Description and life cycle
Chrompodellids are a phylum of unicellular Protist, protists containing two functionally different groups: the Photosynthesis, photosynthetic "chromerids" and the predatory Phagocytosis, phagotrophic "colpodellids". Like other Alveolate, Alveolata, they present tubular crista, mitochondrial cristae and highly flattened Cortical alveolum, cortical alveoli with microtubules underneath. They exhibit a myzocytosis, conoid-like structure similar to that of apicomplexans, with an Apicomplexa, apical complex, a micropore and a rostrum. They live as flagellates with two anisokont (i.e. differently sized) Flagellum, flagella that are heterodynamic (i.e. move in different patterns). Some species exhibit thin mastigonemes in their anterior flagellum, while others bear bulbs. Some species are capable of forming microbial cyst, cysts.Colpodellids
Colpodellids, represented by the genera ''Colpodella'', ''Alphamonas'', ''Voromonas'' and ''Chilovora'', are free-living predatory phagotrophic flagellates. They live as biflagellated single cells, 5–20 μm in length, with an open conoid and rhoptries used to hunt. They present genetic sequences of non-photosynthetic plastids, evidence of their phototroph ancestry. Some species, considered ectoparasites, do not ingest prey cells, but rather fully or partially "suck" their contents, a process known as myzocytosis, common among alveolates. They feed on bacteria and other protozoa, such as bodonids, chrysomonads, bicosoecids, percolomonads and ciliates. After feeding, they internalize their flagella, become microbial cyst, cysts and divide into tetrads, similarly to the development of zoospores in ''Chromera''. The cells conjugate after leaving the cyst, which could imply a sexual reproduction, sexual stage.Chromerids
Chromerids, represented by the genera ''Chromera'' and ''Vitrella'', are photosynthetic protists, and are thus considered algae. They exist in symbiosis, association with corals. For most of their life cycle, they live as round (Coccus, coccoid) brownish immobile vegetative cells called autospores, surrounded by a thick resistant cell wall. They contain one chloroplast in each cell, with chlorophyll a, chlorophyll ''a'', violaxanthin, and beta-carotene, β-carotene. The two genera are markedly different from each other, both in phylogeny and biological life cycle, life cycles. ''Chromera'' autospores are 5–7 μm in diameter. They asexual reproduction, asexually reproduce through binary division to develop autosporangia, which in turn harbor 2–4 autospores under an additional membrane. They also form Zoospore, zoosporangia, up to 15 μm in diameter, capable of generating 2–10 flagellated zoospores that strongly resemble colpodellids. This dispersal process is similar to the schizogony of apicomplexans. Sexual reproduction has not been observed. Under adverse environmental conditions, they form resistant microbial cyst, cysts that remain viable for years. Similarly to apicomplexans, they undergo closed mitosis, without dissolving the nuclear envelope. In addition, ''Chromera'' produces high amounts of an exclusive type of isofucoxanthin. ''Vitrella'' autospores, by contrast, start measuring 3 μm and grow up to 40 μm before transforming into sporangia that generate dozens of autospores or zoospores. There are two types of ''Vitrella'' zoospores: one is generated by budding from the mother cell and exhibits flagella outside the cytoplasm, the other develops axonemes and flagella within their cytoplasm and are ejected from the mother cell after maturing, though both types lack a pseudo-conoid. Some zoospores fuse, possibly representing a sexual reproduction, sexual stage in the life cycle. In addition, ''Vitrella'' produces vaucheriaxanthin.Evolution
Chrompodellids are the closest living relatives of the apicomplexan parasites, which evolved from a photosynthetic myzozoan ancestor, making chromerids the last remaining photosynthetic members of an otherwise parasitic clade within Alveolata. The apicomplexans, chrompodellids, perkinsids and dinoflagellates constitute the clade Myzozoa, characterized by the Apicomplexa, apical complex and plastids derived from an event of Symbiogenesis#Secondary endosymbiosis, secondary endosymbiosis with a Red algae, red alga. The photosynthetic ability of these plastids was eventually lost in apicomplexans, colpodellids, perkinsids and other groups that transitioned into a predatory or parasitic lifestyle. The following cladogram summarizes alveolate relationships and the internal relationships among most genera within the chrompodellid clade (chromerids marked with asterisks):Systematics
Taxonomic history
In 1993, protozoologist Thomas Cavalier-Smith described the order Colpodellida (under the ICZN, later regularized as Colpodellales in accordance to the ICNafp, ICN) to contain what he considered one of the "most primitive flagellate apicomplexans", the genus ''Colpodella''. This order was introduced in the class Apicomonadea along with the Perkinsida. Cavalier-Smith treats this class as a member of the phylum Apicomplexa, while "true" apicomplexans are united under the name Apicomplexa#Taxonomy, Sporozoa. Although the inclusion of colpodellids within apicomplexans was not supported by other authors, Phylogenetics, phylogenetic studies demonstrated that they were Sister group, sister clades. The first chromerid alga, ''Chromera, Chromera velia'', was discovered and isolated from Australian corals in 2001. It was described in 2008 as the first member of a new phylum Chromerida, followed by ''Vitrella, Vitrella brassicaformis'' in 2012. They showed morphological resemblance to colpodellids and other myzozoans. In the following years, Phylogenetics, phylogenetic studies reported the evolutionary proximity between colpodellids and chromerid algae. This was supported by the discovery of retained vestigial plastids in some colpodellid species. In 2015 there was strong support for a clade containing the two groups, phylogenetically mixed with each other, which rendered both as Polyphyly, polyphyletic. The clade was given the provisional name "chrompodellids", later referred to as Chrompodellida by posterior studies. Between 2004 and 2017, Cavalier-Smith retained the classification scheme of Apicomonadea, from which he excluded Perkinsida, leaving only colpodellids and chromerids across multiple orders. In addition, several genera of Flagellate, flagellates were added on the basis of Morphology (biology), morphological data: ''Algovora'', ''Microvorax'' and ''Dinomonas''. Due to lacking Sequencing, molecular data, these genera have been excluded from later classifications. Two genera, ''Chilovora'' and ''Alphamonas'', were eventually rejected in his classification, but later revisions by other authors maintain them as independent genera supported by molecular data. The treatment of chrompodellids as a subgroup of Apicomplexa, under the name of Apicomonadea, was rejected by the International Society of Protistologists. In a 2019 revision of eukaryotic classification, Protistology, protistologists Emendation (zoology), emended the previous name Colpodellida to contain all chrompodellids, and treated it as a direct subgroup of Alveolata, independent from Apicomplexa. Later, Phycology, phycologists advocated for this treatment as a separate phylum, and regularized it under the name of Chromerida or Chromeridophyta, composed of a single class Colpodellophyceae and a single order Colpodellales, in accordance to the nomenclatural rules of the International Code of Nomenclature for algae, fungi, and plants, ICN. However, other authors consider them a subgroup of the phylum Myzozoa, together with apicomplexans, Perkinsea, perkinsozoans and dinoflagellates.Classification
As of 2023, chrompodellids are divided into four families and seven genera: * Family Alphamonaceae ** ''Alphamonas'' * Family Chromera, Chromeraceae ** ''Chromera'' * Family Colpodellaceae ** ''Chilovora'' ** ''Colpodella'' ** ''Voromonas'' * Family Vitrella, Vitrellaceae ** ''Vitrella'' * Incertae sedis: ''Piridium'' (sister group to ''Vitrella'' but not formalized as a member of Vitrellaceae)References
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