Before molecular analyses recovered this clade, evolutionary biologistThomas Cavalier-Smith had already hypothesized an evolutionary proximity between plants and the remaining groups (collectively known as 'chromalveolates' in his classification system). He coined the termphotokaryotes for these organisms, as they include almost all of the photosynthetic eukaryotes. He later called themcorticates due to the presence ofcortical alveoli in many of their members.
Although Diaphoretickes contains organisms of very different morphologies, they have a few common traits. Ancestrally they are similar toexcavates, with twoflagella and a ventral feeding groove. In addition, the presence of cortical alveoli and flagellar hairs are interpreted as ancestral traits unique to the group. Some traits appearedconvergently in many groups, such as the acquisition ofchloroplasts through primary and secondaryendosymbioses and the presence ofaxopodia and aheliozoan-type cell. In particular, chloroplasts withchlorophyllc and heliozoan cells are exclusive to Diaphoretickes.
Within Diaphoretickes, Cryptista and the heliozoanMicroheliella maris form the cladePancryptista, which is the closest relative to Archaeplastida, together forming theCAM clade. Haptista and SAR are closer to each other and to a clade of flagellates known asTelonemia. In addition, three small groups of protists,Provora,Hemimastigophora andMeteora sporadica, form a clade that may belong to Diaphoretickes.
The name Diaphoretickes derives from Greekδιαφορετικές (diaforetikés) meaning diverse, dissimilar, referring to the widemorphological and cellular diversity among members of this clade.[2]
Eukaryotes, organisms whose cells contain anucleus, have been traditionally grouped into fourkingdoms:animals,plants,fungi andprotists. In the late 20th century,molecular phylogenetic analyses revealed that protists are aparaphyletic assortment of many independent evolutionary lineages orclades, from which animals, fungi and plants evolved.[7][8] However, the relationships between these clades remained difficult to assess due to technological limitations.[9] Starting in the early 2000s, improvements on phylogenetics allowed the classification of most eukaryotes into a small number of diverse clades calledsupergroups.[10][11]
The SAR, haptophytes and cryptomonads were collectively known aschromalveolates[14][15] or kingdomChromista due to a hypothesized common ancestor that obtained the ability to photosynthesize, as algae included in them usually contain a unique pigment,chlorophyllc.[8][12] The relationship between plants and chromalveolates had been described earlier by evolutionary biologistThomas Cavalier-Smith (1942–2021), who referred to the clade containing both groups asphotokaryotes since most of their members are photosynthetic.[16][5] He later called themcorticates, suggesting that they share a common ancestor due to the presence ofcortical alveoli (vesicles underneath thecell membrane) in some of their members (glaucophytes and alveolates).[17][18][19] However, these names became obsolete, largely due to the discovery that chromalveolates are notmonophyletic: these algae evolved the ability to photosynthesizeindependently from one another.[13]
In 2012, a publication by the International Society of Protistologists (ISOP) established a taxonomic name for this clade,Diaphoretickes, with the followingphylogenetic definition:[2]
In the following years, higher quality phylogenetic analyses recovered more protists that fall into this definition (e.g.,telonemids,centrohelids,katablepharids), leading to new clades within Diaphoretickes, such asHaptista (centrohelids and haptophytes) andCryptista (cryptomonads, katablepharids and relatives).[20]
In 2015, Cavalier-Smith and co-authors rejected the name Diaphoretickes proposed by the ISOP, arguing that it was "an entirely unnecessary, and less euphonious third synonym with no intuitive meaning [...] which is destabilising and should not be used". Instead, they suggested converting a pre-existing taxonomic name,Corticata,[a] for the superkingdom containing Chromista and Archaeplastida (Plantae).[4] This did not reach consensus, and Diaphoretickes remains widely accepted by the scientific community as the name of this major eukaryotic clade.[21]
Two examples of large multicellular organisms in Diaphoretickes: thegiant redwood (left) and thegiant kelp (right)
Diversity of single-celled organisms in Diaphoretickes (clockwise from top left): a coccolithophore (Haptista), a katablepharid (Cryptista), a telonemid (Telonemia), and an opalinid (SAR)
Archaeplastida includes organisms withchloroplasts derived directly from aprimary endosymbiosis event with acyanobacterium. They amount to an estimated 450,000–500,000 species. Although known asplants by some authors,[24] archaeplastids include manyprotists that do not belong to the multicellular land plants or embryophytes (such asmosses,conifers,ferns,flowering plants). These protists are primarily thered algae,glaucophyte algae, andgreen algae, from which embryophytes evolved. Archaeplastids also include two small groups of heterotrophic flagellates closely related to red algae:rhodelphids andpicozoans.[25] Embryophytes, green algae and red algae all evolved multicellular forms and complex life cycles independently,[26] but embryophytes are distinguished by the retention of thezygote (fertilizedegg cell) as an embryo, instead of its dispersal as a single cell.[27]
The SAR supergroup is named after its three constituent clades:stramenopiles,alveolates andrhizarians. The stramenopiles gather more than 100,000 species in total[28] and comprise many heterotrophic unicellular or fungus-like organisms (e.g.,oomycetes,labyrinthulids,bicosoecids,opalinids), but the described diversity is concentrated in theochrophytes, the photosynthetic clade (e.g.,diatoms,kelp,golden algae). They are distinguished by the presence of straw-likemastigonemes (flagellar hairs) in one of their two flagella, when present.[29] The alveolates are unicellular protists primarily composed of three large, well-studied groups:ciliates (more than 8,000 species, mostly free-living heterotrophs),[30]dinoflagellates (~4,500 species, many photosynthetic)[31] andapicomplexans (more than 6,000 parasitic species), all of which are unicellular.[32] In particular, dinoflagellates, apicomplexans and various smaller groups (e.g.,chromerids) evolved from a photosynthetic ancestor and are collectively known asmyzozoans.[33] The rhizarians are a diverse group of mostlyamoeboid unicellular organisms of very different lifestyles, such as the free-livingradiolarians (over 1,000 living species)[34] andforams (over 6,700 living species),[35] the fungus-likephytomyxeans, the parasiticascetosporeans, and the photosyntheticchlorarachniophytes.[36]
Haptista is composed of two groups of single-celled organisms with mineralized scales. The first are the photosynthetichaptophytes (e.g., thecalcifyingcoccolithophores), of which there are over 500 living species.[37][38] The second are the heterotrophiccentrohelid amoebae, with around 95 species.[39] Cryptista is a group of fully single-celled flagellated organisms, among which are the photosyntheticcryptomonads (more than 100 species)[40] and related heterotrophs, namely thekatablepharids and the speciesPalpitomonas bilix.[21]
The cell ofColponema resembles the ancestral corticate,[19][4] with its anterior (af) and posterior (pf) flagella. The ventral groove can just be seen at the top of the cell body.
Despite their large diversity of forms, a few morphological traits are common to corticates. They arebiflagellates or bikonts, meaning their cells typically have twoflagella.[19] Their cellsancestrally have a ventral groove for feeding, as observed in early-branching species (e.g., the alveolateColponema and the stramenopilesKaonashia andPlatysulcus).[41][42] These cellulartraits are typical ofexcavates, aparaphyletic group composed of the most basal eukaryotes (i.e.,Discoba,Metamonada andMalawimonada); they are likely the ancestral traits of all eukaryotes.[15][43]
In addition, as opposed to excavates, many Diaphoretickes members havecortical alveoli (flattened vesicles beneath the cell surface), such as glaucophytes, alveolates, haptophytes, telonemids[23] and some early-branching stramenopiles (e.g.,Kaonashia,bigyromonads).[42] Due to the wide occurrence of these alveoli, various researchers consider them an ancestral characteristic of Diaphoretickes.[12][23] Another frequent trait is the presence of flagellar hairs, also considered ancestral and unique to Diaphoretickes.[41]
Multiple lineages within Diaphoretickes have acquired photosyntheticplastidsindependently from each other, evolving intoalgae. They include all eukaryotic algae except foreuglenophytes, which belong to the Discoba. Archaeplastids acquired their plastids directly from primary endosymbiosis with a cyanobacterium, while all other algae have plastids originating from asecondary endosymbiosis with either a red alga (as in ochrophytes, myzozoans, cryptomonads and haptophytes) or a green alga (as in chlorarachniophytes).[13] Red algal-derived plastids are exclusive to the Diaphoretickes clade, while green algal-derived ones are also present in euglenophytes.[44][1] One rhizarian species,Paulinella chromatophora, experienced an event of primary endosymbiosis with a different kind of cyanobacterium.[45]
The exact order of the red algal-derived plastid acquisitions is not yet known. Two main hypotheses agree that cryptophytes were the first to obtain them, and the remaining groups obtained theirs by endosymbiosis with a cryptophyte.[1] A third hypothesis proposed in 2024 suggests that there were two independent endosymbioses of a red alga in cryptophytes and ochrophytes, which in turn originated the plastids of haptophytes and myzozoans, respectively.[46]
Plastid acquisitions across eukaryotes, shown in discontinuous arrows: blue for the primary plastids derived directly from a cyanobacterium, and red and green for the secondary plastids derived from red algae and green algae, respectively. Red arrows are placed according to the 2024 hypothesis;[46] disagreements with previous hypotheses are marked '?'.[1]
Diaphoretickes also includes allamoebae that haveaxopodia, stiff filaments used for feeding that branch radially from the cell, a trait acquired independently in various groups. These were historically known as Actinopoda, and were divided into the marineradiolaria (rhizarians) and the mostly freshwaterheliozoa ("sun animalcules").[47] The heliozoa are primarily the centrohelids (relatives of haptophytes),actinophryids (stramenopiles) anddesmothoracids (rhizarians).[48] There are also some lone heliozoan species such asMicroheliella maris, thesister group of Cryptista.[49] Even heliozoa that have not been genetically sequenced are presumed to belong to Diaphoretickes.[50] Cavalier-Smith argued that the ancestral configuration of thecytoskeleton of corticates was apreadaptation that made it easier for them to evolve axopodia numerous independent times.[51]
Evolutionary relationships are still uncertain between the different clades of Diaphoretickes.Haptista andCryptista, initially hypothesized as relatives of each other (collectively known as the taxonHacrobia), were later revealed to be more distantly related.[52] In particular, Cryptista and the speciesMicroheliella maris form a clade known asPancryptista, which in turn is the closest relative ofArchaeplastida, together forming the proposed 'CAM' clade.[49][53]Telonemia, previously assigned to Hacrobia,[4] is sometimes resolved as the sister clade of theSAR supergroup, forming the hypothesized TSAR clade,[23] while other studies resolve it as more closely related toHaptista.[54]
Three small groups of protists—provorans,hemimastigotes, and the speciesMeteora sporadica—form a clade that may be either related to or inside of Diaphoretickes, depending on the analysis.[54][55][56] Before phylogenomic data fromMeteora and provorans became available, there was already a known affinity between hemimastigotes and Diaphoretickes, although the exact position of hemimastigotes remained unclear.[57] Cavalier-Smith proposed that hemimastigotes were the closest relatives of Diaphoretickes (known by him as corticates), and established the nameeucorta (eu-, 'well-developed' andcortex, 'bark') for their suggested clade, since both groups have a cortical pellicle: withcortical alveoli in corticates, and with microtubules and a proteinaceous thickening in hemimastigotes instead.[6] According to the phylogenetic definition of Diaphoretickes, any organism that is more closely related to them than to Discoba or Amorphea is considered part of them, which renders 'eucorta' a synonym of Diaphoretickes.[21]
The following cladogram summarizes the relationships within Diaphoretickes, according tophylogenomic analyses of the 2020s.[58][55][54][56] Chromalveolates are marked *; clades containing heliozoa are marked **.[4]
There is uncertainty regarding relationships with the remaining eukaryotic clades.[59] Between Diaphoretickes andAmorphea, the two major clades of eukaryotes,[21] there are many smaller clades—Discoba,Metamonada,Malawimonada,Ancyromonadida,CRuMs, and the aforementioned provoran-hemimastigote-Meteora clade—that may branch closer to one or the other, or closer to the root of the eukaryotic tree, depending on the analysis.[55][43] Only some analyses find a closer relationship between Diaphoretickes and theDiscoba clade, together known asDiphoda.[54] According to a 2021molecular clock analysis, Diaphoretickes diverged from other eukaryotes during thePaleoproterozoic (2.2 to 1.6 billion years ago), although the first putative fossils originated during theMesoproterozoic.[1]
^abThe taxonomic name Corticata has changed in composition several times. It was first coined by zoologistEdwin Ray Lankester in 1878 as one of the two categories of theProtozoa (the other being Gymnomyxa), which he interpreted as a subkingdom of animals.[3] In his system, Corticata includedflagellated protists, many of which fall in Diaphoretickes (e.g., heterokonts, dinoflagellates, ciliates),[2][4] while Gymnomyxa (meaning 'naked slime') includedamoebae. In 2002, evolutionary biologistThomas Cavalier-Smith adopted these two names as subkingdoms of his own proposed kingdom Protozoa. He redefined Corticata to groupExcavata andRhizaria, due to similarities in theircytoskeleton.[5] This definition waspolyphyletic and fell out of use. After the description of Diaphoretickes, Cavalier-Smith repurposed Corticata in 2015 as its synonym.[4]
^Keeling, Patrick J.; Burger, Gertraud; Durnford, Dion G.; Lang, B. Franz; Lee, Robert W.; Pearlman, Ronald E.; Roger, Andrew J.; Gray, Michael W. (2005). "The tree of eukaryotes".Trends in Ecology & Evolution.20 (12):670–676.doi:10.1016/j.tree.2005.09.005.PMID16701456.
^Cavalier-Smith, Tom (1999). "Principles of protein and lipid targeting in secondary symbiogenesis: euglenoid, dinoflagellate, and sporozoan plastid origins and the eukaryote family tree".Journal of Eukaryotic Microbiology.46 (4):347–366.doi:10.1111/j.1550-7408.1999.tb04614.x.PMID18092388.
^Cavalier-Smith, Thomas (2003). "Protist phylogeny and the high-level classification of Protozoa".European Journal of Protistology.39 (4):338–348.doi:10.1078/0932-4739-00002.
^Lynn, Denis H. (2017). "Ciliophora". In Archibald, John M.; Simpson, Alastair G.B.; Slamovits, Claudio H. (eds.).Handbook of the Protists. Vol. 1 (2nd ed.). Springer. pp. 679–730.doi:10.1007/978-3-319-28149-0_23.ISBN978-3-319-28147-6.
^Saldarriaga, Juan F.; Taylor, F. J. R. 'Max' (2017). "Dinoflagellata". In Archibald, John M.; Simpson, Alastair G.B.; Slamovits, Claudio H. (eds.).Handbook of the Protists. Vol. 1 (2nd ed.). Springer. pp. 625–678.doi:10.1007/978-3-319-28149-0_22.ISBN978-3-319-28147-6.
^Votýpka, Jan; Modrý, David; Oborník, Miroslav; Šlapeta, Jan; Lukeš, Julius (2017). "Apicomplexa". In Archibald, John M.; Simpson, Alastair G.B.; Slamovits, Claudio H. (eds.).Handbook of the Protists. Vol. 1 (2nd ed.). Springer. pp. 567–624.doi:10.1007/978-3-319-28149-0_20.ISBN978-3-319-28147-6.
^Eikrem, Wenche; Medlin, Linda K.; Henderiks, Jorijntje; Rokitta, Sebastian; Rost, Björn; et al. (2017). "Haptophyta". In Archibald, John M.; Simpson, Alastair G.B.; Slamovits, Claudio H. (eds.).Handbook of the Protists. Vol. 2 (2nd ed.). Springer. pp. 893–954.doi:10.1007/978-3-319-28149-0_38.ISBN978-3-319-28147-6.
^Hoef-Emden, Kerstin; Archibald, John M. (2017). "Cryptophyta (Cryptomonads)". In Archibald, John M.; Simpson, Alastair G.B.; Slamovits, Claudio H. (eds.).Handbook of the Protists. Vol. 2 (2nd ed.). Springer. pp. 851–892.doi:10.1007/978-3-319-28149-0_35.ISBN978-3-319-28147-6.
^abCavalier-Smith, Thomas (May 2013). "Early evolution of eukaryote feeding modes, cell structural diversity, and classification of the protozoan phyla Loukozoa, Sulcozoa, and Choanozoa".European Journal of Protistology.49 (2):115–178.doi:10.1016/j.ejop.2012.06.001.PMID23085100.
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