New light on the enigmatic Xenoturbella (phylum uncertain): ontogeny and phylogeny

Israelsson, Olle

doi:10.1098/rspb.1999.0713

Cited by 68 publications

(52 citation statements)

References 16 publications

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“…The sister group to all acoelomorphs, the Xenoturbellida [8,9,30], is represented by only two nominal species: Xenoturbella bocki [31] and Xenoturbella westbladi [32]; however, newly identified specimens have been collected from the Pacific [33]. These worms are notably larger than the acoelomorphs (up to 4 cm in length) but share with them a relatively simple morphology.…”

Section: Comparative Analysis Of Xenacoelomorpha Nervous System Morphmentioning

confidence: 99%

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Xenacoelomorpha: a case of independent nervous system centralization?

Gavilán

Perea-Atienza

Martı́nez

2016

Phil. Trans. R. Soc. B

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One contribution of 16 to a discussion meeting issue 'Homology and convergence in nervous system evolution'. Centralized nervous systems (NSs) and complex brains are among the most important innovations in the history of life on our planet. In this context, two related questions have been formulated: How did complex NSs arise in evolution, and how many times did this occur? As a step towards finding an answer, we describe the NS of several representatives of the Xenacoelomorpha, a clade whose members show different degrees of NS complexity. This enigmatic clade is composed of three major taxa: acoels, nemertodermatids and xenoturbellids. Interestingly, while the xenoturbellids seem to have a rather 'simple' NS (a nerve net), members of the most derived group of acoel worms clearly have ganglionic brains. This interesting diversity of NS architectures (with different degrees of compaction) provides a unique system with which to address outstanding questions regarding the evolution of brains and centralized NSs. The recent sequencing of xenacoelomorph genomes gives us a privileged vantage point from which to analyse neural evolution, especially through the study of key gene families involved in neurogenesis and NS function, such as G protein-coupled receptors, helixloop-helix transcription factors and Wnts. We finish our manuscript proposing an adaptive scenario for the origin of centralized NSs (brains).

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Section: Comparative Analysis Of Xenacoelomorpha Nervous System Morphmentioning

confidence: 99%

“…It would be interesting, though, to have access to more species within this clade (xenoturbellids) since, right now, our inferences are based on morphological information derived from only two different species [31,32].…”

Section: Transformation From Nerve Nets To Compact Brains In the Xenamentioning

confidence: 99%

Xenacoelomorpha: a case of independent nervous system centralization?

Gavilán

Perea-Atienza

Martı́nez

2016

Phil. Trans. R. Soc. B

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“…Xenoturbella bocki is a small, up to 4cm long, free-living marine animal with an irregular and flattened shape (Israelsson, 1999). Its unusual morphology with an epithelial epidermis and gastrodermis, but without anus and only a "statocyst" as distinct organ (Ehlers, 1991), led to highly divergent phylogenetic interpretations.…”

Section: Introductionmentioning

confidence: 99%

The mitochondrial DNA of Xenoturbella bocki: genomic architecture and phylogenetic analysis

et al. 2007

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The phylogenetic position of Xenoturbella bocki has been a matter of controversy since its description in 1949. We sequenced a second complete mitochondrial genome of this species and performed phylogenetic analyses based on the amino-acid sequences of all 13 mitochondrial proteincoding genes and on its gene order. Our results confirm the deuterostome relationship of Xenoturbella. However, in contrast to a recently published study [Bourlat et al., Nature 444: 85-88 (2006)], our data analysis suggests a more basal branching of Xenoturbella within the deuterostomes, rather than a sister-group relationship to the Ambulacraria (Hemichordata and Echinodermata).

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“….'' (Israelsson, 1999). The extraordinary conclusion reached from these two complementary sources of evidence was that the flatworm-like Xenoturbella was a highly derived bivalve mollusc that, in a radical metamorphosis, had lost all morphological traces of its bivalve ancestry in the adult form.…”

Section: Introductionmentioning

confidence: 97%

“…Corroborating this finding, the second article described a phylogenetic analysis based on a specific characteristic of oocyte morphology shared by Xenoturbella and Protobranch molluscs; the persistence through vitellogenesis of micronucleoli arranged along one side of the germinal vesicle (Israelsson, 1997). In a subsequent article, Israelsson described in great detail the presence within Xenoturbella of developmental stages of typical molluscan pericalymma larvae (Israelsson, 1999). The larvae, which are described as undergoing unequal, holoblastic spiral cleavage, have many features in common with the protobranch pericalymma larva.…”

Section: Introductionmentioning

confidence: 99%

Xenoturbellida: The fourth deuterostome phylum and the diet of worms

Telford

2008

Genesis

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Since the discovery of the marine worm Xenoturbella bocki in 1915 by Sixten Bock and its first published description by Einar Westblad (Westblad,1949, Arkiv Zoologi 1:3–29), Xenoturbella was generally allied to the turbellarian flatworms, perhaps most closely to acoelomorphs. In 1997, however, analyses of ribosomal DNA (Norén and Jondelius, 1997, Nature 390:31–32) and developing oocytes (Israelsson, 1997, Nature 390:32) [and, subsequently, embryos (Israelsson, 1999, Proc R Soc Lond B 266:835–841)] recovered from Xenoturbella specimens led to the surprising conclusion that it was in fact a highly degenerate bivalve mollusc. Bourlat et al. showed in 2003 that this result was due to contamination from bivalves in its diet (Bourlat et al.,2003, Nature 424:925–928). Our analyses showed Xenoturbella is a deuterostome, related to the Ambulacraria (echinoderms and hemichordates). Subsequent work has shown that Xenoturbellida is a separate lineage from the Ambulacraria and therefore constitutes the fourth deuterostome phylum (Bourlat et al.,2006, Nature 444:85–88). I consider this phylogenetic position in the light of what is known of its genetics, morphology, and ontogeny. I examine what this phylogenetic position for Xenoturbella can tell us about its own evolution and what light this might shine on the common ancestor of the deuterostomes and hence on the origins of the chordates. genesis 46:580–586, 2008. © 2008 Wiley‐Liss, Inc.

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New light on the enigmatic Xenoturbella (phylum uncertain): ontogeny and phylogeny

Cited by 68 publications

References 16 publications

Xenacoelomorpha: a case of independent nervous system centralization?

Xenacoelomorpha: a case of independent nervous system centralization?

The mitochondrial DNA of Xenoturbella bocki: genomic architecture and phylogenetic analysis

Xenoturbellida: The fourth deuterostome phylum and the diet of worms

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