Chrompodellid

Chrompodellids are a phylum of single-celled protists belonging to the Alveolata supergroup. It comprises two different polyphyletic groups of flagellates: the colpodellids, phagotrophic predators, and the chromerids, photosynthetic algae that live as symbionts of corals. These groups were independently discovered and described, but molecular phylogenetic analyses demonstrated that they are intermingled in a clade that is the closest relative to Apicomplexa, and they became collectively known as chrompodellids. Due to the history of their research, they are variously known in biological classification as Chromerida or Colpodellida (ICZN)/Colpodellales (ICN).

Chrompodellids are a phylum of unicellular protists containing two functionally different groups: the photosynthetic "chromerids" and the predatory phagotrophic "colpodellids". Like other Alveolata, they present tubular mitochondrial cristae and highly flattened cortical alveoli with microtubules underneath. They exhibit a conoid-like structure similar to that of apicomplexans, with an apical complex, a micropore and a rostrum. They live as flagellates with two anisokont (i.e. differently sized) 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 cysts.

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 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 stage.

Chromerids, represented by the genera Chromera and Vitrella, are photosynthetic protists, and are thus considered algae. They exist in association with corals. For most of their life cycle, they live as round (coccoid) brownish immobile vegetative cells called autospores, surrounded by a thick resistant cell wall. They contain one chloroplast in each cell, with chlorophyll a, violaxanthin, and β-carotene.

The two genera are markedly different from each other, both in phylogeny and life cycles. Chromera autospores are 5–7 μm in diameter. They asexually reproduce through binary division to develop autosporangia, which in turn harbor 2–4 autospores under an additional membrane. They also form 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 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 stage in the life cycle. In addition, Vitrella produces vaucheriaxanthin.

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 apical complex and plastids derived from an event of secondary endosymbiosis with a 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):