| Telonemia | |
|---|---|
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| Telonema rivulare byinterference contrast micrography | |
Scientific classification | |
| Domain: | Eukaryota |
| Clade: | Diaphoretickes |
| Clade: | TSAR |
| Phylum: | Telonemia Shalchian-Tabrizi 2006[2] |
| Class: | Telonemea Cavalier-Smith 1993[1] |
| Order: | Telonemida Cavalier-Smith 1993[1] |
| Family: | Telonemidae Cavalier-Smith 1993[1] |
| Genera | |
| Diversity | |
| 7 species | |
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Telonemia is aphylum of microscopiceukaryotes commonly known astelonemids. They areunicellular free-livingflagellates with a unique combination ofcell structures, including a highly complexcytoskeleton unseen in other eukaryotes.
Telonemia shares several distinctive features with its related group, theSAR supergroup. Among these features arecortical alveoli, small sacs beneath the cell's surface that act as cushions, providing support and helping to maintain the cell's shape. Additionally, they possess tripartitemastigonemes, complex three-part hair-like structures on theirflagella, the whip-like tails used for movement. These structures enhance their swimming capabilities by increasing resistance against water. Furthermore, Telonemia is equipped withfilopodia, very thin, thread-like projections extending from the cell body. These projections can serve various purposes, such as aiding in movement or capturing food particles by wrapping around them. Together, the two lineages compose theTSAR clade.
This phylum ismonotypic, composed of a singleclassTelonemea,orderTelonemida andfamilyTelonemidae. It is classified in threegenera and sevenspecies, although numerous undescribedclades ofenvironmental DNA are known. They are detected in all marine and freshwater environments, where they prey onbacteria and smallphytoplankton by engulfing them in theirplasma membrane (phagotrophy).
The phylum Telonemia comprises microscopicunicellulareukaryotes, orprotists.[3] Most of thediversity of telonemids ismorphologically uncharacterized.[4] The few described species are free-living predatoryphagotrophicflagellates composed of pear-shaped cells with twoflagella. These cells measure approximately 5–10 μm in length and 3–7 μm in width. The flagella have different lengths, with the short one measuring up to 12 μm and the long one measuring up to 16 μm. Between the flagella protrudes a shortproboscis-like structure, known as a rostrum. Theirmitochondrial cristae are tubular. They have a unique multi-layeredcytoskeleton of high complexity, composed of layers ofmicrofilaments andmicrotubules, unseen in any other eukaryote.[4] They exhibit a unique combination of cell traits that were previously believed to beexclusive to differentchromalveolate groups, such as complex tripartite mastigonemes (as instramenopiles),cortical alveoli-like structures (as inalveolates) andfilopodia (as inrhizarians).[4] Despite their evolutionary proximity to chromalveolates, they lackchloroplasts.[2]
Telonemids feed on a wide range of organisms, namelybacteria andphytoplankton ranging in size betweenpico- andnanoplankton. They are widely distributed and are sometimes abundant, implying they may play an important ecological role inaquatic ecosystems.[4] Around one hundredclades ofenvironmental sequences from undescribed telonemids have been recovered in a variety ofmarine locations (Antarctic,Arctic andIndian Oceans;Mediterranean,Baltic,Kara,Marmara andWhite seas), includingdeep sea, andfreshwater bodies from different regions (Norway,France,Antarctica,Finland,Canada,Japan).[5][6][4] Several telonemidclades favor open waters with lower nutrients, such as theCanada Basin and offshore theMackenzie River, suggesting that they are able to thrive in low-productivity ecosystems (i.e.oligotrophic).[7]
The first telonemidgenus andspecies,Telonema subtile, was described by Karl Griessmann in 1913 from crude cultures of thegreen algaUlva and ofred algae off the coast ofRoscoff andNaples.[8] Eighty years later, in 1993, American protistologistThomas Cavalier-Smith created a familyTelonemidae, orderTelonemida and classTelonemea to contain thisprotist. Initially, this group was included within the now obsolete phylumOpalozoa, along with other unrelated groups offlagellates such asapusomonads,jakobids,cercomonads,spongomonads,katablepharids,ebriids,proteomyxids and so on. In this scheme, the class Telonemea was distinguished by the presence of two posteriorcilia of equal length (isokont cilia). It contained an additional order besides Telonemida, Nephromycida, which comprised the genusNephromyces[1] (later treated as anapicomplexan).[9] In 2005 a second species of telonemid was described,T. antarcticum, from the surface waters of theOslofjord.[10]
Since 2006, Telonemea was separated into a new eukaryoticphylumTelonemia by protistologist Kamran Shalchian-Tabrizi and coauthors, on the basis ofphylogenetic analyses that placed it nearchromalveolate groups such asHaptista andCryptista.[2] However, in 2015, Cavalier-Smith and coauthors rejected their treatment as an independent phylum and transferred Telonemea to the phylumCryptista, under the obsolete subphylumCorbihelia. This subphylum included other protists with a pharyngeal basket or radiating axopodia[11] such asPicomonas (later classified as a separate phylumPicozoa closely related tored algae)[12] andMicroheliella (now proposed as the sister group to Cryptista).[13] In addition, they transferredT. antarcticum to a new genusLateronema, on the basis ofphylogenetic distance fromTelonema.[11]
Numerous phylogenetic analyses in the following years solidified the position of Telonemia as thesister clade to theSAR supergroup, both collectively composing theTSAR clade (Telonemia,Stramenopila,Alveolata andRhizaria),[14] which lead Cavalier-Smith to finally consider Telonemia a separate phylum in 2022.[15] In the same year, five more species and a third genus,Arpakorses, were described by protistologist Denis Victorovich Tikhonenkov and coauthors.[4]
Until 2019, only two species had beenformally described,[14] although DNA sequences collected from seawater suggested there were many more species not yet described.[16] In 2022, five additional species were described along with a third new genus, bringing the total number of species to seven.[4]
| Telonemia on the eukaryote tree of life |
| Cladogram of eukaryotes based on revisions of the 2020 decade, showing in bold the position of Telonemia.[17][18] |
Telonemia is aclade of protists that branch independently from othereukaryotic supergroups groups as their own 'micro-kingdom'.[19] Early molecular analyses of Telonemia placed them as an independent branch within theSAR supergroup, a diverse clade of eukaryotes that containRhizaria,Alveolata andStramenopila.[20] Other analyses proposed a close relationship withcentrohelids,katablepharids,cryptomonads andhaptophytes.[21][11] At this time, they were suggested to haveevolutionary significance in being a possible transitional form between ecologically importantheterotrophic andphotosynthetic species among chromalveolates.[2]
The present phylogenetic analyses place them as sister to theSAR supergroup in a clade commonly known asTSAR,[14][4] which is widely accepted by the scientific community.[17][18][nb 1] As thesister clade to SAR, Telonemia has a key position in the tree of eukaryotic life. They are morphologically complex organisms that combine characteristics of different SAR lineages. The main trait uniting each SAR lineage has been described in at least one genus of Telonemia: tripartitemastigonemes in theflagella, typical ofstramenopiles and described inLateronema;cortical alveoli underneath theplasma membrane, typical ofalveolates and described inLateronema; and finepseudopodia (filopodia), typical ofrhizarians and described inTelonema. Moreover,Arpakorses presents a kinetid structure similar to that seen in Rhizaria, andTelonema subtile presents microtubules in a formation superficially resembling the apical complex ofapicomplexans.[4]
All telonemid genera possess a highly intricate multi-layeredcytoskeleton, whose complexity is not found in any other eukaryote. This finding may indicate that telonemids have retained an ancestral cytoskeleton organization that has been lost in other eukaryotes.[4]