| Helkesida | |
|---|---|
Scientific classification | |
| Domain: | Eukaryota |
| Clade: | Diaphoretickes |
| Clade: | SAR |
| Clade: | Rhizaria |
| Phylum: | Cercozoa |
| Superclass: | Eoglissa |
| Class: | Helkesea Cavalier-Smith 2018 |
| Order: | Helkesida Cavalier-Smith 2018[1] |
| Families[1][2] | |
| Diversity | |
| 24 species | |
Helkesida (formerly known asSainouroidea)[1] is a group ofmicroscopicprotists belonging to thesupergroupRhizaria, both discovered throughmolecular phylogenetic analyses. It containsamoeboidflagellates with twoflagella. They are either free-living, mostly onfecal matter, or live inside thegut of animals. Among these amoebae, one lineage has independently evolved aggregativemulticellularity similarly toslime moulds.[3]
The organisms classified as Helkesida commonly have agliding motility in which thecells glide on their posteriorflagellum. They are ancestrallyamoeboid bi-flagellates without scales ortheca.[2] Unlike mostCercozoa which have tubularmitochondrial cristae, they can also present flat cristae or discoid cristae. They are the only group withinRhizaria that present discoid mitochondrial cristae.[4]
These organisms have an amorphous apicalcentrosome attached to the nucleus by arhizoplast. Thekinetid arises from 2–4 very shortcentrioles with dense fibrous roots that attach them to each other and to the nucleus. Their anterior flagellum is reduced to a stub without its 9+2axoneme. The centrosome also generates numerousmicrotubules in larger cells. TheGolgi apparatus is seen attached to thenuclear envelope and the anterior rhizoplast. They have a microbody attached to the posterior end of thenucleus.[5]
One helkesid genus,Guttulinopsis, represents an independent lineage in which aggregativemulticellularity hasevolved to generate "fungi-like" fruiting bodies calledsorocarps, similarly toslime moulds such asDictyostelium.[4]
The helkesidamoebae arebacterivores that can be free-living, mostly associated tofecal environments, orendozoic, associated toanimals.[6] They thrive inaerobic conditions and themicroaerophilicgut environment of animals.Rosculus can thrive inanaerobic culture. It is unknown if their preferred habitat is free-living or endozoic.[3]
Some host species can harbor different helkesid genera and species. One animal can be infected by multiple species simultaneously, and one species can also infect different animal hosts. More sampling of hosts, amoebae and molecular data is needed to better understand the life history and ecology of these protists.[3]
Helkesida is a group initially named Sainouroidea. It was discovered in 2009 as a highly divergent clade withinCercozoa throughphylogenetic analyses that used thesequencing of18S ribosomal RNA fromCholamonas cytrodiopsidis,Sainouron acronematica andHelkesimastix marina. It is a molecularly diverse clade that branches within a group of ancestrallyamoeboid bi-flagellates that usually lack an outer cell coat, known asMonadofilosa.[5] A 2016 study revealed a previously unknown wide diversity of Sainouroidea in fecal environments. Previousenvironmental samplings excluded sequences from Sainouroidea due to their highly divergent 18S rDNA sequences.[6] A 2018 study described several new genera and species.[3]
The initial name for this group, Sainouroidea, had the-oidea suffix for superfamilies, but it was not assigned to any existing classes or orders due to the uncertainty of its phylogenetic position.[5] In a 2018 revision, the classHelkesea and orderHelkesida were created as a substitute for this name. Sainouroidea was then modified to only include one of the three helkesid families,Sainouridae. A second superfamily,Helkesimastigoidea, was created to host the remaining two families,Helkesimastigidae andGuttulinopsidae.[1]
| Phylogeny of Helkesida |
| Cladogram of Helkesida, based on a18S rDNA phylogenetic analysis within a 2018 study.[3] The study names this group "Sainouroidea" due to being published prior to the taxonomic change to Helkesida.[1] |
Currently, Helkesida contains 24species distributed in 9genera, 3families[2] and 2superfamilies.[1] Additionally, manyOTUs found throughenvironmental sequencing may represent undescribed clades.[4][6]