| Bigyra | |
|---|---|
 | |
| Aplanochytrium,SEM showing onevegetative cell and extendedectoplasmic network. | |
Scientific classification | |
| Domain: | Eukaryota |
| Clade: | Sar |
| Clade: | Stramenopiles |
| Phylum: | Bigyra Cavalier-Smith 1998,[1] emend. 2006[2] emend. 2013[3] |
| Classes[4][5][6][7] | |
Bigyra (from Latin bi- 'twice' and gyrus 'circle')[1] is aphylum of microscopiceukaryotes that are found at the base of theStramenopilesclade. It includes three well-knownheterotrophic groupsBicosoecida,Opalinata andLabyrinthulomycetes, as well as several smallclasses initially discovered throughenvironmental DNA samples:Nanomonadea,Placididea,Opalomonadea andEogyrea. The classification of Bigyra has changed several times since its origin, and itsmonophyly remains unresolved.
Bigyra is a diverse group ofheterotrophic, mainlyphagotrophicstramenopiles that lackcell walls.[4] It contains three well-known important groups with widely different ecological functions and morphologies:labyrinthulomycetes,opalines andbicosoecids.[8]
Labyrinthulomycetes is a group ofprotists that absorb nutrients in anosmotrophic orphagotrophic manner. They can behave either as free-livingamoebae or asmycelium-like networks ofcytoplasmic threads. Some of them aresaprotrophicdecomposers of thedetritalfood web; as such, they play a role in making organic matter more accessible to otherorganisms. Others areparasitic, and others are predators ofbacteria. They arecosmopolitan,ubiquitous inmarine,freshwater andestuarine environments. They live in association withalgae, marineplants anddetritus.[8]
Opalinata is a diverse assemblage of modifiedparasitic protists known as 'opalines'. They inhabit theintestines of variousanimals, primarilyamphibians. They are found on everycontinent. Among them, theopalinids are highly unusual protists: their large cells have numerous flagella and from two to hundreds ofnuclei. Theircell surface is delicately folded, giving it aniridescent appearance (hence their name, a reference to the iridescentopal). Another important group of opalines isBlastocystis, a prevalent parasite of humans and other animals.[8]
Bicosoecida is a small group that contains free-livingmarine andfreshwaternanoflagellates that feed onbacteria. They are present in every ecosystem, including extreme environments such as the deep sea or salt flats. They play a crucial role in themicrobial food web by composing the link between bacteria and highertrophic levels. They are also important in biogeochemical cycles by remineralizing the nutrients. Their classification has changed multiple times over the years,[9] and is still an unresolved issue.[10]
Bigyra contains many of the earliest-diverging clades of theStramenopiles.[11] The Stramenopiles are asupergroup ofeukaryotic organisms (protists) characterized by the presence of an anteriorflagellum with tripartite hairs, calledmastigonemes. Together withRhizaria andAlveolata, the Stramenopiles compose theSAR supergroup.[4]
All of Bigyra areheterotrophic microorganisms evolved from the last common ancestor of Stramenopiles, which is thought to have been phototrophic. Following this hypothesis, the bigyran ancestor would havesecondarily lost their photosyntheticplastids. Some characteristics of bigyran groups can be explained by their origin from ancestral plastids. For example,labyrinthulomycetes can produceomega-3 poly-unsaturated fatty acids through adesaturase usually present inchloroplasts.[12]
Bigyra is composed of twosubphyla:Opalozoa andSagenista. Opalozoa is further subdivided into two groups:Placidozoa, which contains theopalines and three clades discovered through the detection ofenvironmental DNA (Nanomonadea,Opalomonadea andPlacididea), and thebicosoecid flagellates. Sagenista contains thelabyrinthulomycetes and two environmental clades grouped under the nameEogyrea.[13][14]
Themonophyly of Bigyra remains uncertain. The positions of the two bigyran clades (Opalozoa and Sagenista) are not consistent between the published studies, because theydiverged from each other very early after the separation from the ancestor of allstramenopiles. This 'deep branching' makes it difficult to find the exact branching order of bigyran clades.[2] Additionally, not all clades are well-represented by molecular data in these studies.[13] Several studies support themonophyly of Bigyra.[13][14] Other studies support itsparaphyly.[15][16]
| Monophyletic Bigyra[13] | Paraphyletic Bigyra[16] | |||||||||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
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| Initial phylogeny of Bigyra (1997)[17] |
Bigyra was first described in 1997 by theprotozoologistThomas Cavalier-Smith as aphylum withinHeterokonta (synonym of Stramenopiles). Bigyra was defined as organisms with thesynapomorphy of aciliary transition region (i.e. a structure that controls protein transport at the base of the flagellum) with structures in the shape of two helices or rings, hence the name 'bigyra' meaning 'double helix'. It contained three subgroups:[1]
Their common ancestor was thought to have evolved fromphotosynthetic heterokonts, but would havesecondarily lost itsplastids, as opposed to the photosyntheticOchrophyta which retain them. Bigyra was, at the time, postulated as amonophyletic group (orclade), evolved from aparaphyleticgrade ofochrophyte classes.[1][17]
Posterior analyses completely changed the phylogeny of Stramenopiles. They revealedPseudofungi andBigyromonadea were more closely related to a monophyleticOchrophyta than they were toOpalinata, meaning that thesynapomorphy of a double helix could have been present in the common ancestor of all heterokonts. This rendered Bigyraparaphyletic. Consequently, Bigyra was revised andmodified in 2006 to comprise a different set of three subphyla:
Bigyra was modified again in 2013 after the discovery of severalenvironmental clades called MAST ('MArineSTramenopiles'). The subphylumOpalozoa assimilated thebicosoecids and an array of new clades:Placididea,Nanomonadea (MAST-3) andOpalomonadea (MAST-12). The subphylumSagenista, on the other hand, received a new classEogyrea that was composed of several MAST lineages not yet described.[3] Later, one of the MAST clades withinEogyrea would be described asPseudophyllomitus (MAST-6).[18]
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