Chromista
Proposed kingdom of eukaryotes with chlorophyll c plastids.
Chromista is a proposed biological kingdom—one that remains controversial—refined from the earlier grouping Chromalveolata. It includes both single-celled and multicellular eukaryotes that share certain features in their photosynthetic organelles, or plastids. Specifically, the kingdom covers all eukaryotes whose plastids contain chlorophyll c and are enclosed by four membranes. If the common ancestor of these organisms already had chloroplasts acquired through endosymbiosis from red algae, then any non-photosynthetic members of Chromista must have lost the ability to photosynthesize later on. It is also possible that the various groups within Chromista arose independently from the last eukaryotic common ancestor.
British biologist Thomas Cavalier-Smith created the kingdom in 1981 to set apart the stramenopiles, haptophytes, and cryptophytes. Originally, the kingdom mostly included photosynthetic eukaryotes (algae), but Cavalier-Smith later added many heterotrophs (protozoa) to the group. By 2022, Chromista was nearly as diverse as the kingdoms Plantae and Animalia, containing nine phyla. Notable members include marine algae, the organism that causes potato blight, dinoflagellates, *Paramecium*, the brain parasite *Toxoplasma*, and the malaria parasite *Plasmodium*.
However, many researchers reject Cavalier-Smith’s idea that chromists form a single evolutionary lineage (monophyly). Instead, they think some chromists acquired their plastids by engulfing another chromist, rather than inheriting them from a common ancestor. This process likely happened repeatedly, so that red plastids spread from one group to another. As a result, the plastids have a different evolutionary history from the hosts that carry them—they appear to have originated in the Rhodophytina, then passed to the Cryptophytina, from there to both the Ochrophyta and the Haptophyta, and finally from the Haptophyta to the Myzozoa.
Biologically, members of Chromista are single-celled or multicellular eukaryotes that typically have one or both of these features: plastids containing chlorophyll c, located inside an extra membrane (the periplastid membrane) within the rough endoplasmic reticulum (often in the perinuclear cisterna); and cilia with rigid tubular hairs that are either tripartite or bipartite. The kingdom includes a wide range of organisms, from algae to malarial parasites. Molecular
- proposed_by
- Thomas Cavalier-Smith
- year_proposed
- 1981
- field
- Biology
- nationality
- British
- known_for
- Controversial kingdom of eukaryotes with chlorophyll c plastids
- key_groups
- Stramenopiles, Haptophytes, Cryptophytes
Lore & Background
Chromista as a taxon was created by the British biologist Thomas Cavalier-Smith in 1981 to distinguish the stramenopiles, haptophytes, and cryptophytes. According to Cavalier-Smith, the kingdom originally consisted mostly of photosynthetic eukaryotes (algae), but he later brought many heterotrophs (protozoa) into the proposed group. Notable members include marine algae, potato blight, dinoflagellates, Paramecium, the brain parasite Toxoplasma, and the malarial parasite Plasmodium.
However, Cavalier-Smith's hypothesis of chromist monophyly has been rejected by many 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.
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. In 2009, Cavalier-Smith gave his reason for making a new kingdom, saying he established Chromista as a kingdom distinct from Plantae and Protozoa because of the evidence that chromist chloroplasts were acquired secondarily by enslavement of a red alga, itself a member of kingdom Plantae, and their unique membrane topology.
Reader's Guide
Chromista represents a significant but contested framework in eukaryotic classification. Its creation by Thomas Cavalier-Smith in 1981 aimed to group organisms whose plastids contain chlorophyll c and are surrounded by four membranes, suggesting a common evolutionary origin. The kingdom's diversity is notable, encompassing algae, parasites like Plasmodium and Toxoplasma, and protozoa such as Paramecium. However, the monophyly of Chromista is widely disputed. Many researchers argue that the plastids were acquired through serial endosymbiosis—where one chromist group obtained plastids from another—rather than from a single common ancestor. Molecular evidence has not resolved the relationships, and some analyses suggest that groups like cryptophytes may be more closely related to plants. The alternative hypothesis of serial endosymbiosis implies that the red plastids spread from one group to another, meaning the plastids have a different history from their hosts. As a result, Chromista remains a provisional grouping, with ongoing debate about whether it represents a natural clade or a collection of independently evolved lineages.
Did You Know?
- Chromista was created by British biologist Thomas Cavalier-Smith in 1981.
- Notable members include marine algae, potato blight, dinoflagellates, Paramecium, Toxoplasma, and Plasmodium.
- Many researchers reject chromist monophyly, proposing instead that plastids were acquired through serial endosymbiosis.
- As of 2022, the kingdom consisted of nine phyla and was nearly as diverse as Plantae and Animalia.
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