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Chromista is a proposed but polyphyletic obsolete Biology kingdom, refined from the Chromalveolata, consisting of single-celled and multicellular eukaryotic species that share similar features in their Photosynthesis organelles (). It includes all eukaryotes whose plastids contain chlorophyll c and are surrounded by four membranes. If the ancestor already possessed chloroplasts derived by Endosymbiont 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.
Chromista as a taxon was created by the British biologist Thomas Cavalier-Smith in 1981 to distinguish the , , and Cryptomonad. 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.
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 Rhodophyte, and to have been transmitted to the Cryptophyte and from them to both the Ochrophyte and the Haptophyte, and then from these last to the Myzozoa.
The kingdom includes diverse organisms from algae to malarial parasites ( Plasmodium). Molecular evidence indicates that the plastids in chromists were derived from red algae through secondary symbiogenesis in a single event. 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.
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:
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 (Haptophyta and Cryptophyta).
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. Fast et al. (2001) supported a single origin for the (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.
A 2020 phylogeny of the eukaryotes states that "the chromalveolate hypothesis is not widely accepted" (noting Cavalier-Smith et al 2018 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. 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:
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