Bigyra (from Latin bi- 'twice', and gyrus 'circle')[1] is a possibly paraphyletic phylum of heterotrophic organisms belonging to the Stramenopile lineage.[5][2] It includes the groups Bicosoecida, Opalinata and Labyrinthulea, as well as several small clades that were initially discovered through environmental DNA samples: Nanomonadea, Placididea, Opalomonadea and Eogyrea.[3][6]
Bigyra is a widely distributed group of stramenopile lineages, characterized by an anterior flagellum with tripartite hairs. It contains well-known ecological groups such as the fungi-like slime nets, the flagellate bicosoecids and the opalines.[7]
The slime nets (known as Labyrinthulea, Labyrinthulomycetes or Labyrinthulomycota) are fungus-like heterotrophic, colorless or yellowish protists that absorb nutrients in an osmotrophic or phagotrophic manner, either as free-living amoebae or as networks of anastomosing cytoplasmic threads that extend from a bothrosome. They are typically saprotrophic decomposers of the detrital food web, making organic matter more accessible to grazing protists. Some are parasitic, and others feed on bacteria. They are cosmopolitan, ubiquitous in marine, freshwater and estuarine environments, associated with algae, marine plants and detritus.[7]
The opalines (known as Opalinata) are a diverse assemblage of modified unicellular protists, consisting of three closely-related groups: proteromonads, opalinids and Blastocystis. They inhabit the intestines of various animals and can be found in all continents.[7]
The bicosoecids (known as Bicosoecida) are a small group of marine or freshwater heterotrophic flagellates that feed on bacteria. Their classification has changed multiple times over the years,[8] and is still an unresolved issue.[9]
Bigyra was first described in 1997 by Thomas Cavalier-Smith, as a phylum within Heterokonta that contained three subphyla: 1) the walled saprotrophic Pseudofungi, 2) the non-phagotrophic gut-symbiotic Opalinata, and 3) the phagotrophic zooflagellate Developayella, which received its own subphylum Bigyromonada. These groups would have originated from a common ancestor that had a double ciliary transition zone helix as its synapomorphy. The common ancestor would have evolved from photosynthetic heterokonts, but would have secondarily lost its plastids, as opposed to the photosynthetic Ochrophyta which retain them. Bigyra was, at the time, postulated as a monophyletic group (or clade), followed by a paraphyletic grade of ochrophyte classes.[1][10]
Heterokonta Bigyra Pseudofungi OpalinataPosterior phylogenetic analyses that used 18S rRNA genes revealed that Pseudofungi and Bigyromonadea were more closely related to Ochrophyta than they were to Opalinata, meaning that the synapomorphy of a double helix could have been present in the common ancestor of all heterokonts. This rendered Bigyra paraphyletic. Consequently, Bigyra was revised and modified in 2006 to comprise a different set of three subphyla: 1) Opalozoa, a previously polyphyletic diverse phylum that was modified to only include Opalinata and Nucleohelea; 2) Bicoecea, containing the bicosoecids; and 3) Sagenista, containing the osmotrophic Labyrinthulea. The phylogeny of Bigyra, however, could not be resolved, and its monophyly was weakly supported. The weak support was thought to be caused by all three bigyran lineages diverging from each other very soon after the separation from other heterokonts; this deep branching makes it difficult to find the exact branching order of bigyran clades.[2]
Heterokonta Ochrophyta Pseudofungi OpalozoaBigyra was modified again in 2013 after the discovery of several heterokont clades called MAST (‘marine stramenopiles’), recovered through environmental rDNA sequencing. The subphylum Opalozoa assimilated the bicosoecids in an infraphylum Bikosia, while another new infraphylum Placidozoa assimilated the Opalinata and an array of new clades: Placididea, Nanomonadea (MAST-3) and Opalomonadea (MAST-12), all three classified under the paraphyletic taxon Wobblata. The subphylum Sagenista, on the other hand, received a new class Eogyrea that was composed of several MAST lineages not yet described.[3] Later, one of the MAST clades within Eogyrea would be described as Pseudophyllomitus (MAST-6).[6]
Stramenopiles Gyrista Opalozoa Placidozoa Opalinata SagenistaDespite modern large-scale phylogenomic analyses that use bigger taxon sampling, the validity of Bigyra remains uncertain. The positions of the deep-branching bigyran clades are not consistent among the published studies, and not all the clades are well-represented by genomic and transcriptomic data.[6] Several studies support the monophyly of Bigyra through multi-gene phylogenetic analysis and a rich taxon sampling.[6][11] There is also support for its paraphyly from more recent phylogenetic studies of the 2020s decade.[12][13]
The present classification of Bigyra is as follows:[4][11]
Bigyra (from Latin bi- 'twice', and gyrus 'circle') is a possibly paraphyletic phylum of heterotrophic organisms belonging to the Stramenopile lineage. It includes the groups Bicosoecida, Opalinata and Labyrinthulea, as well as several small clades that were initially discovered through environmental DNA samples: Nanomonadea, Placididea, Opalomonadea and Eogyrea.
