The Mesenchyme-Like Layer of the Fiber Cells of Trichoplax adhaerens (Placozoa), a Syncytium

Buchholz, Katja; Ruthmann, August

doi:10.1515/znc-1995-3-418

Cited by 19 publications

(22 citation statements)

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“…Placozoans are composed of a ciliated epithelium that is differentiated dorsally and ventrally, between which is a mesenchymal syncytium (30). Acoel flatworms also possess a central syncytium (31).…”

Section: Discussionmentioning

confidence: 99%

Evaluating multiple alternative hypotheses for the origin of Bilateria: An analysis of 18S rRNA molecular evidence

Collins

1998

Proc. Natl. Acad. Sci. U.S.A.

238

137

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Six alternative hypotheses for the phylogenetic origin of Bilateria are evaluated by using complete 18S rRNA gene sequences for 52 taxa. These data suggest that there is little support for three of these hypotheses. Bilateria is not likely to be the sister group of Radiata or Ctenophora, nor is it likely that Bilateria gave rise to Cnidaria or Ctenophora. Instead, these data reveal a close relationship between bilaterians, placozoans, and cnidarians. From this, several inferences can be drawn. Morphological features that previously have been identified as synapomorphies of Bilateria and Ctenophora, e.g., mesoderm, more likely evolved independently in each clade. The endomesodermal muscles of bilaterians may be homologous to the endodermal muscles of cnidarians, implying that the original bilaterian mesodermal muscles were myoepithelial. Placozoans should have a gastrulation stage during development. Of the three hypotheses that cannot be falsified with the 18S rRNA data, one is most strongly supported. This hypothesis states that Bilateria and Placozoa share a more recent common ancestor than either does to Cnidaria. If true, the simplicity of placozoan body architecture is secondarily derived from a more complex ancestor. This simplification may have occurred in association with a planula-type larva becoming reproductive before metamorphosis. If this simplification took place during the common history that placozoans share with bilaterians, then placozoan genes that contain a homeobox, such as Trox2, should be explored, for they may include the gene or genes most closely related to Hox genes of bilaterians.Despite numerous speculations and analyses concerning the evolutionary relationships of the major animal clades, until recently only three distinct hypotheses have been offered concerning the phylogenetic position of Bilateria within the other basal metazoan groups (Fig. 1 A-C). Two of these hypotheses are commonly found in zoology textbooks. One view, an idea that goes back to Haeckel (1) and was championed by Hyman (2, 3), is that Bilateria is the sister group to Radiata (Cnidaria and Ctenophora). A second hypothesis, preferred by Harbison (4) and Wilmer (5), holds that Bilateria and Ctenophora share a more recent common ancestor than either does to Cnidaria. Recent cladistic studies based on morphological characters (6, 7) have supported this second view. A third alternative, which appears to have fallen out of favor, has Bilateria as a nonmonophyletic group, with both cnidarians and ctenophorans derived from flatworm ancestors (8). The strength of these hypotheses is difficult to weigh on morphological evidence alone. The diploblastic groups have disparate body plans with many derived features, only a few of which are potentially informative as to their relative phylogenetic positions. Furthermore, homoplasies are difficult to infer when dealing with such general features as mouths, mesoderm, and symmetry.Molecular sequence studies provide sets of characters that can be used to both evaluate previ...

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Section: Discussionmentioning

confidence: 99%

Evaluating multiple alternative hypotheses for the origin of Bilateria: An analysis of 18S rRNA molecular evidence

Collins

1998

Proc. Natl. Acad. Sci. U.S.A.

238

137

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“…Traditionally only four cell types have been described in Trichoplax, upper and lower epithelial cells, gland cells within the lower, feeding epithelium, and fiber cells sandwiched between the epithelia. (7)(8)(9)(10)(11) No organs or specialized nerve or muscular cells are present. A basal lamina and extracellular matrix are likewise lacking.…”

Section: My First Contact With Trichoplaxmentioning

confidence: 99%

My favorite animal, Trichoplax adhaerens

Schierwater¹

2005

BioEssays

152

109

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Trichoplax adhaerens is more simply organized than any other living metazoan. This tiny marine animal looks like a irregular "hairy plate" ("tricho plax") with a simple upper and lower epithelium and some loose cells in between. After its original description by F.E. Schulze 1883, it attracted particular attention as a potential candidate representing the basic and ancestral state of metazoan organization. The lack of any kind of symmetry, organs, nerve cells, muscle cells, basal lamina and extracellular matrix originally left little doubt about the basal position of T. adhaerens. Nevertheless, the interest of zoologists and evolutionary biologists suddenly vanished for more than half a century when Trichoplax was claimed to be an aberrant hydrozoan planula larva. Recently, Trichoplax has been rediscovered as a key species for unraveling early metazoan evolution. For example, research on regulatory genes and whole genome sequencing promise insights into the genetics underlying the origin and development of basal metazoan phyla. Trichoplax offers unique potential for understanding the minimal requirements of metazoan animal organization.

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“…Based on Schulze's early morphological studies and Grell and colleagues' ultrastructural research [2], [3], [5], [13], [17], [19]–[22], [24], [26], [28] Trichoplax possesses a three-layered sandwich organization with morphologically different upper (protective) and lower (nutritive) epithelia. These two layers enclose a meshwork of connected fiber cells that are responsible for the changes in shape of Trichoplax .…”

Section: Introductionmentioning

confidence: 99%

Global Diversity of the Placozoa

et al. 2013

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The enigmatic animal phylum Placozoa holds a key position in the metazoan Tree of Life. A simple bauplan makes it appear to be the most basal metazoan known and genetic evidence also points to a position close to the last common metazoan ancestor. Trichoplax adhaerens is the only formally described species in the phylum to date, making the Placozoa the only monotypic phylum in the animal kingdom. However, recent molecular genetic as well as morphological studies have identified a high level of diversity, and hence a potential high level of taxonomic diversity, within this phylum. Different taxa, possibly at different taxonomic levels, are awaiting description. In this review we firstly summarize knowledge on the morphology, phylogenetic position and ecology of the Placozoa. Secondly, we give an overview of placozoan morphological and genetic diversity and finally present an updated distribution of placozoan populations. We conclude that there is great potential and need to erect new taxa and to establish a firm system for this taxonomic tabula rasa.

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The Mesenchyme-Like Layer of the Fiber Cells of Trichoplax adhaerens (Placozoa), a Syncytium

Cited by 19 publications

References 0 publications

Evaluating multiple alternative hypotheses for the origin of Bilateria: An analysis of 18S rRNA molecular evidence

Evaluating multiple alternative hypotheses for the origin of Bilateria: An analysis of 18S rRNA molecular evidence

My favorite animal, Trichoplax adhaerens

Global Diversity of the Placozoa

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