The Rhizaria are a diverse and species-rich supergroup of mostly
unicellular[3]eukaryotes.[4] Except for the
Chlorarachniophytes and three species in the genus Paulinella in the phylum
Cercozoa, they are all non-photosynthethic, but many foraminifera and radiolaria have a symbiotic relationship with unicellular algae.[5] A multicellular form, Guttulinopsis vulgaris, a cellular
slime mold, has been described.[6] This group was used by
Cavalier-Smith 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-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 (SiO2),
celestite (SrSO4), or
calcite (CaCO3). 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.[7]
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
Cavalier-Smith in 2002,[9] who also included the
centrohelids and
Apusozoa.
A noteworthy order that belongs to
Ascetosporea is the
Mikrocytida.[10] These are parasites of
oysters. This includes the causative agent of Denman Island Disease, Mikrocytos mackini a small (2−3 μm diameter) amitochondriate protistan.[11]
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
Foraminifera and
Gromiida on the basis of their very similar thin, reticulose pseudopodia with granules circulating inside.[12] 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.[13]
Rhizaria are part of the
SAR supergroup (Stramenopiles, Alveolates, Rhizaria), a grouping that had been presaged in 1993 through a study of mitochondrial morphologies.[14] SAR is currently placed in the
Diaphoretickes along with
Archaeplastida,
Cryptista,
Haptista, and several minor clades.
Historically, many rhizarians were considered
animals 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. When scientists began examining the evolutionary relationships among eukaryotes in the 1970s, it became clear that the kingdom
Protista was
paraphyletic. Rhizaria appear to share a common ancestor with
Stramenopiles and
Alveolates forming part of the SAR super assemblage.[15] Rhizaria has been supported by molecular phylogenetic studies as a monophyletic group.[16] Biosynthesis of
24-isopropyl cholestane precursors in various rhizaria[17] suggests a relevant ecological role already during the
Ediacaran.
Phylogeny
Rhizaria is a
monophyletic group composed of two sister phyla:
Cercozoa and
Retaria. Subsequently, Cercozoa and Retaria are also
monophyletic.[18][19] The following cladogram depicts the evolutionary relationships between all rhizarian
classes, and is made after the works of
Cavalier-Smithet al. (2018),[1] and Irwin et al. (2019).[20]
^Moreira D, von der Heyden S, Bass D, López-García P, Chao E, Cavalier-Smith T (July 2007). "Global eukaryote phylogeny: Combined small- and large-subunit ribosomal DNA trees support monophyly of Rhizaria, Retaria and Excavata". Mol. Phylogenet. Evol. 44 (1): 255–66.
doi:
10.1016/j.ympev.2006.11.001.
PMID17174576.
^Seravin LN. Osnovnye tipy i formy tonkogo stroeniia krist mitokhondriĭ: stepen' ikh évoliutsionnoĭ stabil'nosti (sposobnost' k morfologicheskim transformatsiiam) [The basic types and forms of the fine structure of mitochondrial cristae: the degree of their evolutionary stability (capacity for morphological transformations)]. Tsitologiia. 1993;35(4):3-34. Russian. PMID 8328023.
^Hallmann, Christian; Stuhr, Marleen; Kucera, Michal; et al. (2019-03-04). "Putative sponge biomarkers in unicellular Rhizaria question an early rise of animals". Nature Ecology & Evolution. 3 (4): 577–581.
doi:
10.1038/s41559-019-0806-5.
PMID30833757.
S2CID71148672.
^Bass, D.; Chao, E.E.; Nikolaev, S.; et al. (February 2009). "Phylogeny of Novel Naked Filose and Reticulose Cercozoa: Granofilosea cl. n. and Proteomyxidea Revised". Protist. 160 (1): 75–109.
doi:
10.1016/j.protis.2008.07.002.
PMID18952499.
^>Howe, Alexis T.; Bass, David; Scoble, Josephine M.; et al. (2011). "Novel Cultured Protists Identify Deep-branching Environmental DNA Clades of Cercozoa: New Genera Tremula, Micrometopion, Minimassisteria, Nudifila, Peregrinia". Protist. 162 (2): 332–372.
doi:
10.1016/j.protis.2010.10.002.
PMID21295519.
^Irwin, Nicholas A. T.; Tikhonenkov, Denis V.; Hehenberger, Elisabeth; et al. (2019-01-01). "Phylogenomics supports the monophyly of the Cercozoa". Molecular Phylogenetics and Evolution. 130: 416–423.
doi:
10.1016/j.ympev.2018.09.004.
ISSN1055-7903.
PMID30318266.
S2CID52982396.
^
abcLahr DJ, Parfrey LW, Mitchell EA, Katz LA, Lara E. The chastity of amoebae: re-evaluating evidence for sex in amoeboid organisms. Proc Biol Sci. 2011 Jul 22;278(1715):2081-90. doi: 10.1098/rspb.2011.0289. Epub 2011 Mar 23. PMID 21429931; PMCID: PMC3107637