| Chrompodellids | |
|---|---|
 | |
| Typical life cycle stages ofVitrella brassicaformis, a chromerid (vc: vegetative cell, zs: zoosporangium, as: autosporangium) | |
Scientific classification | |
| Domain: | Eukaryota |
| Clade: | Diaphoretickes |
| Clade: | SAR |
| Clade: | Alveolata |
| Phylum: | Myzozoa |
| Class: | Chromeridophyceae Molinari & Guiry 2023[3] |
| Subclass: | Chromeridophycidae Mylnikov et al. 2000[4] |
| Order: | Colpodellales Cavalier-Smith 1993[1] emend. Adl et al. 2005, 2019[2][3] |
| Type genus | |
| Colpodella Cienkowsky 1865[1] | |
| Families[2][3] | |
| Synonyms | |
Chrompodellids are aclade of single-celledprotists belonging to theAlveolata supergroup. It comprises two differentpolyphyletic groups offlagellates: thecolpodellids,phagotrophicpredators, and thechromerids,photosyntheticalgae that live as symbionts ofcorals. These groups were independently discovered and described, but molecularphylogenetic analyses demonstrated that they are intermingled in a clade that is the closest relative toApicomplexa, and they became collectively known as chrompodellids. Due to the history of their research, they are variously known inbiological classification asChromerida orColpodellida (ICZN)/Colpodellales (ICN).
Chrompodellids are aclade of unicellularprotists containing two functionally different groups: thephotosynthetic "chromerids" and the predatoryphagotrophic "colpodellids". Like otherAlveolata, they present tubularmitochondrial cristae and highly flattenedcortical alveoli[8] withmicrotubules underneath. They exhibit aconoid-like structure similar to that ofapicomplexans, with anapical complex, a micropore and a rostrum. They live asflagellates with two anisokont (i.e. differently sized)flagella[1] that are heterodynamic (i.e. move in different patterns).[9] Some species exhibit thinmastigonemes in their anterior flagellum, while others bear bulbs.[9] Some species are capable of formingcysts.[2]
Colpodellids, represented by the generaColpodella,Alphamonas,Voromonas andChilovora, are free-livingpredatoryphagotrophicflagellates.[9] They live as biflagellated single cells, 5–20 μm in length, with an openconoid andrhoptries used to hunt. They present genetic sequences of non-photosyntheticplastids, evidence of theirphototroph ancestry.[8]
Some species, consideredectoparasites, do notingest prey cells, but rather fully or partially "suck" their contents, a process known asmyzocytosis, common amongalveolates.[9][10] They feed onbacteria and otherprotozoa, such asbodonids,chrysomonads,bicosoecids,percolomonads andciliates.[11] After feeding, they internalize their flagella, becomecysts and divide into tetrads, similarly to the development of zoospores inChromera. The cells conjugate after leaving the cyst, which could imply asexual stage.[8]
Chromerids, represented by the generaChromera andVitrella, arephotosyntheticprotists, and are thus consideredalgae. They exist inassociation withcorals. For most of their life cycle, they live as round (coccoid) brownish immobilevegetative cells called autospores, surrounded by a thick resistantcell wall. They contain onechloroplast in each cell,[8] withchlorophylla,[12]violaxanthin, andβ-carotene.[8]
The two genera are markedly different from each other, both inphylogeny andlife cycles.Chromera autospores are 5–7 μm in diameter. Theyasexually reproduce through binary division to develop autosporangia, which in turn harbor 2–4 autospores under an additional membrane. They also formzoosporangia, up to 15 μm in diameter, capable of generating 2–10 flagellatedzoospores that strongly resemble colpodellids. This dispersal process is similar to theschizogony of apicomplexans. Sexual reproduction has not been observed. Under adverse environmental conditions, they form resistantcysts that remain viable for years. Similarly to apicomplexans, they undergo closedmitosis, without dissolving thenuclear envelope.[8] In addition,Chromera produces high amounts of an exclusive type ofisofucoxanthin.[6]
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 ofVitrella zoospores: one is generated by budding from the mother cell and exhibits flagella outside thecytoplasm, 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 asexual stage in the life cycle.[8] In addition,Vitrella producesvaucheriaxanthin.[13]
Chrompodellids are the closest living relatives of theapicomplexan parasites, which evolved from aphotosyntheticmyzozoan ancestor, making chromerids the last remaining photosynthetic members of an otherwise parasiticclade withinAlveolata.[14] The apicomplexans, chrompodellids, perkinsids and dinoflagellates constitute the cladeMyzozoa, characterized by theapical complex andplastids derived from an event ofsecondary endosymbiosis with ared 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.[8] The following cladogram summarizes alveolate relationships and the internal relationships among most genera within the chrompodellid clade (chromerids marked with asterisks):[15][12][16]
In 1993, protozoologistThomas Cavalier-Smith described the orderColpodellida (under theICZN, later regularized asColpodellales in accordance to theICN)[3] to contain what he considered one of the "most primitive flagellate apicomplexans", the genusColpodella. This order was introduced in the class Apicomonadea along with thePerkinsida.[1] Cavalier-Smith treats this class as a member of the phylumApicomplexa, while "true" apicomplexans are united under the nameSporozoa.[5] Although the inclusion of colpodellids within apicomplexans was not supported by other authors,phylogenetic studies demonstrated that they weresister clades.[17]
The first chromerid alga,Chromera velia, was discovered and isolated fromAustraliancorals in 2001. It was described in 2008 as the first member of a new phylumChromerida, followed byVitrella brassicaformis in 2012.[13] They showed morphological resemblance to colpodellids and othermyzozoans.[12] In the following years,phylogenetic studies reported the evolutionary proximity between colpodellids and chromerid algae.[6] This was supported by the discovery of retained vestigialplastids in some colpodellid species.[18] In 2015 there was strong support for aclade containing the two groups, phylogenetically mixed with each other, which rendered both aspolyphyletic. The clade was given the provisional name "chrompodellids",[15] later referred to asChrompodellida by posterior studies.[19]
Between 2004 and 2017, Cavalier-Smith retained the classification scheme ofApicomonadea, from which he excluded Perkinsida, leaving only colpodellids and chromerids across multiple orders. In addition, several genera offlagellates were added on the basis ofmorphological data:Algovora,Microvorax andDinomonas.[20] Due to lackingmolecular data, these genera have been excluded from later classifications.[2] Two genera,Chilovora andAlphamonas, were eventually rejected in his classification,[5] but later revisions by other authors maintain them as independent genera supported by molecular data.[2]
The treatment of chrompodellids as a subgroup ofApicomplexa, under the name of Apicomonadea, was rejected by the International Society of Protistologists. In a 2019 revision ofeukaryotic classification,protistologistsemended the previous name Colpodellida to contain all chrompodellids, and treated it as a direct subgroup ofAlveolata, independent from Apicomplexa.[2] Later,phycologists advocated for this treatment as a separatephylum, and regularized it under the name ofChromerida orChromeridophyta, composed of a single classChromeridophyceae and a single orderColpodellales, in accordance to the nomenclatural rules of theICN.[3][7] However, other authors consider them a subgroup of the phylumMyzozoa, together with apicomplexans,perkinsozoans anddinoflagellates.[15][5][8]
As of 2023, chrompodellids are divided into four families and seven genera:[2][3]