Thecofilosea is a class of unicellulartestate amoebae belonging to the phylum Cercozoa. They are amoeboflagellates, organisms with flagella and pseudopodia, distinguished from other cercozoa by their scale-lacking test composed of organic material. They are closely related to the Imbricatea, a group of testate amoebae with tests composed of inorganic silica scales.
The thecofilosean theca or test has perforations for flagella and pseudopodia. In the phaeodarian amoebae, the test has three perforations. Although they lack silica scales, unlike many Imbricatea, they present a hollow silica endoskeleton in all ebriids and most phaeodarians.[1][2]
Evolution
External evolution
Thecofilosea is a clade or monophyletic group. It belongs to the subphylum Monadofilosa, a group of cercozoan classes that evolved after the early divergence of Reticulofilosa. Thecofilosea is the only group of testate cercozoan amoebae with ancestrally organic scale-lacking shells, while all the Imbricatea (to which the scale-bearing Kraken belongs) have non-organic silica scaled tests. The Thecofilosea appear to have evolved from the Imbricatea, a group previously considered monophyletic but based on new analyses seems paraphyletic to Thecofilosea. Their common ancestor carried scales in its theca and no organic cement, a trait lost in Thecofilosea in favour of an organic theca without scales.[3][4][5]
An earlier analysis that recovers the Kraken within Sarcomonadea, a group of naked amoebae, proposes the unlikely possibility that not only the common ancestor of Imbricatea+Thecofilosea, but also the common ancestor of all Filosa, already bore inorganic silica scales in its test. In this scenario, both Thecofilosea and Sarcomonadea would have lost their scales independently.[6]
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^ abcHowe AT, Bass D, Scoble JM, Lewis R, Vickerman K, Arndt H, Cavalier-Smith T (2011). "Novel Cultured Protists Identify Deep-branching Environmental DNA Clades of Cercozoa: New Genera Tremula, Micrometopion, Minimassisteria, Nudifila, Peregrinia". Protist. 162 (2): 332–372. doi:10.1016/j.protis.2010.10.002. ISSN1434-4610.
^Dumack, Kenneth; Mylnikov, Alexander P.; Bonkowski, Michael (2017). "Evolutionary Relationship of the Scale-Bearing Kraken (incertae sedis, Monadofilosa, Cercozoa, Rhizaria): Combining Ultrastructure Data and a Two-Gene Phylogeny". Protist. 168 (3): 362–373. doi:10.1016/j.protis.2017.04.004.
^Cavalier-Smith, Thomas; Chao, Ema E. (2012). "Oxnerella micra sp. n. (Oxnerellidae fam. n.), a Tiny Naked Centrohelid, and the Diversity and Evolution of Heliozoa". Protist. 163 (4): 574–601. doi:10.1016/j.protis.2011.12.005.