| Katablepharid | |
|---|---|
 | |
| Light (above) andtransmission electron (below) micrographs ofRoombia truncata | |
Scientific classification | |
| Phylum: | Cryptista |
| Subphylum: | Rollomonadia |
| Superclass: | Leucocrypta Cavalier-Smith 2004 stat. nov. 2015[4] |
| Class: | Leucocryptea Cavalier-Smith 2004[3] |
| Order: | Kathablepharidida Okamoto & Inouye 2005[2] |
| Family: | Kathablepharididae Vørs 1992 emend. Clay & Kugrens 1999[1] |
| Genus | |
| Synonyms | |
Thekathablepharids orkatablepharids (from Greek kata 'downwards' and blepharis 'eyelash') are a group ofheterotrophicflagellates closely related tocryptomonads. First described byHeinrich Leonhards Skuja in 1939, kathablepharids were named after thegenusKathablepharis. This genus is corrected toKatablepharis underbotanical nomenclature, but the original spelling is maintained underzoological nomenclature. They are single-celled protists with two anteriorly directedflagella, an anteriorcytostome for ingesting eukaryotic prey, and a sheath that covers thecell membrane. They haveextrusomes known as ejectisomes, as well as tubularmitochondrial cristae.
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| Relationships between katablepharids and closely related protists.[7][8]Hacrobian taxa marked with asterisks. |
Besides the known katablepharid diversity, dozens ofenvironmental DNA sequences (both freshwater and marine) seem to represent further katablepharids which have not been cultured or formally described. Throughmolecular phylogenetic analyses, they are consistently recovered as thesister clade tocryptomonads, an assemblage of flagellates containing thephagotrophicgoniomonads and the photosyntheticcryptophytes. Initially, both groups were placed in theHacrobia, a tentative group that also containedhaptophytes,centrohelids,biliphytes andtelonemids.[9][10] However, the Hacrobia hypothesis was later disproven. Instead, haptophytes and centrohelids belong to the phylumHaptista, while cryptomonads and katablepharids remain as sister groups within the phylumCryptista together withPalpitomonas.[11] Haptista is more closely related to theTSAR clade, which includes telonemids, while Cryptista is more closely related to theArchaeplastida clade, which includesred algae,green algae,plants,glaucophytes andbiliphytes.[12][7][8]
Within katablepharids,Roombia is the earliest branching genus, followed byHatena. This genus is in turn the sister group to the remaining genera:Leucocryptos andKatablepharis. The fifth genus,Platychilomonas, is absent in all phylogenetic analyses due to lack of molecular data.[13]
Katablepharids areflagellates,unicellularprotists capable of swimming freely by using two hairlessflagella inserted subapically or medially in the cell. The flagella are both projected forward (anteriorly), or only one flagellum is projected while the other trails. Theircell membrane is thickened by a sheath composed of two layers containing lamellae. The sheath also encases the flagella. Each cell has anucleus in a central position, aGolgi apparatus in the anterior region, and afood vacuole in the posterior region. Theirmitochondria have tubularcristae. Near thekinetosomes they haveextrusomes known as 'ejectisomes' of various sizes, each composed of a single coiled ribbon or 'scroll', unlikecryptomonads which have ejectisomes composed of two scrolls.[1][11]
These flagellates feed by ingesting othereukaryotes through acytostome supported by bands of longitudinal microtubules.[11] One species,Kathablepharis hyalurus, has secondarily lost the cytostome.[14] The speciesHatena arenicola has a unique life history in comparison: it feeds onNephroselmis algae, temporarily retains theirchloroplasts, enlarges them, and utilizes them forphotosynthesis, which allows it to divide and reproduce. This process is known askleptoplasty.[15]
The botanistHeinrich Leonhards Skuja in 1939 described the family Kathablepharidaceae to accommodate colourless flagellates that had two divergentflagella and a longitudinal groove.[5] He included four genera in this family:Kathablepharis,Leucocryptos,Cryptaulax andPhyllomitus.[16] He considered katablepharids as closely related tocryptomonads, and placed them in classCryptophyceae on the basis of morphological features seen throughlight microscopy.[17]
In 1992, the protozoologist Naja Vørs created the zoological variant of the family, Kathablepharidae and corrected the botanical variant as Katablepharidaceae,[a] redefined to only include three genera:Katablepharis,Leucocryptos andPlatychilomonas.[16] However, she did not assign this family to any higher taxon, and instead treated it asincertae sedis protists, thereby removing them from Cryptophyceae.[19]
An alternative to Vørs' classification was proposed by the protozoologistThomas Cavalier-Smith in 1993. Through observations of a single speciesKathablepharis ovalis, he classified katablepharids as part of the phylumOpalozoa, on the basis of tubularmitochondrial cristae and the absence of ejectisomes that are characteristic of cryptomonads. He erected a new classCyathobodonea and placedKathablepharis andLeucocryptos in a new orderKathablepharida, defined by two anterior flagella encased by a surface sheath, lack of cytopharynx, and an anterior cytostome supported by four bands ofmicrotubules.[20][21] The phylum Opalozoa was highly non-monophyletic, and in 1997 Cavalier-Smith separated katablepharids into a new phylumNeomonada which was another broad non-monophyletic assemblage. Katablepharids were placed in a new subphylum Isomita which also containedTelonemea.[22] Because this scheme was based on the observations on a single speciesK. ovalis, it was not considered valid.[19]
In 1999, Brec Clay and Paul Kugrens reviewed the systematics of katablepharids and rejected Cavalier-Smith's classification. Instead, they adopted Vørs' family, corrected the zoological spelling toKathablepharididae,emended thediagnosis to include onlyKatablepharis andLeucocryptos, and postponed any higher classification untilmolecular phylogenetics could resolve their true placement.[1]
Eventually, molecular data andelectron microscopy studies revealedcryptophytes and katablepharids to be related. In 2004, Cavalier-Smith included both group as subphyla under the phylumCryptista. For katablepharids, he proposed a new classLeucocryptea and subphylumLeucocrypta, named afterLeucocryptos.[23] The following year, Noriko Okamoto and Isao Inouye interpreted the molecular and morphological gap between the two groups sufficient to propose them as two separate phyla. They also argued that the treatment of both groups asdivisions (=botanical phylum) agrees with the widely accepted system whereCryptophyta is a division. They described higher taxa for both nomenclature codes: phylumKathablepharida, classKathablepharidea and orderKathablepharidida underzoological nomenclature, and divisionKatablepharidophyta, classKatablepharidophyceae and orderKatablepharidales underbotanical nomenclature.[24][a] In the following years, two new genera of katablepharids were described:Hatena in 2006[25] andRoombia in 2009.[9]
Following his own classification, Cavalier-Smith continued considering both groups as members of phylum Cryptista. In 2015, he lowered Leucocrypta to a superclass included within the subphylumRollomonadia (equivalent to Cryptophyta), along with cryptomonads (under the name ofCryptomonada), and added additional subphylaPalpitia andCorbihelia to the phylum.[4] As of 2024, katablepharids are generally accepted as a subgroup of the Cryptista or Cryptophyta, instead of an independent phylum or division, together with cryptomonads.[11][26]
There are five accepted genera of katablepharids:[11]