Xenacoelomorpha[2](/ˌzɛnəˌsɛloʊˈmɔːrfə/) is a small phylum ofbilaterianinvertebrateanimals,consisting of twosister groups:xenoturbellidsandacoelomorphs.This new phylum was named in February 2011 and suggested based on morphologicalsynapomorphies(physical appearances shared by the animals in theclade),[3]which was then confirmed byphylogenomicanalyses of molecular data (similarities in the DNA of the animals within the clade).[2][4]
| Xenacoelomorpha | |
|---|---|
| |
| Xenoturbella japonica,a xenacoelomorph member (xenoturbellids) | |
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| Proporussp., another xenacoelomorph member (acoelomorphs) | |
Scientific classification
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| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Subkingdom: | Eumetazoa |
| Clade: | ParaHoxozoa |
| Clade: | Bilateria |
| Phylum: | Xenacoelomorpha Philippe et al. 2011[1] |
| Subphyla | |
Phylogenetics
editPrior to molecular studies, xenacoelomorphs were considered to beflatwormsbased on their superficial similarities. Like flatworms, they do not have a coelom and aredorsoventrallyflattened.[5]With the advent ofphylogenetics,Xenoturbellaand Acoelomorpha were found to be sister groups and only distantly related to flatworms.[4]Initially thisphylumwas considered to be a member of thedeuterostomes,[2]but because of recenttranscriptomeanalyses, it was concluded that phylum Xenacoelomorpha is the sister group to theNephrozoa,which includes both theprotostomesand the deuterostomes, which makes the phylum thebasalmostbilaterian clade.[6][7]This would mean they are neither deuterostomes nor protostomes.
Their larvae show similarities withcnidarianplanulalarvae and poriferan parenchyma larvae, but it is not clear if the similarities are ancestral or derived.[8]
However, some studies point out that their basal placement may be caused by highmutationrates leading tolong branch attraction(LBA). These analyses suggest that the xenacoelomorphs are instead the sister group ofAmbulacrariaforming the cladeXenambulacrariaand that despite their simple body plans, they actually derive from a more complex ancestor.[9][10]Having a larger number of species within this group would allow for better conclusions and analysis to be made within the phylum and in groups closely related to the phylum.
| Bilateria |
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Internal phylogeny
editFor multiple decades, the genusXenoturbellacontained only one species,X. bocki.In 2016 however, a team reported the discovery of four new species from theGulf of Californiaand sequenced each new species'mitogenomeand upon analysis found that the two species that lived in shallow water (X. bockiandX. hollandorum) formed a "shallow" clade and that three deep water species formed a "deep" clade.[11]The following year another team discovered a sixth species,X. japonica,found off the coast of Japan. Their phylogenetic analysis confirmed the first team's hypothesis and placedX. japonicawithin the shallow clade.[12]
The other two groups,NemertodermatidaandAcoela,have less clear relationships as species-level phylogenies have not been conducted. Nemertodermatida only has two families and six total genera.Ascopariidaecontains two of these genera, whileNemertodermatidaehas the other four. A 2016 study analyzed three of the four Nemertodermatid genera and found thatSterreriaandMearaare closer to each other than toNemertoderma,whileNemertinoideswas left unplaced.[6]Acoela phylogeny is even less certain as it is by far the most diverse part of the phylum and is very understudied. A 2011 study attempted to solve this problem and recovered numerous traditional families aspolyphyletic.They also recovered a tentative clade of various species fromActinoposthiidaeandIsodiametridaewhich is not shown in the below cladogram.[13]Several small basal families were not included in their study and their position is still uncertain.
| Xenacoelomorpha |
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Characteristics
editThe phylum consists of small, flat and worm-like creatures found inmarineand sometimesbrackish waterenvironments,on thesediments.There are species that are variously free-living,parasitic,andsymbiotic.They can be found at depths of almost 4 km (2.5 mi) and nearhydrothermal vents.
The phylum ishermaphroditic(all individuals have both male and female sex organs) and reproducessexuallywithdirect development,meaning they skip a potentially vulnerablelarvalstage.Xenoturbellahave external fertilization, and Acoelomorpha has internal fertilization.[14][15][16]All xenacoelomorphs are bilateral, meaning they have a central front-to-back body axis with mirror image right and left sides. They aretriploblasts(meaning they have the threegerm layers:ectoderm,endoderm,andmesoderm). Theirbody planisacoelomate– they lack acoelom– do not have a true body cavity. Also an excretory system is absent, yet all genes related to the excretory system are present except for Osr, which is essential for the development for such a system. In acoelomorphs, which has gone through rapid evolutionary rates and chromosomic rearrangements, about 60% of the genes shared between protostomes and deuterostomes are missing. How many of these genes which are present or absent in Xenoturbella will require a whole genome sequencing.[17]
While other animals that arediploblastic(only have two germ layers: ectoderm and endoderm) also lack a coelom, those technically do not have anacoelomatebody plan because they lack the mesoderm germ layer. Inacoels,the mouth opens directly into a large endodermalsyncytium,while innemertodermatidsand xenoturbellids there is a sack-like gut lined by unciliated cells.[18]
A defining feature is a digestive system lacking nerve cells. Because anenteric nervous system,also called the stomatogastric nervous system, is also found in many cnidarians, its absence is most likely a derived trait.[19]
Their nervous systems arebasiepidermal– located right under theepidermis– and they have no brain. The xenoturbellids' nervous system consists of a simplenerve net,with no special concentration of neurons. In acoelomorphs the nervous system is arranged in a series of longitudinal bundles, united in the anterior region by a ring comissure of variable complexity.[20]
The sensory organs include astatocyst(for balance). Some groups have two unicellularocelli(simple eyes).[18][20]
The epidermis of all species within the phylum is ciliated. Theciliaare composed of a set of nine pairs of peripheralmicrotubulesand one or two central microtubules (patterns 9+1 and 9+2, respectively). The pairs 4–7 terminate before the tip, creating a structure called a "shelf".[21]
See also
editReferences
edit- ^Tyler, S.; Schilling, S.; Hooge, M.; Bush, L.F. (2006–2016)."Xenacoelomorpha".Turbellarian taxonomic database. Version 1.7.Archived fromthe originalon 9 February 2019.Retrieved3 February2016.
- ^abcPhilippe, H.; Brinkmann, H.; Copley, R. R.; Moroz, L. L.; Nakano, H.; Poustka, A. J.; Wallberg, A.; Peterson, K. J.; Telford, M. J. (10 February 2011)."Acoelomorph flatworms are deuterostomes related toXenoturbella".Nature.470(7333): 255–258.Bibcode:2011Natur.470..255P.doi:10.1038/nature09676.PMC4025995.PMID21307940.
- ^Lundin, K (1998). "The epidermal ciliary rootlets ofXenoturbella bocki(Xenoturbellida) revisited: new support for a possible kinship with the Acoelomorpha (Platyhelminthes) ".Zoologica Scripta.27(3): 263–270.doi:10.1111/j.1463-6409.1998.tb00440.x.S2CID85324766.
- ^abHejnol, A.; Obst, M.; Stamatakis, A.; Ott, M.; Rouse, G. W.; Edgecombe, G. D.; et al. (2009)."Assessing the root of bilaterian animals with scalable phylogenomic methods".Proceedings of the Royal Society B: Biological Sciences.276(1677): 4261–4270.doi:10.1098/rspb.2009.0896.PMC2817096.PMID19759036.
- ^What is Xenoturbella? | Zoological Letters - BioMed Central
- ^abPerseke, M.; Hankeln, T.; Weich, B.; Fritzsch, G.; Stadler, P.F.; Israelsson, O.; Bernhard, D.; Schlegel, M. (August 2007)."The mitochondrial DNA of Xenoturbella bocki: genomic architecture and phylogenetic analysis"(PDF).Theory Biosci.126(1): 35–42.CiteSeerX10.1.1.177.8060.doi:10.1007/s12064-007-0007-7.PMID18087755.S2CID17065867.Archived fromthe original(PDF)on 24 April 2019.Retrieved10 December2013.
- ^Cannon, J.T.; Vellutini, B.C.; Smith, J.; Ronquist, F.; Jondelius, U.; Hejnol, A. (4 February 2016)."Xenacoelomorpha is the sister group to Nephrozoa".Nature.530(7588): 89–93.Bibcode:2016Natur.530...89C.doi:10.1038/nature16520.PMID26842059.S2CID205247296.
- ^Nakano, Hiroaki; Lundin, Kennet; Bourlat, Sarah J.; Telford, Maximilian J.; Funch, Peter; Nyengaard, Jens R.; Obst, Matthias; Thorndyke, Michael C. (2013)."Xenoturbella bocki exhibits direct development with similarities to Acoelomorpha".Nature Communications.4:1537.Bibcode:2013NatCo...4.1537N.doi:10.1038/ncomms2556.PMC3586728.PMID23443565.
- ^Philippe, Hervé; et al. (2019). "Mitigating Anticipated Effects of Systematic Errors Supports Sister-Group Relationship between Xenacoelomorpha and Ambulacraria".Current Biology.29(11): 1818–1826.e6.Bibcode:2019CBio...29E1818P.doi:10.1016/j.cub.2019.04.009.hdl:21.11116/0000-0004-DC4B-1.ISSN0960-9822.PMID31104936.S2CID155104811.
- ^Kapli, Paschalia; Telford, Maximilian J. (11 December 2020)."Topology-dependent asymmetry in systematic errors affects phylogenetic placement of Ctenophora and Xenacoelomorpha".Science Advances.6(10): eabc5162.Bibcode:2020SciA....6.5162K.doi:10.1126/sciadv.abc5162.PMC7732190.PMID33310849.
- ^Rouse, Greg W.; Wilson, Nerida G.; Carvajal, Jose I.; Vrijenhoek, Robert C. (4 February 2016). "New deep-sea species ofXenoturbellaand the position of Xenacoelomorpha ".Nature.530(7588): 94–97.Bibcode:2016Natur.530...94R.doi:10.1038/nature16545.ISSN0028-0836.PMID26842060.S2CID3870574.
- ^Nakano, Hiroaki; Miyazawa, Hideyuki; Maeno, Akiteru; Shiroishi, Toshihiko; Kakui, Keiichi; Koyanagi, Ryo; Kanda, Miyuki; Satoh, Noriyuki; Omori, Akihito; Kohtsuka, Hisanori (18 December 2017)."A new species ofXenoturbellafrom the western Pacific Ocean and the evolution ofXenoturbella".BMC Evolutionary Biology.17(1): 245.doi:10.1186/s12862-017-1080-2.ISSN1471-2148.PMC5733810.PMID29249199.
- ^Jondelius, Ulf; Wallberg, Andreas; Hooge, Matthew; Raikova, Olga (December 2011)."How the Worm Got its Pharynx: Phylogeny, Classification and Bayesian Assessment of Character Evolution in Acoela".Systematic Biology.60(6).doi:10.1093/sysbio/syr073.
- ^Pontarotti, Pierre (1 October 2019).Evolution, Origin of Life, Concepts and Methods.Springer Nature.ISBN978-3-030-30363-1.
- ^Nakano, H. (2019)."Development of Xenoturbellida".Evo-Devo: Non-model Species in Cell and Developmental Biology.Results and Problems in Cell Differentiation. Vol. 68. pp. 251–258.doi:10.1007/978-3-030-23459-1_11.ISBN978-3-030-23458-4.PMID31598860.S2CID204033850.
- ^Achatz, J. G.; Chiodin, M.; Salvenmoser, W.; Tyler, S.; Martinez, P. (2012)."The Acoela: On their kind and kinships, especially with nemertodermatids and xenoturbellids (Bilateria incertae sedis)".Organisms, Diversity & Evolution.13(2): 267–286.doi:10.1007/s13127-012-0112-4.PMC3789126.PMID24098090.
- ^Abalde, Samuel; Tellgren-Roth, Christian; Heintz, Julia; Vinnere Pettersson, Olga; Jondelius, Ulf (2023)."The draft genome of the microscopic Nemertoderma westbladi sheds light on the evolution of Acoelomorpha genomes".Frontiers in Genetics.14:1244493.doi:10.3389/fgene.2023.1244493.PMC10565955.PMID37829276.
- ^abAchatz, Johannes G.; Chiodin, Marta; Salvenmoser, Willi; Tyler, Seth; Martinez, Pedro (June 2013)."The Acoela: on their kind and kinships, especially with nemertodermatids and xenoturbellids (Bilateria incertae sedis)".Organisms Diversity & Evolution.13(2): 267–286.doi:10.1007/s13127-012-0112-4.ISSN1439-6092.PMC3789126.PMID24098090.
- ^The digestive system of xenacoelomorphs - CORE
- ^abPerea-Atienza, E.; Gavilan, B.; Chiodin, M.; Abril, J.F.; Hoff, K.J.; Poustka, A.J.; Martinez, P. (2015)."The nervous system of Xenacoelomorpha: A genomic perspective".Journal of Experimental Biology.218(4): 618–628.doi:10.1242/jeb.110379.hdl:2445/192702.ISSN0022-0949.PMID25696825.
- ^Franzen, Ake; Afzelius, Bjorn A. (January 1987). "The ciliated epidermis of Xenoturbella bocki (Platyhelminthes, Xenoturbellida) with some phylogenetic considerations".Zoologica Scripta.16(1): 9–17.doi:10.1111/j.1463-6409.1987.tb00046.x.ISSN0300-3256.S2CID85675105.