Nucleariid

Nucleariids are single-celled amoebae that lack flagella and have radiating filopodia (i.e., thread-like pseudopodia). They have a spherical or sometimes flattened cell bodies with one or few conspicuous nuclei, each with a prominent central nucleolus except for Fonticula and Parvularia: Parvularia can also have peripheral nucleoral material instead, and Fonticula has an indistinct nucleolus. The cytoplasm contains multiple vacuoles, including food vesicles, contractile vacuoles in freshwater species, and lipid globules. They have relatively simple cells in comparison to other protists: they lack flagella, cytoplasmic microtubules,{{efn|The putative nucleariid genus Vampyrellidium has cytoplasmic microtubules, but the remaining ultrastructural traits have supported its placement among nucleariids.}} extrusomes, and special organelles. Exceptionally, some species contain endosymbiotic proteobacteria (most frequently members of the genus Rickettsia).

Most nucleariids have some kind of mucous coat, with or without coverings. The coverings can be made with endogenous silica-based particles (known as idiosomes) or with exogenous particles (known as xenosomes). These particles are developed into hollow siliceous scales or spines.{{cite journal |vauthors=Patterson DJ |title=On the Organization and Affinities of the Amoeba, Pompholyxophrys punicea Archer, Based on Ultrastructural Examination of Individual Cells from Wild Material 1 |journal=J. Protozool. |volume=32 |issue=2 |pages=241–6 |date=May 1985 |doi=10.1111/j.1550-7408.1985.tb03044.x }} The mucous coat itself—sometimes called glycocalyx—is enigmatic, as it can be present or absent in the same organism depending on the conditions. It appears to be made of one or two layers fibrous material running parallel to the cell membrane, and it often houses bacterial ectosymbionts. Surrounding the cell periphery, the characteristic hyaline (i.e., transparent) filopodia are found, originating from any point of the cell surface, sometimes branching or tapering but are never stiff or anastomosing (fusing with one another). Unlike Heliozoa, these filopodia are not supported by microtubules and do not contain extrusomes.

Most species develop a resting cyst during their life cycle consisting of a smooth spherical cell covered by one or more thick layers of a translucent material.

Nucleariids thrive in water bodies worldwide. Most live in a variety of freshwater environments, including hot spring waters of around 30 °C.{{cite journal |vauthors=Yoshida M, Nakayama T, Inouye I |title=Nuclearia thermophila sp. nov. (Nucleariidae), a new nucleariid species isolated from Yunoko Lake in Nikko (Japan) |journal=European Journal of Protistology |volume=45 |issue=2 |pages=147–155 |date=January 2009 |pmid=19157810 |doi=10.1016/j.ejop.2008.09.004 }} Others are found in marine environments (e.g., Lithocolla), and others inhabit faeces (Fonticula).

Nucleariids are free-living phagotrophs, and preferably consume cyanobacteria and other algae.{{cite journal|title=The nucleariid amoebae: more protists at the animal-fungal boundary|first1=Linda A. Amaral|last1=Zettler|first2=Thomas A.|last2=Nerad|first3=Charles J.|last3=O'Kelly|first4=Mitchell L.|last4=Sogin|journal=Journal of Eukaryotic Microbiology|date=11 July 2005|issue=3|pmid=11411837|volume=48|doi=10.1111/j.1550-7408.2001.tb00317.x|pages=293–297|s2cid=44548329}} Small-celled species like those of Parvularia and Fonticula feed on small bacteria, while larger cells such as Nuclearia, Pompholyxophrys and Lithocolla can also feed on detritus and unicellular eukaryotic algae (e.g., diatoms). All of them are slow-paced grazers{{cite journal|first1=Sebastian|last1=Dirren|first2=Gianna|last2=Pitsch|first3=Marisa O.D.|last3=Silva|first4=Thomas|last4=Posch|title=Grazing of Nuclearia thermophila and Nuclearia delicatula (Nucleariidae, Opisthokonta) on the toxic cyanobacterium Planktothrix rubescens|journal=European Journal of Protistology|volume=60|date=August 2017|pages=87–101|doi=10.1016/j.ejop.2017.05.009|pmid=28675820 |url=https://www.zora.uzh.ch/id/eprint/144930/1/Dirren_2017_EJP.pdf }} that probably grow in response to the availability of their food sources, such as after algal blooms.

File:Macro_photograph_of_an_F._alba_colony_radius_growing_on_a_plate.jpg developing into sorocarps. Scale bar: 1 mm.]]

Nucleariids are the closest relatives of fungi,{{cite journal|last1=Steenkamp|first1=Emma T.|last2=Wright|first2=Jane|last3=Baldauf|first3=Sandra L.|date=8 September 2005|title=The Protistan Origins of Animals and Fungi|journal=Molecular Biology and Evolution|volume=23|issue=1|pages=93–106|doi=10.1093/molbev/msj011|pmid=16151185|doi-access=free}} together forming the clade Holomycota.{{Cite journal|last1=Brown|first1=Matthew W.|last2=Spiegel|first2=Frederick W.|last3=Silberman|first3=Jeffrey D.|date=2009-12-01|title=Phylogeny of the "Forgotten" Cellular Slime Mold, Fonticula alba, Reveals a Key Evolutionary Branch within Opisthokonta|journal=Molecular Biology and Evolution|language=en|volume=26|issue=12|pages=2699–2709|doi=10.1093/molbev/msp185|issn=0737-4038|pmid=19692665|doi-access=free}}{{cite journal|last1=Liu|first1=Yu|last2=Steenkamp|first2=Emma T.|last3=Brinkmann|first3=Henner|last4=Forget|first4=Lisa|first5=Hervé|last5=Philippe|first6=B. Franz|last6=Lang|title=Phylogenomic analyses predict sistergroup relationship of nucleariids and Fungi and paraphyly of zygomycetes with significant support|journal=BMC Evolutionary Biology|volume=9|pages=272|date=2009|issue=1 |doi=10.1186/1471-2148-9-272|doi-access=free|pmc=2789072|pmid=19939264|bibcode=2009BMCEE...9..272L }}{{Cite journal|last1=Tedersoo|first1=Leho|last2=Sánchez-Ramírez|first2=Santiago|last3=Kõljalg|first3=Urmas|last4=Bahram|first4=Mohammad|last5=Döring|first5=Markus|last6=Schigel|first6=Dmitry|last7=May|first7=Tom|last8=Ryberg|first8=Martin|last9=Abarenkov|first9=Kessy|date=2018|title=High-level classification of the Fungi and a tool for evolutionary ecological analyses|journal=Fungal Diversity|language=en|volume=90|issue=1|pages=135–159|doi=10.1007/s13225-018-0401-0|issn=1560-2745|doi-access=free}}{{cite journal|first1=Toni|last1=Gabaldón|first2=Eckhard|last2=Völcker|first3=Guifré|last3=Torruella|title=On the Biology, Diversity and Evolution of Nucleariid Amoebae (Amorphea, Obazoa, Opisthokonta)|journal=Protist|volume=173|issue=4|date=17 June 2022|pages=125895|doi=10.1016/j.protis.2022.125895|doi-access=free|pmid=35841659|hdl=2117/369912|hdl-access=free}} This clade is, in turn, closely related to Holozoa, the clade containing animals and their closest protist relatives. Together, they form the clade Opisthokonta. After animals and fungi, nucleariids include the third known occurrence of multicellularity among opisthokonts:{{efn|Excluding the colony formation found across holozoan protists, particularly choanoflagellates and ichthyosporeans, which can be considered multicellular to some degree.}} the species Fonticula alba, a type of slime mold, capable of aggregative multicellular fruiting that develops sorocarps (stalks with masses of spores) for dispersal. The existence of Fonticula alba suggests that opisthokonts have a great propensity toward multicellularity.

{{clade|label1=Obazoa|1={{clade

|1=Apusomonadida, Breviatea 70px

|label2=Opisthokonta|2={{clade

|1=Holozoa (animals and relatives) 50px

|label2=Holomycota|2={{clade

|1=Fungi 50px

|2=Nucleariids 50px

}}

}}

}}

}}

Nucleariids are unique within their greater evolutionary context. Apusomonads (relatives of opisthokonts), holozoans and fungi all evolved from ancestors that were single-celled phagotrophic flagellates. Opisthokonts, in particular, are characterized by a single posterior flagellum. Even the most basal-branching fungi (aphelids, rozellids and microsporidia){{efn|The aphelids, rozellids and microsporidia are sometimes interpreted as protists{{Cite journal |last1=Galindo |first1=Luis Javier |last2=Torruella |first2=Guifré |last3=López-García |first3=Purificación |last4=Ciobanu |first4=Maria |last5=Gutiérrez-Preciado |first5=Ana |last6=Karpov |first6=Sergey A |last7=Moreira |first7=David |date=2023 |title=Phylogenomics Supports the Monophyly of Aphelids and Fungi and Identifies New Molecular Synapomorphies |url=https://doi.org/10.1093/sysbio/syac054 |journal=Systematic Biology|volume=72 |issue=3 |pages=505–515 |doi=10.1093/sysbio/syac054 |pmid=35900180 }} instead of fungi,{{Cite journal |display-authors=6 |vauthors=Wijayawardene NN, Hyde KD, Al-Ani LK, Tedersoo L, Haelewaters D, Rajeshkumar KC, Zhao RL, Aptroot A, Leontyev DV, Saxena RK, Tokarev YS, Dai DQ, Letcher PM, Stephenson SL, Ertz D, Lumbsch HT, Kukwa M, Issi IV, Madrid H, Phillips AJ, Selbmann L, Pfliegler WP, Horváth E, Bensch K, Kirk PM, Kolaříková K, Raja HA, Radek R, Papp V, Dima B, Ma J, Malosso E, Takamatsu S, Rambold G, Gannibal PB, Triebel D, Gautam AK, Avasthi S, Suetrong S, Timdal E, Fryar SC, Delgado G, Réblová M, Doilom M, Dolatabadi S, Pawłowska J, Humber RA, Kodsueb R, Sánchez-Castro I, Goto BT, Silva DK, de Souza FA, Oehl F, da Silva GA, Silva IR, Błaszkowski J, Jobim K, Maia LC, Barbosa FR, Fiuza PO, Divakar PK, Shenoy BD, Castañeda-Ruiz RF, Somrithipol S, Lateef AA, Karunarathna SC, Tibpromma S, Mortimer PE, Wanasinghe DN, Phookamsak R, Xu J, Wang Y, Tian F, Alvarado P, Li DW, Kušan I, Matočec N, Maharachchikumbura SS, Papizadeh M, Heredia G, Wartchow F, Bakhshi M, Boehm E, Youssef N, Hustad VP, Lawrey JD, Santiago AL, Bezerra JD, Souza-Motta CM, Firmino AL, Tian Q, Houbraken J, Hongsanan S, Tanaka K, Dissanayake AJ, Monteiro JS, Grossart HP, Suija A, Weerakoon G, Etayo J, Tsurykau A, Vázquez V, Mungai P, Damm U, Li QR, Zhang H, Boonmee S, Lu YZ, Becerra AG, Kendrick B, Brearley FQ, Motiejūnaitė J, Sharma B, Khare R, Gaikwad S, Wijesundara DS, Tang LZ, He MQ, Flakus A, Rodriguez-Flakus P, Zhurbenko MP, McKenzie EH, Stadler M, Bhat DJ, Liu JK, Raza M, Jeewon R, Nassonova ES, Prieto M, Jayalal RG, Erdoğdu M, Yurkov A, Schnittler M, Shchepin ON, Novozhilov YK, Silva-Filho AG, Liu P, Cavender JC, Kang Y, Mohammad S, Zhang LF, Xu RF, Li YM, Dayarathne MC, Ekanayaka AH, Wen TC, Deng CY, Pereira OL, Navathe S, Hawksworth DL, Fan XL, Dissanayake LS, Kuhnert E, Grossart HP, Thines M |title = Outline of Fungi and fungus-like taxa |journal=Mycosphere |year=2020 |volume=11 |issue=1 |pages=1060–1456 |url=http://www.mycosphere.org/pdf/MYCOSPHERE_11_1_8-1.pdf |issn=2077-7019 |doi=10.5943/mycosphere/11/1/8 |pmid= |doi-access=free }} depending on the authors.}} are single-celled flagellates that prey on other eukaryotes. The last common ancestor of nucleariids, however, had lost the opisthokont flagellum and its cell polarity, and had gained the characteristic mucous coat. The presence of filopodia is more common among opisthokonts, shared with aphelids and most holozoans.{{cite journal|last1=Shalchian-Tabrizi|first1=Kamran|last2=Minge|first2=Marianne A.|last3=Espelund|first3=Mari|last4=Orr|first4=Russell|last5=Ruden|first5=Torgeir|last6=Jakobsen|first6=Kjetill S.|last7=Cavalier-Smith|first7=Thomas|author-link7=Thomas Cavalier-Smith|date=7 May 2008|title=Multigene Phylogeny of Choanozoa and the Origin of Animals|journal=PLOS ONE|volume=3|issue=5|pages=e2098|doi=10.1371/journal.pone.0002098|doi-access=free |pmid=18461162 |pmc=2346548 |bibcode=2008PLoSO...3.2098S }}

The history of the classification of nucleariids is full of incongruence between morphology and molecular phylogeny. Toward the end of the 19th century, most nucleariid species had already been described, and were classified with other naked or scaled filose amoebae.{{cite book|last1=Penard|first1=Eugène|author-link1=Eugène Penard|title=Les héliozoaires d'eau douce|trans-title=The freshwater heliozoans|language=French|publisher=W. Kündig & fils|date=1904|location=Genève|doi=10.5962/bhl.title.1407|oclc=8236527|id={{BHL page|1095193}}|doi-access=free|url=https://archive.org/details/leshliozoaires00pena}} During the second half of the 20th century, naturalist Heinrich Rainer described a subgroup of heliozoans, Rotosphaeridia, to accommodate non-flagellated, scaled, filose amoebae without axopodia (the nucleariids Pompholyxophrys, Pinaciophora, Lithocolla and Rhabdiophrys).{{cite book|last1=Rainer|first1=Heinrich|title=Urtiere, Protozoa; Wurzelfüẞler, Rhizopoda; Sonnentierchen, Heliozoa: Systematik und Taxonomie, Biologie, Verbreitung und Ökologie der Arten der Erde|trans-title=Protozoa; Rhizopods; Heliozoa: systematics and taxonomy, biology, distribution and ecology of the species of the Earth|language=German|date=1968|series=Die Tierwelt Deutschlands und der angrenzenden Meeresteile [The animal world of Germany and the adjacent sea areas]|volume=56|publisher=VEB Gustav Fischer Verlag|location=Jena|oclc=5630582}}

Protozoologists John P. Cann and Frederick C. Page established the family Nucleariidae to include the naked genera Nuclearia, Gobiella and Nucleosphaerium{{cite journal|last1=Cann|first1=J. P.|last2=Page|first2=F. C.|title=Nucleosphaerium tuckeri nov. gen. nov. sp. – a new freshwater filose amoeba without motile form in a new family Nucleariidae (Filosea: Aconchulinida) feeding by ingestion only|journal=Archiv für Protistenkunde|volume=122|date=1979|issue=3–4|pages=226–240|doi=10.1016/S0003-9365(79)80034-2}} (later synonymized with Nuclearia).{{cite journal|last1=Cann|first1=John P.|date=August 1986|title=The feeding behavior and structure of Nuclearia delicatula|journal=Journal of Protozoology|volume=33|issue=3|pages=392–396|doi=10.1111/j.1550-7408.1986.tb05629.x}} Through studies of their fine cellular ultrastructure via transmission electron microscopy, the order Cristidiscoidida was established within the class Filosea, to accommodate both families Nucleariidae and the silica-scaled Pompholyxophryidae (e.g., Pompholyxophrys, Pinaciophora and Rhabdiophrys), because they all shared disc-shaped mitochondrial cristae as a common characteristic (i.e., they were discicristate).{{cite journal|last=Page|first=Frederick C.|title=The classification of 'naked' amoebae (Phylum Rhizopoda)|journal=Archiv für Protistenkunde|volume=133|issue=3–4|date=1987|pages=199–217|doi=10.1016/S0003-9365(87)80053-2}}{{cite book|chapter=Amoebae of uncertain affinities|first1=D. J.|last1=Patterson|first2=A. G. B.|last2=Simpson|first3=A.|last3=Rogerson|pages=804–827|chapter-url=https://protistologists.org/wp-content/uploads/2023/07/19AMOEBAE_OF_UNCERTAIN_AFFINITIES.pdf|title=The Illustrated Guide to the Protozoa: organisms traditionally referred to as protozoa, or newly discovered groups|editor-first1=John J.|editor-last1=Lee|editor-first2=Gordon F.|editor-last2=Leedale|editor-first3=Phyllis|editor-last3=Bradbury|volume=II|edition=2nd|publisher=Society of Protozoologists|url=https://protistologists.org/publications/illustrated-guide-to-the-protozoa/|author-link1=David J. Patterson|date=2000}}

The genus Fonticula was continually excluded, as it was considered an acrasid{{cite journal|first1=Ann C.|last1=Worley|first2=Kenneth B.|last2=Raper|first3=Marianne|last3=Hohl|title=Fonticula alba: a new cellular slime mold (Acrasiomycetes)|journal=Mycologia|volume=71|date=1979|issue=4|pages=746–760|doi=10.1080/00275514.1979.12021068}} or a slime mold,{{cite journal|first1=Mary C.|last1=Deasey|first2=Lindsay S.|last2=Olive|title=Role of Golgi apparatus in sorogenesis by the cellular slime mold Fonticula alba|journal=Science|volume=213|issue=4507|pages=561–563|doi=10.1126/science.213.4507.561|date=31 July 1981|pmid=17794844 |bibcode=1981Sci...213..561D }} until 1993, when protozoologist Thomas Cavalier-Smith created the subclass Cristidiscoidia to house two orders: Nucleariida (with Nucleariidae and Pompholyxophryidae) and Fonticulida (with Fonticulidae).{{cite journal|last1=Cavalier-Smith|first1=Thomas|author-link=Thomas Cavalier-Smith|title=Kingdom Protozoa and its 18 phyla|journal=Microbiological Reviews|date=1993|volume=57 |pages=953–994|pmc=372943|pmid=8302218|doi= 10.1128/mr.57.4.953-994.1993|issue=4}} However, he later considered scaled nucleariids (Pompholyxophryidae) as members of the Cercozoa, completely separate from naked ones,{{cite journal|last1=Cavalier-Smith|first1=Thomas|author-link=Thomas Cavalier-Smith|last2=Chao|first2=Ema E.|title=Oxnerella micra sp. n. (Oxnerellidae fam. n.), a tiny naked centrohelid, and the diversity and evolution of Heliozoa|volume=163|issue=4|date=July 2012|pages=574–601|doi=10.1016/j.protis.2011.12.005|journal=Protist|pmid=22317961 }} but this is no longer accepted. In a 1999 taxonomic revision by Kirill A. Mikrjukov, Cristidiscoidida was regarded as a junior synonym of Rotosphaerida, which finally united all discicristate filose amoebae. He also included Belonocystis{{cite journal|first1=Kirill A.|last1=Mikrjukov|title=Taxonomic revision of scale-bearing heliozoon-like amoebae (Pompholyxophryidae, Rotosphaerida)|journal=Acta Protozoologica|volume=38|issue=2|pages=119–131|date=1999|url=https://rcin.org.pl/dlibra/publication/17718/edition/13534/content}} and Micronuclearia in this order,{{cite journal|last1=Mikrjukov|first1=Kirill A.|last2=Mylnikov|first2=Alexander P.|title=A study of the fine structure and the mitosis of a lamellicristate amoeba, Micronuclearia podoventralis gen. et sp. nov. (Nucleariidae, Rotosphaerida)|journal=European Journal of Protistology|volume=37|issue=1|date=2001|doi=10.1078/0932-4739-00783|pages=15–24}} which now are known to belong to Amoebozoa and CRuMs, respectively.

At present, different conflicting classifications for nucleariids remain in use depending on the authors. Cavalier-Smith maintained his system through the years, using the name Cristidiscoidea as a class of his paraphyletic phylum Choanozoa, which included all protists most closely related to animals and fungi.{{cite journal|first1=Michael A.|last1=Ruggiero|first2=Dennis P.|last2=Gordon|first3=Thomas M.|last3=Orrell|first4=Nicolas|last4=Bailly|first5=Thierry|last5=Bourgoin|first6=Richard C.|last6=Brusca|first7=Thomas|last7=Cavalier-Smith|author-link7=Thomas Cavalier-Smith|first8=Michael D.|last8=Guiry|author-link8=Michael D. Guiry|first9=Paul M.|last9=Kirk|title=A higher level classification of all living organisms|journal=PLOS ONE|date=29 April 2015|doi-access=free|doi=10.1371/journal.pone.0119248|volume=10|issue=4|pages=e0119248|pmid=25923521|pmc=4418965|bibcode=2015PLoSO..1019248R }} Mycologists have proposed a separate kingdom Nucleariae with lower ranks for Cavalier-Smith's two orders (phyla Nuclearida and Fonticulida, classes Nuclearidea and Fonticulidea), but without specifying their taxonomic composition. Generally, protistologists prefer using the order Rotosphaerida instead, as it has priority over more recent names.{{cite journal | url=https://doi.org/10.1111%2FJEU.12691 | doi=10.1111/JEU.12691 | title=Revisions to the Classification, Nomenclature, and Diversity of Eukaryotes | date=26 September 2018| last1=Adl | first1=Sina M. | last2=Bass | first2=David | last3=Lane | first3=Christopher E. | last4=Lukeš | first4=Julius | last5=Schoch | first5=Conrad L. | last6=Smirnov | first6=Alexey | last7=Agatha | first7=Sabine | last8=Berney | first8=Cedric | last9=Brown | first9=Matthew W. | last10=Burki | first10=Fabien | last11=Cárdenas | first11=Paco | last12=Čepička | first12=Ivan | last13=Chistyakova | first13=Lyudmila | last14=Del Campo | first14=Javier | last15=Dunthorn | first15=Micah | last16=Edvardsen | first16=Bente | last17=Eglit | first17=Yana | last18=Guillou | first18=Laure | last19=Hampl | first19=Vladimír | last20=Heiss | first20=Aaron A. | last21=Hoppenrath | first21=Mona | last22=James | first22=Timothy Y. | last23=Karnkowska | first23=Anna | last24=Karpov | first24=Sergey | last25=Kim | first25=Eunsoo | last26=Kolisko | first26=Martin | last27=Kudryavtsev | first27=Alexander | last28=Lahr | first28=Daniel J.G. | last29=Lara | first29=Enrique | last30=Le Gall | first30=Line | journal=The Journal of Eukaryotic Microbiology | volume=66 | issue=1 | pages=4–119 | pmid=30257078 | pmc=6492006}}

Most studies using the name Cristidiscoidea have only included Nuclearia or environmental sequences, while those using Rotosphaerida have been used when studying the scale-bearing amoebae and, more recently, the naked ones as well.

At the family-level rank, although historically both Nucleariidae and Pompholyxophryidae have been used separately for naked and scale-bearing nucleariids respectively, and Fonticulidae solely for Fonticula, protistologists tend to use only Nucleariidae now, as there is no clear evolutionary separation between the three families.

Of all nucleariids, only a few have been isolated and molecularly analyzed.{{cite journal|first1=Luis Javier|last1=Galindo|first2=Guifré|last2=Torruella|first3=David|last3=Moreira|first4=Yana|last4=Eglit|first5=Alastair G. B.|last5=Simpson|first6=Eckhard|last6=Völcker|first7=Steffen|last7=Clauẞ|first8=Purificación|last8=López-García|title=Combined cultivation and single-cell approaches to the phylogenomics of nucleariid amoebae, close relatives of fungi|journal=Philosophical Transactions of the Royal Society B. Biological Sciences|date=7 October 2019|volume=374|issue=1786 |pages=20190094|doi=10.1098/rstb.2019.0094|pmid=31587649 |pmc=6792443 }} Many genera remain as incertae sedis due to the lack of molecular data,{{cite journal|first1=Sina M.|last1=Adl|first2=Alastair G. B.|last2=Simpson|first3=Christopher E.|last3=Lane|first4=Julius|last4=Lukeš|first5=David|last5=Bass|first6=Samuel S.|last6=Bowser|first7=Matthew W.|last7=Brown|first8=Fabien|last8=Burki|first9=Micah|last9=Dunthorn|first10=Vladimir|last10=Hampl|first11=Aaron|last11=Heiss|first12=Mona|last12=Hoppenrath|first13=Enrique|last13=Lara|first14=Line|last14=le Gall|first15=Denis H.|last15=Lynn|first16=Hilary|last16=McManus|first17=Edward A. D.|last17=Mitchell|first18=Sharon E.|last18=Mozley-Stanridge|first19=Laura W.|last19=Parfrey|first20=Jan|last20=Pawlowski|first21=Sonja|last21=Rueckert|first22=Laura|last22=Shadwick|first23=Conrad L.|last23=Schoch|first24=Alexey|last24=Smirnov|first25=Frederick W.|last25=Spiegel|title=The Revised Classification of Eukaryotes|journal=The Journal of Eukaryotic Microbiology|date=28 September 2012|volume=59|issue=2|pages=429–514|doi=10.1111/j.1550-7408.2012.00644.x|doi-access=free|pmc=3483872|pmid=23020233}} as it is difficult to confirm their evolutionary position other than by morphological similarity. The following is a list of nucleariid genera with definite placement:

• Fonticula {{au|Worley, Raper & Hohl 1979}}
• Lithocolla {{au|Schulze 1874}}
• Nuclearia Cienkowski 1865 (=Nuclearella {{au|Frenzel 1897}}; Nuclearina {{au|Frenzel 1897}}; Nucleosphaerium {{au|Cann & Page 1979}})
• Parvularia López-Escardó 2017{{Cite journal|last1=López-Escardó|first1=David|last2=López-García|first2=Purificación|last3=Moreira|first3=David|last4=Ruiz-Trillo|first4=Iñaki|last5=Torruella|first5=Guifré|title=Parvularia atlantis gen. et sp. nov., a Nucleariid Filose Amoeba (Holomycota, Opisthokonta)|journal=Journal of Eukaryotic Microbiology|volume=65|issue=2|language=en|pages=170–179|doi=10.1111/jeu.12450|pmid=28741861|issn=1550-7408|year=2017|pmc=5708529}}
• Pompholyxophrys {{au|Archer 1869}} (=Hyalolampe {{au|Greeff 1869}}

The following are genera regarded as possibly belonging to this order:

• Elaeorhanis {{au|Greef, 1873}} (=Lithosphaerella {{au|Frenzel 1897}}; Estrella {{au|Frenzel 1897}}) – may also be related to Diplophrys, a member of the labyrinthulomycete order Amphitremida.{{cite journal|first1=Yuki|last1=Takahashi|first2=Masaki|last2=Yoshida|first3=Isao|last3=Inouye|first4=Makoto M.|last4=Watanabe|title=Fibrophrys columna gen. nov., sp. nov: a member of the family Amphifilidae|volume=56|date=October 2016|pages=41–50|doi=10.1016/j.ejop.2016.06.003|journal=European Journal of Protistology|pmid=27468745 }}
• Pinaciophora {{au|Greeff 1869}} (=Pinacocystis {{au|Hertwig & Lesser 1874}}; Pinaciocystis {{au|Roskin 1929}}; Potamodiscus {{au|Gerloff 1968}})
• Rabdiaster {{au|Mikrjukov 1999}}
• Rabdiophrys {{au|Rainer 1968}}
• Thomseniophora {{au|Nicholls 2012}}{{cite journal|first1=Kenneth H.|last1=Nicholls|date=17 September 2012|title=New and little-known marine species of Pinaciophora, Rabdiaster and Thomseniophora gen. nov. (Rotosphaerida: Pompholyxophryidae)|journal=Journal of the Marine Biological Association of the United Kingdom|volume=93|issue=5|pages=1211–1229|doi=10.1017/S002531541200135X}}
• Vampyrellidium {{au|Zopf 1885}}

{{Notelist|25em}}

{{Reflist}}

{{Life on Earth}}

{{Eukaryota}}

{{Opisthokont protists}}

{{Taxonbar|from1=Q15129670|from2=Q15129663|from3=Q55711231|from4=Q21447207|from5=Q1048747|from6=Q13403500}}

Category:Holomycota

Category:Nucleariids