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About Xenacoelomorpha[edit]

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Etymology[edit]

The term "xenacoelomorph" derives from the four Ancient Greek words ξένος (xénos), meaning "strange, unusual", ἀ (a), the alpha privative, expressing negation or absence, κοιλία (koilía), meaning "cavity", and μορφή (morphḗ), meaning "form".[1][2] This refers to the fact that this taxonomic group contains xenoturbellids, hence the prefix 'xéno-', and acoelomorphs, which have a structure lacking a fluid-filled body cavity.

The term Xenoturbella derives from the Ancient Greek word ξένος (xénos), meaning "strange, unusual",[3][4] and from the Latin word turbella meaning "stir, bustle".[5] This refers to the unusual taxonomic status of the animal, initially considered as related to turbellarians, a group of flatworms whose aquatic species stir microscopic particles close to their ciliated epidermis.[6]


Phylogeny[edit]

In addition, comparative analyses of morphological, developmental, and molecular characters raised two points.

  • Acoelomorpha is the sister group to Xenoturbellida, and both taxa consitute the so-called Xenacoelomorpha clade.[7][8][9] The close evolutionary relationship between Xenoturbella and Acoelomorpha is supported by their morphology (structure of epidermal cilia[10]), embryology (direct development without a feeding larval stage[11]), and by hundreds of proteins concatenates.[12][7][13]
  • The phylogenetic placement of Xenacoelomorpha among bilaterian animals is not yet well defined, despite increased taxon and gene sampling, (re)-analyses of published data sets, and use of more sophisticated models of sequence evolution in phylogenomic studies. There is a conflict between two evolutionary hypotheses, with Xenacoelomorpha being the sister group to Ambulacraria within deuterostomes (i.e., the Xenambulacraria hypothesis) on the one hand,[14] and Xenacoelomorpha as sister group to all other bilaterians (i.e., the Nephrozoa hypothesis) on the other.[13][15][16][17] However, the Nephrozoa hypothesis might reflect methodological errors resulting from model violations in the phylogenomic inference.[14]

Philippe et al. (2019)[edit]

"Highlights:[14]

  • Model violations hinder inference of the phylogeny of Bilateria
  • Methods predicted to cause errors place Xenacoelomorpha outside Nephrozoa
  • Xenacoelomorpha is the sister group of Ambulacraria (hemichordates and echinoderms)
  • Monophyletic deuterostomes (Chordata plus Xenambulacraria) are not clearly supported"

Martín-Durán et al. (2018)[edit]

Our findings restrict the use of molecular patterns to explain nervous system evolution, and suggest that the similarities in dorsoventral patterning and trunk neuroanatomies evolved independently in Bilateria.[18]

About Xenambulacraria[edit]

"First is Reisinger’s proposed idea of an association between Xenoturbella and archicoelo- mate deuterostomes (5 Ambulacraria: hemichordates and echinoderms) (Reisinger, 1960). Westblad had al- ready noted detailed similarities of the epidermis of Xen- oturbella and of hemichordates. Reisinger emphasised this resemblance and further noted similarities of the sta- tocyst of Xenoturbella to that found in certain sea cucumbers (Holothuria, Echinodermata) and proposed that Xenoturbella might represent the sexually mature larva of an animal related to the echinoderms and hemi- chordates. He went further and even speculated that it might belong to the life cycle of a known type of archi- coelomate and subsequently develop into an animal sim- ilar to enteropneusts, which would later become sexu- ally mature for a second time." [19]


NewRef[edit]

About NewTaxa[edit]

NewRef[edit]

Phylogenetic hypotheses[edit]

Inlusion and exclusion status of 4 bilaterian taxa with respect to different hypotheses of evolutionary groupings.[20]
Metazoan taxa Deuterostomia
grouping 		Xenambulacraria
grouping 		Nephrozoa
grouping 		Bilateria
clade
Ambulacraria Included Included Included Included
Chordata Included Excluded Included Included
Protostomia Excluded Excluded Included Included
Xenacoelomorpha Uncertain status Included Excluded Included

About flatworms[edit]

The "traditional" view before the 1990s was that Platyhelminthes formed the sister group to all the other bilaterians, which include, for instance, arthropods, molluscs, annelids and chordates. Since then, molecular phylogenetics, which aims to work out evolutionary "family trees" by comparing different organisms' biochemicals such as DNA, RNA and proteins, has radically changed scientists' view of evolutionary relationships between animals.[21] Detailed morphological analyses of anatomical features in the mid-1980s, as well as molecular phylogenetics analyses since 2000 using different sections of DNA, agree that Acoelomorpha, consisting of Acoela (traditionally regarded as very simple "turbellarians"[22]) and Nemertodermatida (another small group previously classified as "turbellarians"[23]) are the sister group to all other bilaterians, including the rest of the Platyhelminthes.[21][24] However, a 2007 study concluded that Acoela and Nemertodermatida were two distinct groups of bilaterians, although it agreed that both are more closely related to cnidarians (jellyfish, etc.) than other bilaterians are.[25]

Xenoturbella, a bilaterian whose only well-defined organ is a statocyst, was originally classified as a "primitive turbellarian".[26] Later studies suggested it may instead be a [[deuterostome],[27][28] but more detailed molecular phylogenetics have led to its classification as sister-group to the Acoelomorpha.[29]

References[edit]

  1. ^ Bailly, Anatole (1981-01-01). Abrégé du dictionnaire grec français. Paris: Hachette. ISBN 2010035283. OCLC 461974285.
  2. ^ Bailly, Anatole. "Greek-french dictionary online". www.tabularium.be. Retrieved 2020-03-13.
  3. ^ Bailly, Anatole (1981-01-01). Abrégé du dictionnaire grec français. Paris: Hachette. ISBN 2010035283. OCLC 461974285.
  4. ^ Bailly, Anatole. "Greek-french dictionary online". www.tabularium.be. Retrieved 13 March 2020.
  5. ^ Gaffiot, Félix (1934). Dictionnaire illustré Latin-Français (in French). Paris: Librairie Hachette. p. 1613. Retrieved 13 March 2020.
  6. ^ Ruppert, E.E.; Fox, R.S.; Barnes, R.D. (2004). Invertebrate Zoology (7 ed.). Brooks / Cole. p. 228. ISBN 0-03-025982-7.
  7. ^ a b Philippe, H.; Brinkmann, H.; Copley, R. R.; Moroz, L. L.; Nakano, H.; Poustka, A. J.; Wallberg, A.; Peterson, K. J.; Telford, M. J. (2011). "Acoelomorph flatworms are deuterostomes related to Xenoturbella". Nature. 470 (7333): 255–258. Bibcode:2011Natur.470..255P. doi:10.1038/nature09676. PMC 4025995. PMID 21307940.
  8. ^ "Scientists reorganise the animal phylogenetic tree". domain-b.com. 2011-02-18. Retrieved 2020-03-03.
  9. ^ "Simple marine worms distantly related to humans". phys.org. 2011-02-09. Retrieved 2020-03-03.
  10. ^ Lundin, K (1998). "The epidermal ciliary rootlets of Xenoturbella 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.
  11. ^ Nakano, H.; Lundin, K.; Bourlat, S. J.; Telford, M. J.; Funch, P.; Nyengaard, J. R.; Obst, M.; Thorndyke, M. C. (2013). "Xenoturbella bocki exhibits direct development with similarities to Acoelomorpha". Nature Communications. 4: 1537–. Bibcode:2013NatCo...4.1537N. doi:10.1038/ncomms2556. PMC 3586728. PMID 23443565.
  12. ^ Hejnol, Andreas; Obst, Matthias; Stamatakis, Alexandros; Ott, Michael; Rouse, Greg W.; Edgecombe, Gregory D.; Martinez, Pedro; Baguñà, Jaume; Bailly, Xavier; Jondelius, Ulf; Wiens, Matthias (2009-12-22). "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. PMC 2817096. PMID 19759036.{{cite journal}}: CS1 maint: PMC format (link)
  13. ^ a b Edgecombe, G. D.; Giribet, G.; Dunn, C. W.; Hejnol, A.; Kristensen, R. M.; Neves, R. C.; Rouse, G. W.; Worsaae, K.; Sørensen, M. V. (2011). "Higher-level metazoan relationships: Recent progress and remaining questions". Organisms Diversity & Evolution. 11 (2): 151–172. doi:10.1007/s13127-011-0044-4.
  14. ^ a b c Philippe, Hervé; Poustka, Albert J.; Chiodin, Marta; Hoff, Katharina J.; Dessimoz, Christophe; Tomiczek, Bartlomiej; Schiffer, Philipp H.; Müller, Steven; Domman, Daryl (2019). "Mitigating Anticipated Effects of Systematic Errors Supports Sister-Group Relationship between Xenacoelomorpha and Ambulacraria". Current Biology. 29 (11): 1818–1826.e6. doi:10.1016/j.cub.2019.04.009. hdl:21.11116/0000-0004-DC4B-1. PMID 31104936.
  15. ^ Srivastava, M.; Mazza-Curll, K. L.; Van Wolfswinkel, J. C.; Reddien, P. W. (2014). "Whole-Body Acoel Regeneration is Controlled by Wnt and Bmp-Admp Signaling". Current Biology. 24 (10): 1107–13. doi:10.1016/j.cub.2014.03.042. PMID 24768051.
  16. ^ Cannon, Johanna Taylor; Vellutini, Bruno Cossermelli; Smith, Julian; Ronquist, Fredrik; Jondelius, Ulf; Hejnol, Andreas (2016). "Xenacoelomorpha is the sister group to Nephrozoa". Nature. 530 (7588): 89–93. doi:10.1038/nature16520. PMID 26842059.
  17. ^ "The return of the flatworm". phys.org. 2016-02-04. Retrieved 2020-03-03.
  18. ^ Martín-Durán, José M.; Pang, Kevin; Børve, Aina; Lê, Henrike Semmler; Furu, Anlaug; Cannon, Johanna Taylor; Jondelius, Ulf; Hejnol, Andreas (2018-01-04). "Convergent evolution of bilaterian nerve cords". Nature. 553 (7686): 45–50. doi:10.1038/nature25030. ISSN 1476-4687. PMC 5756474. PMID 29236686.{{cite journal}}: CS1 maint: PMC format (link)
  19. ^ Reisinger, Erich (1960). "Was ist Xenoturbella?". Z. Wiss. Zool. 164: 188–198.
  20. ^ Marlétaz, Ferdinand (2019-06-17). "Zoology: Worming into the Origin of Bilaterians". Current Biology. 29 (12): R577–R579. doi:10.1016/j.cub.2019.05.006. ISSN 0960-9822. PMID 31211978.
  21. ^ a b Cite error: The named reference Halanych2004NewViewOfAnimalPhylogeny was invoked but never defined (see the help page).
  22. ^ Cite error: The named reference RuppertBarnes2004Platyhelminthes was invoked but never defined (see the help page).
  23. ^ Jondelius, U.; Ruiz-Trillo, I.; Baguñà, J.; Riutort, M. (April 2002). "The Nemertodermatida are basal bilaterians and not members of the Platyhelminthes". Zoologica Scripta. 31 (2): 201–215. doi:10.1046/j.1463-6409.2002.00090.x.
  24. ^ Larsson, K.; Jondelius, U. (20 December 2008). "Phylogeny of Catenulida and support for Platyhelminthes". Organisms Diversity & Evolution. 8 (5): 378–387. doi:10.1016/j.ode.2008.09.002.
  25. ^ Wallberg, A.; Curini-Galletti, M.; Ahmadzadeh, A.; Jondelius, U. (September 2007). "Dismissal of Acoelomorpha: Acoela and Nemertodermatida are separate early bilaterian clades". Zoologica Scripta. 36 (5): 509–523. doi:10.1111/j.1463-6409.2007.00295.x. {{cite journal}}: Unknown parameter |last-author-amp= ignored (|name-list-style= suggested) (help)
  26. ^ Westblad, E. (1949). "Xenoturbella bocki n.g., n.sp., a peculiar, primitive turbellarian type". Arkiv för Zoologi. 1: 3–29.
  27. ^ Bourlat SJ, Nielsen C, Lockyer AE, Littlewood DT, Telford MJ (21 August 2003). "Xenoturbella is a deuterostome that eats molluscs". Nature. 424 (6951): 925–928. Bibcode:2003Natur.424..925B. doi:10.1038/nature01851. PMID 12931184.
  28. ^ Bourlat SJ, Juliusdottir T, Lowe CJ, Freeman R, Aronowicz J, Kirschner M, Lander ES, Thorndyke M, Nakano H, Kohn AB, Heyland A, Moroz LL, Copley RR, Telford MJ (2006). "Deuterostome phylogeny reveals monophyletic chordates and the new phylum Xenoturbellida". Nature. 444 (7115): 85–88. Bibcode:2006Natur.444...85B. doi:10.1038/nature05241. PMID 17051155.
  29. ^ 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. PMID 26842059.
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