Chromista is a proposed but controversial 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 2022, the kingdom was nearly as diverse as the kingdoms Plantae and Animalia, consisting of nine phyla. Notable members include marine algae, potato blight, dinoflagellates, Paramecium, the brain parasite Toxoplasma, and the malarial parasite Plasmodium.

Biology

thumb|upright=2.5|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: In contrast, plants acquired their plastids from cyanobacteria through primary symbiogenesis. 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. Additional symbiogenesis of green algae has provided genes retained in some members (such as heterokonts), and bacterial chlorophyll (indicated by the presence of ribosomal protein L36 gene, rpl36) in haptophytes and cryptophytes.

History and groups

Some examples of classification of the groups involved, which have overlapping but non-identical memberships, are shown below.

Chromophycées (Chadefaud, 1950)

The Chromophycées (Chadefaud, 1950), renamed Chromophycota (Chadefaud, 1960), 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 and 1989)

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)

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)

The name Chromista was first introduced by Cavalier-Smith in 1981; Chromophyta (or Chromobiota) corresponds to non-cryptophyte chromists.

  • Heterokonts or Stramenopiles: brown algae, diatoms, water moulds, etc.
  • Haptophytes
  • Cryptomonads

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:

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 <!--which includes Heliozoa--> and Cryptista). 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. Harper & Keeling (2003) described haptophyte homologs and considered them further evidence of a single endosymbiotic event involving the ancestor of all chromists.

Chromalveolata (Adl et al., 2005)

The Chromalveolata included Stramenopiles, Haptophyta, Cryptophyta and Alveolata. However, in 2008 the group was found not to be monophyletic, and later studies confirmed this.

Classification

Ruggiero et al., 2015

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:

Cavalier-Smith, 2018

thumb|upright=1.5|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):

Polyphyly and serial endosymbiosis

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. Cryptista specifically may be sister or part of Archaeplastida, though this could be an artefact due to acquisition of genes from red algae by cryptomonads. as an exception), explaining that the host lineages do not appear to be closely related in "most phylogenetic analyses". 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.

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.

::: Rhodophyta → Cryptophyta → Haptophyta

::: Rhodophyta → Ochrophyta → Myzozoa

See also

  • Cabozoa
  • Cavalier-Smith's system of classification
  • List of Chromista by conservation status

References

  • UCMP: Introduction to the Chromista