Cercozoa (now synonymised withFilosa)[2] is a phylum of diverse single-celledeukaryotes.[4][5] They lack shared morphological characteristics at the microscopic level,[6] and are instead united bymolecular phylogenies ofrRNA andactin orpolyubiquitin.[7] They were the first majoreukaryotic group to be recognized mainly throughmolecular phylogenies.[8] They are the natural predators of many species of bacteria. They are closely related to the phylumRetaria, comprising amoeboids that usually have complex shells, and together form a supergroup calledRhizaria.[2]
The group includes mostamoeboids andflagellates that feed by means of filose pseudopods. These may be restricted to part of the cell surface, but there is never a truecytostome or mouth as found in many other protozoa. They show a variety of forms[9] and have proven difficult to define in terms of structural characteristics, although their unity is strongly supported byphylogenetic studies.
Tectofilosids, filose amoebae that produce organic shells.
Cercomonads, common soil-dwelling amoeboflagellates.
Reticulose, meaning they form a reticulating net ofpseudopods. For example:
Chlorarachniophytes, set apart by the presence ofchloroplasts bound by four membranes and still possess a vestigial nucleus, called a nucleomorph. As such, they have been of great interest to researchers studying the endosymbiotic origins of organelles.
Soil-dwelling cercozoans are one of the dominant groups of free-living eukaryotic microorganisms found intemperate soils, accounting for around 30% of identifiable protozoanDNA in arid or semi-arid soils and 15% in more humid soils. Intranscriptomic analyses they account for 40-60% of all identifiable protozoanRNA found in forest and grassland soils. They also comprise 9-24% of alloperational taxonomic units found in theocean floor.[12]
Some cercozoa are coprophilic orcoprozoic, meaning they usefeces as a source ofnutrients or as transport through animal hosts. The faecal habitat is an understudied reservoir of microbial eukaryotic diversity, dominated by amoeboflagellates from the phylum Cercozoa. Strongly coprophilic examples of cercozoa are the flagellatesCercomonas,Proleptomonas andHelkesimastix, and thesorocarpic amoebaGuttulinopsis. Many new cercozoan lineages, especially amongsarcomonads, have been discovered through phylogenetic sampling of feces because they appear preferentially in this medium.[13]
Cercozoanbacterivores (i.e.predators ofbacteria) are highly diverse and important in the plantphyllosphere, the leaf surfaces of plants. Particularlysarcomonads, with their ability tocyst, feed and multiply within hours, are perfectly adapted to the fluctuating environmental factors in the phyllosphere. Their predation causes shifts in the bacterial communities: they reduce populations ofalphaproteobacteria andbetaproteobacteria, which are less resistant to their grazing, in favour of other bacterial populations such asgammaproteobacteria.[14]
Originally, Cercozoa contained both Filosa andEndomyxa, according tophylogenetic analyses usingribosomal RNA andtubulin. These analyses also confirmed Cercozoa as the sister group of Retaria within the supergroup Rhizaria.[10][16]
However, themonophyly of the group was still uncertain. Posterior multigene phylogenetic analyses consistently found Cercozoa to be paraphyletic, becauseEndomyxa clustered next toRetaria instead of Filosa.[17][18][19] Because of this, Endomyxa was excluded from Cercozoa, which became a synonym of Filosa.[2]
More recent phylogenomic analyses with better sampling recovered a sister relationship between Filosa (=Cercozoa) andEndomyxa once again,[15] although the modern classification of eukaryotes retainsEndomyxa, Cercozoa andRetaria as separate phyla withinRhizaria.[20]
The phylum Cercozoa previously contained both Filosa andEndomyxa, but in the latest classifications Endomyxa has been excluded, and Cercozoa is now synonymous with Filosa. It is composed of twosubphyla:Monadofilosa andReticulofilosa. According to multigenephylogenetic analyses, Monadofilosa is a robustclade, in which the deepest branching group isMetromonadea, followed byHelkesea as the second group (together forming the paraphyleticEoglissa) before the divergence of the cladeVentrifilosa (Imbricatea,Sarcomonadea andThecofilosea). On the other hand, Reticulofilosa is probably paraphyletic, withGranofilosea diverging earlier thanChlorarachnea, which makes Chlorarachnea the sister group of Monadofilosa.[2]
A more recentphylogenomic analysis recovered both Monadofilosa and Reticulofilosa as monophyletic within the clade Filosa.[15]
In addition to the knownGranofilosea,Chlorarachnea andMonadofilosa, a variety ofclades inside Cercozoa have been discovered in other analyses and have slowly been described and named, such asTremulida (previously known as Novel Clade 11)[16] andAquavolonida (Novel Clade 10),[21] although their specific positions among the two main cercozoan subphyla have yet to be refined. These two orders have been classified as the classSkiomonadea, within Reticulofilosa.[2]
^abcBass D, Chao EE, Nikolaev S, et al. (February 2009). "Phylogeny of Novel Naked Filose and Reticulose Cercozoa: Granofilosea cl. n. and Proteomyxidea Revised".Protist.160 (1):75–109.doi:10.1016/j.protis.2008.07.002.PMID18952499.
^Bass D, Silberman JD, Brown MW, Pearce RA, Tice AK, Jousset A, Geisen S, Hartikainen H (23 February 2016). "Coprophilic amoebae and flagellates, including Guttulinopsis, Rosculus and Helkesimastix, characterise a divergent and diverse rhizarian radiation and contribute to a large diversity of faecal-associated protists".Environmental Microbiology.18 (5):1604–1619.doi:10.1111/1462-2920.13235.PMID26914587.
^Flues S, Bass D, Bonkowski M (15 June 2017). "Grazing of leaf-associated Cercomonads (Protists: Rhizaria: Cercozoa) structures bacterial community composition and function".Environmental Microbiology.19 (8):3297–3309.doi:10.1111/1462-2920.13824.PMID28618206.
^abcIrwin, Nicholas A.T.; Tikhonenkov, Denis V.; Hehenberger, Elisabeth; Mylnikov, Alexander P.; Burki, Fabien; Keeling, Patrick J. (2019-01-01). "Phylogenomics supports the monophyly of the Cercozoa".Molecular Phylogenetics and Evolution.130:416–423.doi:10.1016/j.ympev.2018.09.004.ISSN1055-7903.PMID30318266.S2CID52982396.
^Adl SM, Bass D, Lane CE, Lukeš J, Schoch CL, Smirnov A, Agatha S, Berney C, Brown MW, Burki F, Cárdenas P, Čepička I, Chistyakova L, del Campo J, Dunthorn M, Edvardsen B, Eglit Y, Guillou L, Hampl V, Heiss AA, Hoppenrath M, James TY, Karnkowska A, Karpov S, Kim E, Kolisko M, Kudryavtsev A, Lahr DJG, Lara E, Le Gall L, Lynn DH, Mann DG, Massana R, Mitchell EAD, Morrow C, Park JS, Pawlowski JW, Powell MJ, Richter DJ, Rueckert S, Shadwick L, Shimano S, Spiegel FW, Torruella G, Youssef N, Zlatogursky V, Zhang Q (2019)."Revisions to the Classification, Nomenclature, and Diversity of Eukaryotes".Journal of Eukaryotic Microbiology.66 (1):4–119.doi:10.1111/jeu.12691.PMC6492006.PMID30257078.
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