The Rhizaria are a diverse and species-rich

clade In biology, a clade (), also known as a Monophyly, monophyletic group or natural group, is a group of organisms that is composed of a common ancestor and all of its descendants. Clades are the fundamental unit of cladistics, a modern approach t ...

of mostly unicellular

eukaryote The eukaryotes ( ) constitute the Domain (biology), domain of Eukaryota or Eukarya, organisms whose Cell (biology), cells have a membrane-bound cell nucleus, nucleus. All animals, plants, Fungus, fungi, seaweeds, and many unicellular organisms ...

s. Except for the Chlorarachniophytes and three species in the genus '' Paulinella'' in the phylum

Cercozoa Cercozoa (now synonymised with Filosa) is a phylum of diverse single-celled eukaryotes. They lack shared morphological characteristics at the microscopic level, and are instead united by phylogeny, molecular phylogenies of rRNA and actin or Ubiqu ...

, they are all non-photosynthetic, but many

Foraminifera Foraminifera ( ; Latin for "hole bearers"; informally called "forams") are unicellular organism, single-celled organisms, members of a phylum or class (biology), class of Rhizarian protists characterized by streaming granular Ectoplasm (cell bio ...

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 Thomas (Tom) Cavalier-Smith, Royal Society, FRS, Royal Society of Canada, FRSC, Natural Environment Research Council, NERC Professorial Fellow (21 October 1942 – 19 March 2021), was a professor of evolutionary biology in the Departme ...

in 2002, although the term "Rhizaria" had been long used for clades within the currently recognized taxon. Being described mainly from rDNA sequences, they vary considerably in form, having no clear morphological distinctive characters ( synapomorphies), but for the most part they are amoeboids with filose, reticulose, or

microtubule Microtubules are polymers of tubulin that form part of the cytoskeleton and provide structure and shape to eukaryotic cells. Microtubules can be as long as 50 micrometres, as wide as 23 to 27 nanometer, nm and have an inner diameter bet ...

-supported pseudopods. In the absence of an apomorphy, the group is ill-defined, and its composition has been very fluid. Some Rhizaria possess mineral exoskeletons ( thecae or loricas), which are in different clades within Rhizaria made out of

opal Opal is a hydrated amorphous form of silicon dioxide, silica (SiO2·''n''H2O); its water content may range from 3% to 21% by weight, but is usually between 6% and 10%. Due to the amorphous (chemical) physical structure, it is classified as a ...

(),

celestite Celestine (the IMA-accepted name) or celestite is a mineral consisting of strontium sulfate ( Sr S O). The mineral is named for its occasional delicate blue color. Celestine and the carbonate mineral strontianite are the principal sources o ...

(), or

calcite Calcite is a Carbonate minerals, carbonate mineral and the most stable Polymorphism (materials science), polymorph of calcium carbonate (CaCO3). It is a very common mineral, particularly as a component of limestone. Calcite defines hardness 3 on ...

(). Certain species can attain sizes of more than a centimeter with some species being able to form cylindrical colonies approximately 1 cm in diameter and greater than 1 m in length. They feed by capturing and engulfing prey with the extensions of their pseudopodia; forms that are symbiotic with unicellular algae contribute significantly to the total primary production of the ocean.

Groups

The three main groups of Rhizaria are: *

– various amoebae and flagellates, usually with filose pseudopods and common in soil *

– amoeboids with reticulose pseudopods, common as marine benthos * Radiolaria – amoeboids with axopods, common as marine

plankton Plankton are the diverse collection of organisms that drift in Hydrosphere, water (or atmosphere, air) but are unable to actively propel themselves against ocean current, currents (or wind). The individual organisms constituting plankton are ca ...

A few other groups may be included in the Cercozoa, but some trees appear closer to the Foraminifera. These are the Phytomyxea and Ascetosporea, parasites of plants and animals, respectively, and the peculiar amoeba '' Gromia''. The different groups of Rhizaria are considered close relatives based mainly on genetic similarities, and have been regarded as an extension of the Cercozoa. The name Rhizaria for the expanded group was introduced by

in 2002, who also included the centrohelids and Apusozoa. A noteworthy order that belongs to Ascetosporea is the Mikrocytida. These are parasites of

oyster Oyster is the common name for a number of different families of salt-water bivalve molluscs that live in marine or brackish habitats. In some species, the valves are highly calcified, and many are somewhat irregular in shape. Many, but no ...

s. This includes the causative agent of Denman Island Disease, ''Mikrocytos mackini'' a small (2−3 μm diameter) amitochondriate protistan.

History

Similarities between various Rhizaria organisms have been noticed since the 19th century. In his 1861 classification of the Rhizopoda (amoebae), the zoologist William B. Carpenter proposed the order Reticularia, which consisted of

and Gromiida on the basis of their very similar thin, reticulose pseudopodia with granules circulating inside. However, the idea that these organisms and others such as Radiolaria were all related to one another emerged rather recently, with the help of molecular phylogenetics and advanced microscopy techniques in the late 20th century.

Evolutionary relationships

Rhizaria are part of the SAR supergroup (Stramenopiles, Alveolates, Rhizaria), a grouping that had been presaged in 1993 through a study of mitochondrial morphologies. SAR is currently placed in the

Diaphoretickes Diaphoretickes is a major group of eukaryote, eukaryotic organisms spanning over 400,000 species. The majority of the earth's biomass that carries out photosynthesis belongs to Diaphoretickes. In older classification systems, members of the Diaph ...

along with Archaeplastida,

Cryptista Cryptista is a clade of alga-like eukaryotes. It is most likely related to Archaeplastida which includes plants and many algae, within the larger group Diaphoretickes. Other characteristic features of cryptophyte mtDNAs include large syntenic ...

Haptista Haptista is a proposed group of protists made up of centrohelids and haptophytes. Phylogenomic studies indicate that Haptista, together with ''Ancoracysta twista'', forms a sister clade to the SAR supergroup#Internal phylogeny, TSAR Supergroup (b ...

, and several minor clades. Historically, many rhizarians were considered

animal Animals are multicellular, eukaryotic organisms in the Biology, biological Kingdom (biology), kingdom Animalia (). With few exceptions, animals heterotroph, consume organic material, Cellular respiration#Aerobic respiration, breathe oxygen, ...

s because of their motility and heterotrophy. However, when a simple animal-plant dichotomy was superseded by a recognition of additional kingdoms, taxonomists generally placed amoebae in the kingdom

Protista A protist ( ) or protoctist is any Eukaryote, eukaryotic organism that is not an animal, Embryophyte, land plant, or fungus. Protists do not form a Clade, natural group, or clade, but are a Paraphyly, paraphyletic grouping of all descendants o ...

. When scientists began examining the evolutionary relationships among eukaryotes in the 1970s, it became clear that the kingdom

was

paraphyletic Paraphyly is a taxonomic term describing a grouping that consists of the grouping's last common ancestor and some but not all of its descendant lineages. The grouping is said to be paraphyletic ''with respect to'' the excluded subgroups. In co ...

. Rhizaria appear to share a common ancestor with

Stramenopiles The stramenopiles, also called heterokonts, are Protist, protists distinguished by the presence of stiff tripartite external hairs. In most species, the hairs are attached to flagella, in some they are attached to other areas of the cellular sur ...

and Alveolates forming part of the SAR super assemblage. Rhizaria has been supported by molecular phylogenetic studies as a monophyletic group. Biosynthesis of 24-isopropyl cholestane precursors in various rhizaria suggests a relevant ecological role already during the

Ediacaran The Ediacaran ( ) is a geological period of the Neoproterozoic geologic era, Era that spans 96 million years from the end of the Cryogenian Period at 635 Million years ago, Mya to the beginning of the Cambrian Period at 538.8 Mya. It is the last ...

Phylogeny

Rhizaria is a monophyletic group composed of two sister phyla:

and Retaria. Subsequently, Cercozoa and Retaria are also

monophyletic In biological cladistics for the classification of organisms, monophyly is the condition of a taxonomic grouping being a clade – that is, a grouping of organisms which meets these criteria: # the grouping contains its own most recent co ...

.> The following cladogram depicts the evolutionary relationships between all rhizarian

class Class, Classes, or The Class may refer to: Common uses not otherwise categorized * Class (biology), a taxonomic rank * Class (knowledge representation), a collection of individuals or objects * Class (philosophy), an analytical concept used d ...

es, and is made after the works of

''et al.'' (2018), and Irwin ''et al.'' (2019).

Sexual cycle

Complete sexual life cycles have been demonstrated for two lineages (

and '' Gromia'') and direct evidence for karyogamy or

meiosis Meiosis () is a special type of cell division of germ cells in sexually-reproducing organisms that produces the gametes, the sperm or egg cells. It involves two rounds of division that ultimately result in four cells, each with only one c ...

has been observed in five lineages ('' Euglyphida'', '' Thecofilosea'', ''Chlorarachniophyta'', ''Plasmodiophorida'' and '' Phaeodarea''). In particular, the Foraminifera are marine amoebae that are defined by a dynamic network of pseudopodia, and the production of intricate shells. These amoeba have complex sexual life cycles with meiosis and

gamete A gamete ( ) is a Ploidy#Haploid and monoploid, haploid cell that fuses with another haploid cell during fertilization in organisms that Sexual reproduction, reproduce sexually. Gametes are an organism's reproductive cells, also referred to as s ...

production occurring at separate stages.

References

External links

Rhizaria
at UniEuk Taxonomy Map

Tree of Life Eukaryota
{{Taxonbar, from=Q855740 Neoproterozoic first appearances Taxa named by Thomas Cavalier-Smith Infrakingdoms