Myxozoa (etymology:Greek: μύξαmyxa "slime" or "mucus"[2] + thematic vowel o + ζῷονzoon "animal"[3]) is a subphylum of aquaticcnidariananimals – all obligateparasites. It contains thesmallest animals ever known to have lived. Over 2,180 species have been described and some estimates have suggested at least 30,000 undiscovered species.[1] Many have a two-host lifecycle, involving afish and anannelid worm or abryozoan. The average size of amyxosporeanspore 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.
Myxozoans are highlyderivedcnidarians that have undergone dramaticevolution from a free swimming, self-sufficientjellyfish-like creature into their current form ofobligate parasites composed of very fewcells. As myxozoans evolved into microscopic parasites, they lost manygenes 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]
Myxozoans are endoparasitic animals exhibiting complex life cycles that, in most of the documented cases, involve anintermediate host, usually a fish, but in rare cases amphibians,[7] reptiles,[7] birds,[8] and mammals;[9][10] and a definitive host, usually anannelid or anectoproct.
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 throughvalve spores[clarification needed] that have many forms, but their main morphology is the same: one or twosporoplasts, which are the real infectious agent, surrounded by a layer of flattened cells calledvalve cells, which can secrete a layer protective coating and formfloat appendages. Integrated into the layer of valve cells are two to four specializedcapsulogenic cells (in a few cases, one or even 15), each carrying apolar capsule containing coiledpolar 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 gutepithelium. Opening of the shell valves allows thesporoplasms 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 apansporocyst. 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. Myxosporeantrophozoites 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-sporicpseudoplasmodia, or poly-sporicplasmodia.[14]
Relationships between myxosporeans and their hosts are often highly evolved and do not usually result in severe diseases of the natural host. Infection infish 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 ofaquaculture 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 suchparasites can be severe, especially whereprevalence rates are high; they may also have a severe impact on wild fish stocks.
Myxozoans are very small animals, typically 10–300 μm in length.[15]
Like other cnidarians they possesscnidocysts, 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 lostepithelial structures, anervous system,gut, andcilia. Most lackmuscles, though these are retained in some members ofmalacosporea. Those who have lost their muscles move around inside the host using other forms of locomotion, such as the use offilopodia, 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 truegametes.[1] 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 bybinary fission is rare, and cells divide instead viaendogeny.[15]
Myxozoans were originally considered to beprotozoans,[18] and were included among other non-motile forms in the groupSporozoa.[19] As their distinct nature became clear through18Sribosomal DNA (rDNA) sequencing, they were relocated in themetazoa. Detailed classification within the metazoa was however long hindered by conflicting rDNA evidence: although 18S rDNA suggested an affinity withCnidaria,[20] other rDNA sampled,[21][22] and theHOX genes of two species,[23] were more similar to those of theBilateria.
The discovery thatBuddenbrockia plumatellae, a worm-like parasite ofbryozoans 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 thatBuddenbrockia's longitudinal symmetry is not twofold, but fourfold, casting doubt on thishypothesis.
Further testing resolved the genetic conundrum by sourcing the first three previously identified discrepant HOX genes (Myx1-3) to thebryozoanCristatella mucedo and the fourth (Myx4) tonorthern 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 tofalse positives for a position among the Bilateria. More careful cloning of 50 coding genes fromBuddenbrockia firmly established the clade as severely modified members of the phylumCnidaria, withmedusozoans as their closest relatives.[24] Similarities between myxozoan polar capsules and cnidariannematocysts had been drawn for a long time, but were generally assumed to be the result ofconvergent evolution.
Taxonomists now recognize the outdated subgroup Actinosporea as alife-cycle phase ofMyxosporea.[25]
Molecular clocks suggest that myxozoans and their closest relatives, thepolypodiozoa, shared their last common ancestor withmedusazoans about 600 million years ago, during theEdiacaran period.[1]
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]
^Gruhl, Alexander (2015)."Chapter 7 - Myxozoa". In Wanninger, Andreas (ed.).Evolutionary developmental biology of invertebrates. Vol. 1: Introduction,non–bilateria, acoelomorpha, xenoturbellida, chaetognatha. Springer Verlag Wien. pp. 165–177.doi:10.1007/978-3-7091-1862-7_7.ISBN978-3-7091-1861-0.
^Kent M. L.; Margolis L.; Corliss J.O. (1994). "The demise of a class of protists: taxonomic and nomenclatural revisions proposed for the protist phylum Myxozoa Grasse, 1970".Canadian Journal of Zoology.72 (5):932–937.Bibcode:1994CaJZ...72..932K.doi:10.1139/z94-126.
