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Chromista is a proposed but polyphyletic<ref name="pmid7972066">Template:Cite journal</ref><ref name="Burki Roger Brown Simpson 2020 pp. 43–55"/><ref name="Strassert Irisarri Williams Burki 2021"/> obsolete biological kingdom, refined from the Chromalveolata, consisting of single-celled and multicellular eukaryotic species that share similar features in their photosynthetic organelles (plastids).<ref name="parfrey"/> It includes all eukaryotes whose plastids contain chlorophyll c and are surrounded by four membranes. If the ancestor already possessed chloroplasts derived by endosymbiosis from red algae, all non-photosynthetic Chromista have secondarily lost the ability to photosynthesise. Its members might have arisen independently as separate evolutionary groups from the last eukaryotic common ancestor.<ref name="pmid7972066" />

Chromista as a taxon was created by the British biologist Thomas Cavalier-Smith in 1981 to distinguish the stramenopiles, haptophytes, and cryptophytes.<ref name=bio1981>Template:Cite journal</ref> According to Cavalier-Smith, the kingdom originally consisted mostly of photosynthetic eukaryotes (algae), but he later brought many heterotrophs (protozoa) into the proposed group. As of 2018, the kingdom was nearly as diverse as the kingdoms Plantae and Animalia, consisting of eight phyla. Notable members include marine algae, potato blight, dinoflagellates, Paramecium, the brain parasite Toxoplasma, and the malarial parasite Plasmodium.<ref name="protoplasma18">Template:Cite journal</ref>

However, Cavalier-Smith's hypothesis of chromist monophyly has been rejected by other researchers, who consider it more likely that some chromists acquired their plastids by incorporating another chromist instead of inheriting them from a common ancestor. This is thought to have occurred repeatedly, so that the red plastids spread from one group to another. The plastids, far from characterising their hosts as belonging to a single clade, thus have a different history from their disparate hosts. They appear to have originated in the Rhodophytina, and to have been transmitted to the Cryptophytina and from them to both the Ochrophyta and the Haptophyta, and then from these last to the Myzozoa.<ref name="Strassert Irisarri Williams Burki 2021">Template:Cite journal</ref>

BiologyEdit

File:Chromista structure.jpg
Structure of some types of Chromista compared with plant cell (left). The idea was that the Chromista had arisen, supposedly just once (making them monophyletic, and in Tom Cavalier-Smith's view a separate Kingdom) by enslaving a red alga, ending up with multiple membranes around what became their red plastids. Groups lacking red plastids were supposed to have secondarily lost them. The Cryptophyta are within the Cryptista; the Myzozoa are within the Alveolata.

Members of Chromista are single-celled and multicellular eukaryotes having basically either or both features:<ref name=bio1981/>

  1. plastid(s) that contain chlorophyll c and lie within an extra (periplastid) membrane in the lumen of the rough endoplasmic reticulum (typically within the perinuclear cisterna);
  2. cilia with tripartite or bipartite rigid tubular hairs.

The kingdom includes diverse organisms from algae to malarial parasites (Plasmodium).<ref name="biol2009"/> Molecular evidence indicates that the plastids in chromists were derived from red algae through secondary symbiogenesis in a single event.<ref name="Keeling 2009">Template:Cite journal</ref> In contrast, plants acquired their plastids from cyanobacteria through primary symbiogenesis.<ref name="Ponce-Toledo Deschamps 2017">Template:Cite journal</ref> These plastids are now enclosed in two extra cell membranes, making a four-membrane envelope, as a result of which they acquired many other membrane proteins for transporting molecules in and out of the organelles. The diversity of chromists is hypothesised to have arisen from degeneration, loss or replacement of the plastids in some lineages.<ref name="Keeling 2010">Template:Cite journal</ref> Additional symbiogenesis of green algae has provided genes retained in some members (such as heterokonts),<ref>Template:Cite journal</ref> and bacterial chlorophyll (indicated by the presence of ribosomal protein L36 gene, rpl36) in haptophytes and cryptophytes.<ref>Template:Cite journal</ref>

History and groupsEdit

Some examples of classification of the groups involved, which have overlapping but non-identical memberships, are shown below.<ref>Template:Cite book</ref><ref>Template:Cite journal</ref>

Chromophycées (Chadefaud, 1950)Edit

The Chromophycées (Chadefaud, 1950),<ref>Template:Cite journal</ref> renamed Chromophycota (Chadefaud, 1960),<ref>Template:Cite book</ref> included the current Ochrophyta (autotrophic Stramenopiles), Haptophyta (included in Chrysophyceae until Christensen, 1962), Cryptophyta, Dinophyta, Euglenophyceae and Choanoflagellida (included in Chrysophyceae until Hibberd, 1975).

Chromophyta (Christensen 1962, 1989)Edit

The Chromophyta (Christensen 1962, 2008), defined as algae with chlorophyll c, included the current Ochrophyta (autotrophic Stramenopiles), Haptophyta, Cryptophyta, Dinophyta and Choanoflagellida. The Euglenophyceae were transferred to the Chlorophyta.

Chromophyta (Bourrelly, 1968)Edit

The Chromophyta (Bourrelly, 1968) included the current Ochrophyta (autotrophic Stramenopiles), Haptophyta and Choanoflagellida. The Cryptophyceae and the Dinophyceae were part of Pyrrhophyta (= Dinophyta).

Chromista (Cavalier-Smith, 1981)Edit

The name Chromista was first introduced by Cavalier-Smith in 1981;<ref name=bio1981/> the earlier names Chromophyta, Chromobiota and Chromobionta correspond to roughly the same group. It has been described as consisting of three different groups:<ref name="pmid18296415">Template:Cite journal</ref> It includes all protists whose plastids contain chlorophyll c.<ref name="pmid7972066"/>

In 1994, Cavalier-Smith and colleagues indicated that the Chromista is probably a polyphyletic group whose members arose independently, sharing no more than descent from the common ancestor of all eukaryotes:<ref name="pmid7972066"/>

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In 2009, Cavalier-Smith gave his reason for making a new kingdom, saying:<ref name="biol2009"/>

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Since then Chromista has been defined in different ways at different times. In 2010, Cavalier-Smith reorganised Chromista to include the SAR supergroup (named for the included groups Stramenopiles, Alveolata and Rhizaria) and Hacrobia (Haptista and Cryptista).<ref name="biol2009"/>

Patron et al. (2004) considered the presence of a unique class of FBA (fructose-1,6-biophosphate-aldolase) enzyme not similar to that found in plants as evidence of chromist monophyly.<ref>Template:Cite journal</ref> Fast et al. (2001) supported a single origin for the myzozoan (dinoflagellate + apicomplexan), heterokont and cryptophyte plastids based on their comparison of GAPDH (glyceraldehyde-3-phosphate dehydrogenase) genes.<ref>Template:Cite journal</ref> Harper & Keeling (2003) described haptophyte homologs and considered them further evidence of a single endosymbiotic event involving the ancestor of all chromists.<ref>Template:Cite journal</ref>

Chromalveolata (Adl et al., 2005)Edit

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The Chromalveolata included Stramenopiles, Haptophyta, Cryptophyta and Alveolata.<ref name=Adl2005>Template:Cite journal</ref> However, in 2008 the group was found not to be monophyletic,<ref name=Burki2008a>Template:Cite journal</ref><ref name=KimGraham2008>Template:Cite journal</ref> and later studies confirmed this.<ref name="Burki Kaplan 2016 Untangling"/><ref name="Burki Okamoto 2012 Separate origins">Template:Cite journal</ref>

ClassificationEdit

Cavalier-Smith et al. 2015Edit

In 2015, Cavalier-Smith and his colleagues made a new higher-level grouping of all organisms as a revision of the seven kingdoms model. In it, they classified the kingdom Chromista into 2 subkingdoms and 11 phyla, namely:<ref>Template:Cite journal</ref>

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Cavalier-Smith 2018Edit

File:Chromista classification.jpg
Chromista classification according to Cavalier-Smith, 2018, with supposed events marking group divergences

Cavalier-Smith made a new analysis of Chromista in 2018 in which he classified all chromists into 8 phyla (Gyrista corresponds to the above phyla Ochrophyta and Pseudofungi, Cryptista corresponds to the above phyla Cryptista and "N.N.", Haptista corresponds to the above phyla Haptophyta and Heliozoa):<ref name="protoplasma18"/>

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Polyphyly and serial endosymbiosisEdit

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Molecular trees have had difficulty resolving relationships between the different groups. All three may share a common ancestor with the alveolates (see chromalveolates), but there is evidence that suggests the haptophytes and cryptomonads do not belong together with the heterokonts or the SAR clade, but may be associated with the Archaeplastida.<ref name="parfrey">Template:Cite journal</ref><ref name="pmid17726520">Template:Cite journal</ref> Cryptista specifically may be sister or part of Archaeplastida,<ref name="Burki Kaplan 2016 Untangling">Template:Cite journal</ref> though this could be an artefact due to acquisition of genes from red algae by cryptomonads.<ref name="Strassert Irisarri Williams Burki 2021" />

A 2020 phylogeny of the eukaryotes states that "the chromalveolate hypothesis is not widely accepted" (noting Cavalier-Smith et al 2018<ref name="Cavalier-Smith Chao Lewis 2018 pp. 1517–1574">Template:Cite journal</ref> as an exception), explaining that the host lineages do not appear to be closely related in "most phylogenetic analyses".<ref name="Burki Roger Brown Simpson 2020 pp. 43–55"/><ref name="Burki 2017">Template:Cite book</ref> Further, none of TSAR, Cryptista, and Haptista, groups formerly within Chromalveolata, appear "likely to be ancestrally defined by red secondary plastids". This is because of the many non-photosynthetic organisms related to the groups with chlorophyll c, and the possibility that cryptophytes are more closely related to plants.<ref name="Burki Roger Brown Simpson 2020 pp. 43–55">Template:Cite journal</ref>

The alternative to monophyly is serial endosymbiosis, meaning that the "chromists" acquired their plastids from each other instead of inheriting them from a single common ancestor. Thus the phylogeny of the distinctive plastids, which are agreed to have a common origin in the rhodophytes, is different from the phylogeny of the host cells.<ref name="Strassert Irisarri Williams Burki 2021"/> In 2021, Jürgen Strassert and colleagues modelled the timelines for the presumed spread of the red plastids, concluding that "the hypotheses of serial endosymbiosis are chronologically possible, as the stem lineages of all red plastid-containing groups overlap in time" during the Mesoproterozoic and Neoproterozoic eras. They propose that the plastids were transmitted between groups as follows:<ref name="Strassert Irisarri Williams Burki 2021"/>

RhodophytinaCryptophytinaOchrophyta
 ↘ HaptophytaMyzozoa

See alsoEdit

ReferencesEdit

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External linksEdit

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Template:Eukaryota Template:Taxonbar Template:Authority control