The missing link has gone missing – A new study involving Xenoturbella and Acoels
A new study led by Max Telford completely shook the animal Tree of life and also mark the birth of a new phyla termed “Xenacoelomorpha”.
Xenoturbella and Acoelomorpha are marine worms with highly debated ancestry. Both were originally associated with the flatworms (Platyhelminthes), but molecular data have revised their phylogenetic positions, generally linking xenoturbella to the deuterostomes leaving Acoelomorpha or Acoels as the most basally branching bilaterian group.
The acoels are small ciliated worms without a lumen (coelom) to the gut , which gives the group its name “Acoela”. These animals are bilaterally symmetric triploblastic worms possess a mid-ventral mouth and lack an anus.
Like more basal Cnidarians , acoels also have only one pore for eating and excretion but they differ from jelly fishes in having middle germ layer (mesoderm). Its because of the presence of mesoderm, which is found in all bilaterians including echinoderms and vertebrates, they were always considered as missing link between simple groups such as jelly fish and the rest of the animals, including humans, starfish, insects and molluscs. The Morphology and some molecular phylogenies place the Acoela as basal branches of the Bilateria, but there has been always a debate regarding the proper place of these unique animals in the tree of life.
Max Telford and co workers compared hundreds of genes from both Xenoturbella and the Acoelomorpha with their counterparts from a whole range of animal species to determine their evolutionary relationships.The results show that the two groups constitute a newly classified phylum , which the authors name the ‘Xenacoelomorpha’. The xenacoelomorph phylum joins the three known phyla of deuterostomes: vertebrates (including humans), echinoderms (e.g. starfish) and hemichordates (acorn worms). Previously thought to be an evolutionary link between simple animals such as jellyfish and the rest of animal life — the worms’ new position along with deuterostomes implies that they have not always been as simple as they now appear.
Because the simple Xenacoelomorpha are descended from the same ancestor that gave rise to complex groups such as vertebrates, echinoderms and hemichordates, these simple worms must have lost a lot of the complexity that they originally possessed — Max Telford
However not everyone in the field is happy with the study and you can get a detailed version about the views of different experts from the field by reading an excellent article by Amy Maxmen in Nature news published in same issue of Nature journal.
1) Acoelomorph flatworms are deuterostomes related to Xenoturbella.
Philippe H, Brinkmann H, Copley RR, Moroz LL, Nakano H, Poustka AJ, Wallberg A, Peterson KJ, Telford MJ.
Nature. 2011 Feb 10;470(7333):255-8
2) Xenoturbella is a deuterostome that eats molluscs.
Bourlat SJ, Nielsen C, Lockyer AE, Littlewood DT, Telford MJ.
Nature. 2003 Aug 21;424(6951):925-8.
3) Deuterostome phylogeny reveals monophyletic chordates and the new phylum Xenoturbellida.
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
Nature. 2006 Nov 2;444(7115):85-8.
Bourlat SJ, Hejnol A.
Curr Biol. 2009 Apr 14;19(7):R279-80
5) Evolution: A can of worms