chang, E. Sally; Neuhof, Moran; Rubinstein, Nimrod D.; Diamant, Arik; Philippe, Hervé; Huchon, Dorothée; Cartwright, Paulyn (1 December 2015). "Genomic insights into the evolutionary origin of Myxozoa within Cnidaria". Proceedings of the National Academy of Sciences . 112 (48): 14912–14917. Bibcode:2015PNAS..11214912C. doi: 10.1073/pnas.1511468112 . ISSN   1091-6490. PMC   4672818 . PMID   26627241.
  6. ↑ Yahalomi, D.; Atkinson, S.D.; Neuhof, M.; Chang, E.S.; Philippe, H.; Cartwright, P.; Bartholomew, J.L.; Huchon, D. (24 February 2020). "A cnidarian parasite of salmon (Myxozoa: Henneguya) lacks a mitochondrial genome". Proceedings of the National Academy of Sciences . 117 (10): 5358–5363. Bibcode:2020PNAS..117.5358Y. doi: 10.1073/pnas.1909907117 . PMC   7071853 . PMID   32094163.
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  14. ↑ el Matbouli, M.; Hoffmann, R.W.; Mandok, C. (1995). "Light and electron microscopic observations on the route of the triactinomyxon-sporoplasm of Myxobolus cerebralis from epidermis into rainbow trout (Oncorhynchus mykiss) cartilage". Journal of Fish Biology. 46 (6): 919–935. doi:10.1111/j.1095-8649.1995.tb01397.x.
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  21. 1 2 A.S. Monteiro; et al. (1 June 2002). "Orphan worm finds a home: Buddenbrockia is a Myxozoan". Mol. Biol. Evol. 19 (6): 968–71. doi: 10.1093/oxfordjournals.molbev.a004155 . PMID   12032254.
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