Xenacoelomorpha
| Xenacoelomorpha | |
|---|---|
| |
| Xenoturbella japonica,a xenacoelomorph member (xenoturbellids) | |
| |
| Proporussp., another xenacoelomorph member (acoelomorphs) | |
Scientific classification
| |
| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Subkingdom: | Eumetazoa |
| Clade: | ParaHoxozoa |
| Clade: | Bilateria |
| Phylum: | Xenacoelomorpha Philippe et al. 2011[1] |
| Subphyla | |
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]
Phylogenetics[edit]
ThecladeXenacoelomorpha groups the Acoelomorpha and thegenusXenoturbellabased on molecular studies.[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.[5][6]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.[7]
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.[8][9]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 |
| ||||||||||||||||||
Characteristics[edit]
The 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.[10][11][12]All xenacoelomorphs are bilateral, meaning they have a central front-to-back body axis with mirror image right and left sides. While 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.[13]
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.[14]
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.[15]
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.[16]
The sensory organs include astatocyst(for balance). Some groups have two unicellularocelli(simple eyes).[14][16]
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".[17]
See also[edit]
References[edit]
- ^Tyler, S.; Schilling, S.; Hooge, M.; Bush, L.F. (2006–2016)."Xenacoelomorpha".Turbellarian taxonomic database. Version 1.7.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.
- ^Perseke, 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.
- ^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.
- ^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.