Cercozoa (now synonymised with Filosa)[2] is a phylum of diverse single-celled
eukaryotes.[4][5] They lack shared morphological characteristics at the microscopic level,[6] and are instead united by
molecular phylogenies of
rRNA and
actin or
polyubiquitin.[7] They were the first major
eukaryotic group to be recognized mainly through
molecular phylogenies.[8] They are the natural predators of many species of bacteria. They are closely related to the phylum
Retaria, comprising amoeboids that usually have complex shells, and together form a supergroup called
Rhizaria.[2]
Characteristics
The group includes most
amoeboids and
flagellates that feed by means of filose pseudopods. These may be restricted to part of the cell surface, but there is never a true
cytostome 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 by
phylogenetic studies.
Diversity
Some cercozoans are grouped by whether they are "filose" or "reticulose" in the behavior of their
cytoskeleton when moving:[10]
Tectofilosids, filose amoebae that produce organic shells.
Cercomonads, common soil-dwelling amoeboflagellates.
Reticulose, meaning they form a reticulating net of
pseudopods. For example:
Chlorarachniophytes, set apart by the presence of
chloroplasts 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 in
temperate soils, accounting for around 30% of identifiable protozoan
DNA in arid or semi-arid soils and 15% in more humid soils. In
transcriptomic analyses they account for 40-60% of all identifiable protozoan
RNA found in forest and grassland soils. They also comprise 9-24% of all
operational taxonomic units found in the
ocean floor.[12]
Some cercozoa are coprophilic or
coprozoic, meaning they use
feces as a source of
nutrients 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 flagellates Cercomonas, Proleptomonas and Helkesimastix, and the
sorocarpic amoeba Guttulinopsis. Many new cercozoan lineages, especially among
sarcomonads, have been discovered through phylogenetic sampling of feces because they appear preferentially in this medium.[13]
Cercozoan
bacterivores (i.e.
predators of
bacteria) are highly diverse and important in the plant
phyllosphere, the leaf surfaces of plants. Particularly
sarcomonads, with their ability to
cyst, 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 of
alphaproteobacteria and
betaproteobacteria, which are less resistant to their grazing, in favour of other bacterial populations such as
gammaproteobacteria.[14]
Originally, Cercozoa contained both Filosa and
Endomyxa, according to
phylogenetic analyses using
ribosomal RNA and
tubulin. These analyses also confirmed Cercozoa as the sister group of Retaria within the supergroup Rhizaria.[10][16]
However, the
monophyly of the group was still uncertain. Posterior multigene phylogenetic analyses consistently found Cercozoa to be paraphyletic, because
Endomyxa clustered next to
Retaria 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) and
Endomyxa once again,[15] although the modern classification of eukaryotes retains
Endomyxa, Cercozoa and
Retaria as separate phyla within
Rhizaria.[20]
Internal evolution
The phylum Cercozoa previously contained both Filosa and
Endomyxa, but in the latest classifications Endomyxa has been excluded, and Cercozoa is now synonymous with Filosa. It is composed of two
subphyla:
Monadofilosa and
Reticulofilosa. According to multigene
phylogenetic analyses, Monadofilosa is a robust
clade, in which the deepest branching group is
Metromonadea, followed by
Helkesea as the second group (together forming the paraphyletic
Eoglissa) before the divergence of the clade
Ventrifilosa (
Imbricatea,
Sarcomonadea and
Thecofilosea). On the other hand, Reticulofilosa is probably paraphyletic, with
Granofilosea diverging earlier than
Chlorarachnea, which makes Chlorarachnea the sister group of Monadofilosa.[2]
A more recent
phylogenomic analysis recovered both Monadofilosa and Reticulofilosa as monophyletic within the clade Filosa.[15]
In addition to the known
Granofilosea,
Chlorarachnea and
Monadofilosa, a variety of
clades inside Cercozoa have been discovered in other analyses and have slowly been described and named, such as
Tremulida (previously known as Novel Clade 11)[16] and
Aquavolonida (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 class
Skiomonadea, within Reticulofilosa.[2]
Classification
The classification of Cercozoa was revised in 2018:[2]
^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.
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^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.
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