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Telonemia is a phylum of microscopic commonly known as telonemids. They are unicellular free-living with a unique combination of cell structures, including a highly complex cytoskeleton unseen in other eukaryotes.
Telonemia shares several distinctive features with its related group, the SAR supergroup. Among these features are cortical 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 tripartite , complex three-part hair-like structures on their flagella, the whip-like tails used for movement. These structures enhance their swimming capabilities by increasing resistance against water. Furthermore, Telonemia is equipped with filopodia, 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 the TSAR clade.
This phylum is monotypic, composed of a single class Telonemea, order Telonemida and family Telonemidae. It is classified in three Genus and seven species, although numerous undescribed clades of environmental DNA are known. They are detected in all marine and freshwater environments, where they prey on bacteria and small phytoplankton by engulfing them in their plasma membrane (phagotrophy).
Since 2006, Telonemea was separated into a new eukaryotic phylum Telonemia by protistologist Kamran Shalchian-Tabrizi and coauthors, on the basis of Phylogenetics that placed it near chromalveolate groups such as Haptista and Cryptista. However, in 2015, Cavalier-Smith and coauthors rejected their treatment as an independent phylum and transferred Telonemea to the phylum Cryptista, under the obsolete subphylum Corbihelia. This subphylum included other protists with a pharyngeal basket or radiating axopodia such as Picomonas (later classified as a separate phylum Picozoa closely related to red algae) and Microheliella (now proposed as the sister group to Cryptista). In addition, they transferred T. antarcticum to a new genus Lateronema, on the basis of genetic distance from Telonema.
Numerous phylogenetic analyses in the following years solidified the position of Telonemia as the sister clade to the SAR supergroup, both collectively composing the TSAR clade ( Telonemia, Stramenopila, Alveolata and Rhizaria), which lead Cavalier-Smith to finally consider Telonemia a separate phylum in 2022. In the same year, five more species and a third genus, Arpakorses, were described by protistologist Denis Victorovich Tikhonenkov and coauthors.
The present phylogenetic analyses place them as sister to the SAR supergroup in a clade commonly known as TSAR, which is widely accepted by the scientific community. As the sister 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: tripartite in the flagella, typical of stramenopiles and described in Lateronema; cortical alveoli underneath the plasma membrane, typical of and described in Lateronema; and fine pseudopodia (filopodia), typical of and described in Telonema. Moreover, Arpakorses presents a kinetid structure similar to that seen in Rhizaria, and Telonema subtile presents microtubules in a formation superficially resembling the apical complex of .
All telonemid genera possess a highly intricate multi-layered cytoskeleton, 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.
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