Helkesida | |
---|---|
Scientific classification | |
Domain: | Eukaryota |
Clade: | Diaphoretickes |
Clade: | SAR |
Phylum: | Cercozoa |
Superclass: | Eoglissa |
Class: | Helkesea Cavalier-Smith 2018 |
Order: | Helkesida Cavalier-Smith 2018 [1] |
Families [1] [2] | |
Diversity | |
24 species |
Helkesida (formerly known as Sainouroidea) [1] is a group of microscopic protists belonging to the supergroup Rhizaria, both discovered through molecular phylogenetic analyses. It contains amoeboid flagellates with two flagella. They are either free-living, mostly on fecal matter, or live inside the gut of animals. Among these amoebae, one lineage has independently evolved aggregative multicellularity similarly to slime moulds. [3]
The organisms classified as Helkesida commonly have a gliding motility in which the cells glide on their posterior flagellum. They are ancestrally amoeboid bi-flagellates without scales or theca. [2] Unlike most Cercozoa which have tubular mitochondrial cristae, they can also present flat cristae or discoid cristae. They are the only group within Rhizaria that present discoid mitochondrial cristae. [4]
These organisms have an amorphous apical centrosome attached to the nucleus by a rhizoplast. The kinetid arises from 2–4 very short centrioles 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+2 axoneme. The centrosome also generates numerous microtubules in larger cells. The Golgi apparatus is seen attached to the nuclear envelope and the anterior rhizoplast. They have a microbody attached to the posterior end of the nucleus. [5]
One helkesid genus, Guttulinopsis , represents an independent lineage in which aggregative multicellularity has evolved to generate "fungi-like" fruiting bodies called sorocarps, similarly to slime moulds such as Dictyostelium . [4]
The helkesid amoebae are bacterivores that can be free-living, mostly associated to fecal environments, or endozoic, associated to animals. [6] They thrive in aerobic conditions and the microaerophilic gut environment of animals. Rosculus can thrive in anaerobic 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 within Cercozoa through phylogenetic analyses that used the sequencing of 18S ribosomal RNA from Cholamonas cytrodiopsidis , Sainouron acronematica and Helkesimastix marina . It is a molecularly diverse clade that branches within a group of ancestrally amoeboid bi-flagellates that usually lack an outer cell coat, known as Monadofilosa. [5] A 2016 study revealed a previously unknown wide diversity of Sainouroidea in fecal environments. Previous environmental 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 class Helkesea and order Helkesida 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 and Guttulinopsidae. [1]
Phylogeny of Helkesida | ||||||||||||||||||||||||||||||||||||||||||||||||
| ||||||||||||||||||||||||||||||||||||||||||||||||
Cladogram of Helkesida, based on a 18S 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 24 species distributed in 9 genera, 3 families [2] and 2 superfamilies. [1] Additionally, many OTUs found through environmental sequencing may represent undescribed clades. [4] [6]
A flagellate is a cell or organism with one or more whip-like appendages called flagella. The word flagellate also describes a particular construction characteristic of many prokaryotes and eukaryotes and their means of motion. The term presently does not imply any specific relationship or classification of the organisms that possess flagella. However, several derivations of the term "flagellate" are more formally characterized.
Nucleariida is a group of amoebae with filose pseudopods, known mostly from soils and freshwater. They are distinguished from the superficially similar vampyrellids mainly by having mitochondria with discoid cristae, in the absence of superficial granules, and in the way they consume food.
The family Vampyrellidae is a subgroup of the order Vampyrellida within the supergroup Rhizaria. Based on molecular sequence data, the family currently comprises the genus Vampyrella, and maybe several other vampyrellid amoebae. The cells are naked and characterised by radiating, filose pseudopodia and an orange colouration of the main cell body.
Heliozoa, commonly known as sun-animalcules, are microbial eukaryotes (protists) with stiff arms (axopodia) radiating from their spherical bodies, which are responsible for their common name. The axopodia are microtubule-supported projections from the amoeboid cell body, and are variously used for capturing food, sensation, movement, and attachment. They are similar to Radiolaria, but they are distinguished from them by lacking central capsules and other complex skeletal elements, although some produce simple scales and spines. They may be found in both freshwater and marine environments.
Cercozoa is a phylum of diverse single-celled eukaryotes. They lack shared morphological characteristics at the microscopic level, and are instead united by molecular phylogenies of rRNA and actin or polyubiquitin. They were the first major eukaryotic group to be recognized mainly through molecular phylogenies. They are the natural predators of many species of bacteria. They are closely related to the phylum Retaria, comprising amoeboids that usually have complex shells, and together form a supergroup called Rhizaria.
The Rhizaria are a diverse and species-rich supergroup of mostly unicellular eukaryotes. Except for the Chlorarachniophytes and three species in the genus Paulinella in the phylum Cercozoa, they are all non-photosynthetic, but many foraminifera and radiolaria have a symbiotic relationship with unicellular algae. A multicellular form, Guttulinopsis vulgaris, a cellular slime mold, has been described. This group was used by Cavalier-Smith in 2002, although the term "Rhizaria" had been long used for clades within the currently recognized taxon.
Phaeodarea or Phaeodaria is a group of amoeboid cercozoan organisms. They are traditionally considered radiolarians, but in molecular trees do not appear to be close relatives of the other groups, and are instead placed among the Cercozoa. They are distinguished by the structure of their central capsule and by the presence of a phaeodium, an aggregate of waste particles within the cell.
Lobosa is a taxonomic group of amoebae in the phylum Amoebozoa. Most lobosans possess broad, bluntly rounded pseudopods, although one genus in the group, the recently discovered Sapocribrum, has slender and threadlike (filose) pseudopodia. In current classification schemes, Lobosa is a subphylum, composed mainly of amoebae that have lobose pseudopods but lack cilia or flagella.
The tectofilosids are a group of filose amoebae with shells. These are composed of organic materials and sometimes collected debris, in contrast to the euglyphids, which produce shells from siliceous scales. The shell usually has a single opening, but in Amphitrema and a few other genera it has two on opposite ends. The cell itself occupies most of the shell. They are most often found on marsh plants such as Sphagnum.
Monadofilosa is a grouping of Cercozoa. These organisms are single-celled amoeboid protists.
Thecofilosea is a class of unicellular testate amoebae belonging to the phylum Cercozoa. They are amoeboflagellates, organisms with flagella and pseudopodia, distinguished from other cercozoa by their scale-lacking test composed of organic material. They are closely related to the Imbricatea, a group of testate amoebae with tests composed of inorganic silica scales.
Rosculus is a genus of parasitic organisms which are poorly studied. Taxonomically, Rosculus is currently accepted to be in the family Sainouroidea, which contains a sister genus and Rosculus. Many species in this genus are aquatic, but genomic data shows that some species are terrestrial. Rosculus is thought to thrive in anaerobic and aerobic environments. This protist if very small in size, and it contains a massive genome. One defining characteristic of Rosculus is its discoidal cristae but morphologically.
The sarcomonads or class Sarcomonadea are a group of amoeboid biciliate protists in the phylum Cercozoa. They are characterized by a propensity to move through gliding on their posterior cilium or through filopodia, a lack of scales or external theca, a soft cell surface without obvious cortical filamentous or membranous skeleton, two cilia without scales or hairs, tubular mitochondrial cristae, near-spherical extrusomes, and a microbody attached to the nucleus.
Leucodictyids are heterotrophic amoeboid protists that comprise the order Leucodictyida in the phylum Cercozoa.
Cryptofilida is an order of small heterotrophic protists in the phylum Cercozoa. They are filose amoebae that lack cilia and gliding, and are instead characterized by movement through branching or unbranched granular filopodia that are appressed to the substrate during their feeding.
Mesofila is a genus of freshwater heterotrophic protists of the phylum Cercozoa. It is the only genus in the family Mesofilidae. It is a monotypic genus, with the sole species M. limnetica.
The glissomonads are a group of bacterivorous gliding flagellated protists that compose the order Glissomonadida, in the amoeboflagellate phylum Cercozoa. They comprise a vast, largely undescribed diversity of soil and freshwater organisms. They are the sister group to cercomonads; the two orders form a solid clade of gliding soil-dwelling flagellates called Pediglissa.
Pediglissa is a subclass of phagotrophic protists that inhabit soil or freshwater habitats. They were defined in 2018 according to phylogenetic analyses that showed a clade containing the orders Cercomonadida and Glissomonadida. They're the sister group of Paracercomonadida.
Viridiraptoridae, previously known as clade X, is a clade of heterotrophic protists in the phylum Cercozoa. They're a family of glissomonads, a group containing a vast, mostly undescribed diversity of soil and freshwater organisms.
An amoeboflagellate is any eukaryotic organism capable of behaving as an amoeba and as a flagellate at some point during their life cycle. Amoeboflagellates present both pseudopodia and at least one flagellum, often simultaneously.