TheRhizaria are a diverse and species-richsupergroup of mostlyunicellular[3]eukaryotes.[4] Except for theChlorarachniophytes and three species in the genusPaulinella in the phylumCercozoa, they are all non-photosynthetic, but manyForaminifera andRadiolaria have a symbiotic relationship with unicellular algae.[5] A multicellular form,Guttulinopsis vulgaris, a cellularslime mold, has been described.[6] This group was used byCavalier-Smith in 2002, although the term "Rhizaria" had been long used for clades within the currently recognized taxon.
Being described mainly fromrDNA sequences, they vary considerably in form, having no clear morphological distinctive characters (synapomorphies), but for the most part they areamoeboids withfilose,reticulose, ormicrotubule-supportedpseudopods. In the absence of an apomorphy, the group is ill-defined, and its composition has been very fluid. Some Rhizaria possess mineral exoskeletons (thecae orloricas), which are in different clades within Rhizaria made out ofopal (SiO2),celestite (SrSO4), orcalcite (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 thePhytomyxea andAscetosporea, parasites of plants and animals, respectively, and the peculiar amoebaGromia. 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 byCavalier-Smith in 2002,[9] who also included thecentrohelids andApusozoa.
A noteworthy order that belongs toAscetosporea is theMikrocytida.[10] These are parasites ofoysters. This includes the causative agent of Denman Island Disease,Mikrocytos mackini a small (2−3 μm diameter) amitochondriate protistan.[11]
Similarities between various Rhizaria organisms have been noticed since the 19th century. In his 1861 classification of theRhizopoda (amoebae), the zoologistWilliam B. Carpenter proposed the orderReticularia, which consisted ofForaminifera andGromiida on the basis of their very similar thin, reticulose pseudopodia with granules circulating inside.[12] However, the idea that these organisms and others such asRadiolaria 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 theSAR supergroup (Stramenopiles, Alveolates, Rhizaria), a grouping that had been presaged in 1993 through a study of mitochondrial morphologies.[14] SAR is currently placed in theDiaphoretickes along withArchaeplastida,Cryptista,Haptista, and several minor clades.
Historically, many rhizarians were consideredanimals because of their motility andheterotrophy. However, when a simple animal-plant dichotomy was superseded by a recognition of additional kingdoms, taxonomists generally placed amoebae in the kingdomProtista. When scientists began examining the evolutionary relationships among eukaryotes in the 1970s, it became clear that the kingdomProtista wasparaphyletic. Rhizaria appear to share a common ancestor withStramenopiles andAlveolates forming part of the SAR super assemblage.[15] Rhizaria has been supported by molecular phylogenetic studies as a monophyletic group.[16] Biosynthesis of24-isopropyl cholestane precursors in various rhizaria[17] suggests a relevant ecological role already during theEdiacaran.
Rhizaria is amonophyletic group composed of two sister phyla:Cercozoa andRetaria. Subsequently, Cercozoa and Retaria are alsomonophyletic.[18][19] The following cladogram depicts the evolutionary relationships between all rhizarianclasses, and is made after the works ofCavalier-Smithet al. (2018),[1] and Irwinet al. (2019).[20]
^Gast, Rebecca J.; Caron, David A. (2001-10-01). "Photosymbiotic associations in planktonic foraminifera and radiolaria".Hydrobiologia.461 (1):1–7.doi:10.1023/A:1012710909023.S2CID1387879.
^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.Bibcode:2007MolPE..44..255M.doi:10.1016/j.ympev.2006.11.001.PMID17174576.
^Seravin LN (1993). "[The basic types and forms of the fine structure of mitochondrial cristae: the degree of their evolutionary stability (capacity for morphological transformations)]".Tsitologiia (in Russian).35 (4):3–34.PMID8328023.
^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.
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