Taxonomy of Protista

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A modern non-hierarchical interpretation of the phylogeny of eukaryotes, including protists

A protist (/ˈproʊtɪst/) is any eukaryotic organism (one with cells containing a nucleus) that is not an animal, plant, or fungus. The protists do not form a natural group, or clade, since they exclude certain eukaryotes with whom they share a common ancestor;[a] but, like algae or invertebrates, the grouping is used for convenience. In some systems of biological classification, such as the popular five-kingdom scheme proposed by Robert Whittaker in 1969, the protists make up a kingdom called Protista, composed of "organisms which are unicellular or unicellular-colonial and which form no tissues".[1][b] In the 21st century, the classification shifted toward a two-kingdom system of protists: Chromista (containing the chromalveolate, rhizarian and hacrobian groups) and Protozoa (containing excavates and all protists more closely related to animals and fungi).[2]

The following groups contain protists. The clade Opisthokonta also contains the animals and the fungi, and the kingdom Archaeplastida also contains algae and plants.

Key to symbols used:

(P) = Paraphyletic group.
† = Extinct group.
? = Group of uncertain taxonomic position.
* = Group lacking molecular data.
[ ] = Synonyms and alternative names.

Current classification[edit]

This article largely follows the efforts of the scientific community and the International Society of Protistologists to revise the taxonomy of protists in a manner that reflects their phylogeny and evolution, striving away from the use of historical paraphyletic taxa and relying exclusively on clades as the basis of the classification. The reference used for the general arrangement of groups is the publication Revisions to the Classification, Nomenclature and Diversity of Eukaryotes, published in 2019 by researcher Sina Adl and coauthors.[3] Other studies are used for some specific groups of protists that have been revised after 2018, such as slime moulds,[4] glaucophyte algae,[5] arcellinid testate amoebae[6] and planktonic foraminifera.[7]

The following cladogram shows the relationships between the major groups of protists, as well as the kingdoms of animals, plants and fungi, according to the most recent phylogenetic analyses.[8][9][10][11][12]

Eukaryota
Choanozoa (P)
sensu Cavalier-Smith

Amorphea Adl et al. 2012[edit]

Obazoa Brown et al. 2013[edit]

Cladogram of Obazoa based on several analyses.[11][13] It includes the earliest diverging 'fungi' (microsporidians, aphelids and rozellids), which are only sometimes considered protists under the name of Opisthosporidia.[14]

Incertae sedis Obazoa:

Opisthokonta Cavalier-Smith 1987, emend. Adl et al. 2005[edit]

Amoebozoa Lühe 1913, sensu Cavalier-Smith 1998[edit]

Cladogram of Amoebozoa based on an 824-gene phylogenetic analysis published in 2022.[21]

Incertae sedis Amoebozoa: Belonocystis, Boveella, Biomyxa, Corallomyxa, Gibbodiscus, Hartmannia, Malamoeba, Malpighamoeba, Microglomus, Oscillosignum, Pseudothecamoeba, Rhabdamoeba, Schoutedamoeba, Stereomyxa, Subulamoeba, Thecochaos, Triaenamoeba, Unda.[3]

Tubulinea Smirnov et al. 2005[edit]

Evosea Kang et al. 2017[edit]

Discosea Cavalier-Smith et al. 2004 sensu Smirnov et al. 2011[edit]

Diaphoretickes Adl et al. 2012[edit]

Archaeplastida Adl et al. 2005[edit]

Stramenopiles Patterson 1989, emend. Adl et al. 2005[edit]

Stramenopiles incertae sedis:

Bigyra Cavalier-Smith 1998, emend. 2006[edit]

Gyrista Cavalier-Smith 1998[edit]

Alveolata Cavalier-Smith 1991[edit]

Cladogram of Alveolata, based on a SSU rDNA phylogeny of basal alveolates[53] and a 203-gene phylogeny of apicomplexans and their relatives[54] published in 2023.

Myzozoa Cavalier-Smith & Chao 2004[edit]

Ciliophora Doflein 1901[edit]

Rhizaria Cavalier-Smith 2002[edit]

Rhizaria incertae sedis: Gymnosphaerida Poche 1913, emend. Mikrjukov 2000. Actinocoryne, Cienkowskya, Gymnosphaera, Hedraiophrys [possible synonym of Cienkowskya], Wagnerella.

Cercozoa Cavalier-Smith 1998, emend. Adl et al. 2005; emend. Cavalier-Smith 2018[edit]

Endomyxa Cavalier-Smith 2002, emend. Bass & Berney in Adl et al. 2019[edit]

Retaria Cavalier-Smith 2002[edit]

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