Diaphoretickes () is a major evolutionary lineage, or clade, of eukaryote organisms spanning over 600,000 species. They comprise an enormous diversity of life forms, from single-celled protozoa to multicellular and numerous types of algae. The clade was discovered through phylogenetic analyses in the 21st century that revealed a close relationship between the supergroups Archaeplastida (or in a broad sense), Haptista, Cryptista, and SAR supergroup (Stramenopiles, Alveolata and Rhizaria).
Before molecular analyses recovered this clade, evolutionary biologist Thomas Cavalier-Smith had already hypothesized an evolutionary proximity between plants and the remaining groups (collectively known as '' in his classification system). He coined the term photokaryotes for these organisms, as they include almost all of the photosynthetic eukaryotes. He later called them corticates due to the presence of cortical alveoli in many of their members.
Although Diaphoretickes contains organisms of very different morphologies, they have a few common traits. Ancestrally they are similar to excavata, with two flagella 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 appeared convergently in many groups, such as the acquisition of through primary and secondary endosymbiosis and the presence of axopodia and a heliozoan-type cell. In particular, chloroplasts with chlorophyll c and heliozoan cells are exclusive to Diaphoretickes.
Within Diaphoretickes, Cryptista and the heliozoan Microheliella maris form the clade Pancryptista, which is the closest relative to Archaeplastida, together forming the CAM clade. Haptista and SAR are closer to each other and to a clade of flagellates known as Telonemia. In addition, three small groups of protists, Provora, Hemimastigophora and Meteora sporadica, form a clade that may belong to Diaphoretickes.
In 2008, a close evolutionary relationship was discovered between some of these clades: Archaeplastida (plants and relatives; sometimes known as kingdom Plantae), SAR supergroup (, and ), and two smaller groups of algae, and . This collection of organisms contains almost all eukaryotes capable of photosynthesis.
The SAR, haptophytes and cryptomonads were collectively known as or kingdom Chromista due to a hypothesized common ancestor that obtained the ability to photosynthesize, as algae included in them usually contain a unique pigment, chlorophyll c. The relationship between plants and chromalveolates had been described earlier by evolutionary biologist Thomas Cavalier-Smith (1942–2021), who referred to the clade containing both groups as photokaryotes since most of their members are photosynthetic. He later called them corticates, suggesting that they share a common ancestor due to the presence of cortical alveoli (vesicles underneath the cell membrane) in some of their members ( and alveolates). However, these names became obsolete, largely due to the discovery that chromalveolates are not monophyletic: these algae evolved the ability to photosynthesize independently from one another.
In 2012, a publication by the International Society of Protistologists (ISOP) established a taxonomic name for this clade, Diaphoretickes, with the following phylogenetic definition:
In the following years, higher quality phylogenetic analyses recovered more protists that fall into this definition (e.g., , , ), leading to new clades within Diaphoretickes, such as Haptista (centrohelids and haptophytes) and Cryptista (cryptomonads, katablepharids and relatives).
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, for the superkingdom containing Chromista and Archaeplastida (Plantae). This did not reach consensus, and Diaphoretickes remains widely accepted by the scientific community as the name of this major eukaryotic clade.
Archaeplastida includes organisms with chloroplasts derived directly from a primary endosymbiosis event with a cyanobacterium. They amount to an estimated 450,000–500,000 species. Although known as plants by some authors, archaeplastids include many that do not belong to the multicellular land plants or embryophytes (such as , , , ). These protists are primarily the red algae, glaucophyte algae, and green algae, from which embryophytes evolved. Archaeplastids also include two small groups of heterotrophic flagellates closely related to red algae: rhodelphis and . Embryophytes, green algae and red algae all evolved multicellular forms and complex life cycles independently, but embryophytes are distinguished by the retention of the zygote (fertilized egg cell) as an embryo, instead of its dispersal as a single cell.
The SAR supergroup is named after its three constituent clades: , and . The stramenopiles gather more than 100,000 species in total and comprise many heterotrophic unicellular or fungus-like organisms (e.g., , , , ), but the described diversity is concentrated in the , the photosynthetic clade (e.g., , kelp, golden algae). They are distinguished by the presence of straw-like (flagellar hairs) in one of their two flagella, when present. The alveolates are unicellular protists primarily composed of three large, well-studied groups: (more than 8,000 species, mostly free-living heterotrophs),
Haptista is composed of two groups of single-celled organisms with mineralized scales. The first are the photosynthetic (e.g., the calcifying ), of which there are over 500 living species.
In addition, as opposed to excavates, many Diaphoretickes members have cortical alveoli (flattened vesicles beneath the cell surface), such as glaucophytes, alveolates, haptophytes, telonemids and some early-branching stramenopiles (e.g., Kaonashia, ). Due to the wide occurrence of these alveoli, various researchers consider them an ancestral characteristic of Diaphoretickes. Another frequent trait is the presence of flagellar hairs, also considered ancestral and unique to Diaphoretickes.
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. 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.
Diaphoretickes also includes all amoebae that have axopodia, 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 marine radiolaria (rhizarians) and the mostly freshwater heliozoa ("sun animalcules"). The heliozoa are primarily the centrohelids (relatives of haptophytes), (stramenopiles) and (rhizarians). There are also some lone heliozoan species such as Microheliella maris, the sister group of Cryptista. Even heliozoa that have not been genetically sequenced are presumed to belong to Diaphoretickes. Cavalier-Smith argued that the ancestral configuration of the cytoskeleton of corticates was a preadaptation that made it easier for them to evolve axopodia numerous independent times.
Three small groups of protists—, , and the species Meteora sporadica—form a clade that may be either related to or inside of Diaphoretickes, depending on the analysis. Before phylogenomic data from Meteora and provorans became available, there was already a known affinity between hemimastigotes and Diaphoretickes, although the exact position of hemimastigotes remained unclear. Cavalier-Smith proposed that hemimastigotes were the closest relatives of Diaphoretickes (known by him as corticates), and established the name eucorta ( eu-, 'well-developed' and cortex, 'bark') for their suggested clade, since both groups have a cortical pellicle: with cortical alveoli in corticates, and with microtubules and a proteinaceous thickening in hemimastigotes instead. 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.
The following cladogram summarizes the relationships within Diaphoretickes, according to phylogenomics of the 2020s. Chromalveolates are marked *; clades containing heliozoa are marked **.
There is uncertainty regarding relationships with the remaining eukaryotic clades. Between Diaphoretickes and Amorphea, the two major clades of eukaryotes, 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. Only some analyses find a closer relationship between Diaphoretickes and the Discoba clade, together known as Diphoda. According to a 2021 molecular clock analysis, Diaphoretickes diverged from other eukaryotes during the Paleoproterozoic (2.2 to 1.6 billion years ago), although the first putative fossils originated during the Mesoproterozoic.
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