PZ Myers. 2005 Jun 26. Trichoplax adhaerans. <http://pharyngula.org/index/weblog/trichoplax_adhaerans/>. Accessed 2008 Aug 30.
Posted on M00o93H7pQ09L8X1t49cHY01Z5j4TT91fGfr on Sunday, June 26, 2005
Trichoplax adhaerans
Last week, I cited a paper from Nature on the search for the Urmetazoan, a paper that briefly mentioned Trichoplax adhaerens as "the simplest known living animal". So what happens is that I get a bunch of e-mail asking, "what the heck is a Trichoplax?" It's easiest if I just tell you all at once, so here you go.
Salt-water aquarists may already know what Trichoplax is—it's a common salt-water scavenger that crops up on the glass walls of aquaria, sliding around and eating algae. It's not a particularly attractive aquarium resident, though.
It's a flattened blob, a few millimeters across and made up of a few thousand cells. It's main claim to fame is its remarkable simplicity: it is a multicellular animal that consists of only four apparent cell types, and the only obvious organization is into an upper and lower surface. The upper surface consists of a sheet of covering cells, while the lower surface contains two cell types: the gland cells that secrete digestive enzymes onto whatever the animal is sitting on, and the cylinder cells that absorb whatever nutrients are released. In between is a loose network of fiber cells that are responsible for the animal's movement.
The four classically recognized cell types of Trichoplax: cover cells of the upper epithelium, fiber cells of the intermediate layer, and cylinder cells and gland cells of the lower layer
Trichoplax completely lacks any kind of nervous system. It's genome is tiny, only 50Mb—significantly smaller than our 3Gb, and not much bigger than that of yeast, at 12Mb. It is considered the simplest multicellular animal in existence.
There are subtle hints that some of that simplicity may only be apparent, though: the latest molecular evidence suggests that there is some regional specificity, and that there may be a few other cell types scattered throughout the organism. They do have one Hox-like gene, Trox-2, that is expressed in scattered cells in the marginal zone. That and a few other molecules suggest that there may be some positional information in the animal to distinguish margin from center. There are also a few cells in the margin that secrete RFamide, a common neurotransmitter used by cnidarians (Hydra, for instance).
(C) Secp1, which encodes a putative small secreted protein, is expressed uniformly in the marginal zone, as shown in the top view (upper panel), and in all three layers, as shown in the transverse section (lower panel). (D) The Gsx-type homeobox gene, Trox-2 is also expressed in the marginal zone, but in discrete cells. (E) Another homeobox gene, Not, is expressed in folds in intact animals, as shown here, as well as in regenerating wounds. (F) Birefringent granules, possibly of calcitic calcium carbonate, are also limited to the marginal zone. (G) The T-box transcription factor Brachyury is expressed in a few cells or groups of cells in the marginal outgrowth zones of large Trichoplax individuals.
One other strange thing: in culture, Trichoplax is consistently asexual and reproduces by fission, but older cultures at high density begin to produce small motile presumptive sperm cells, and as individual animals desintegrate, they spew out ova. The two have never been observed to come together, though, so there is no fertilization, and while the ova may divide a half dozen times, they all eventually die. It is possible that there is another stage in the life cycle that is not viable under laboratory conditions and has never been observed.
Another likely possibility hinted at by these scattered scraps of complexity is that Trichoplax is a degenerate form, a descendant of a more complex ancestor that secondarily abandoned complexity. It's not clear who that ancestor would have been, though—some of the molecular evidence rules out the cnidaria, the obvious candidate. The enigmatic status of this animal has prompted the DOE Joint Genome Institute to sponsor an effort to sequence Trichoplax.
The sole representative species of the phylum Placozoa, Trichoplax adhaerens represents the simplest known animal, with the smallest known animal genome. The DNA sequence of the 50-Mbp Trichoplax genome will have far-reaching scientific importance, providing significant genomic insights into our understanding of how animal life evolved. This genome will have enormous utility to the scientific community, becoming the standard basal group for the comparative analysis of animal genomes, genes, and biological processes. Researchers who study the lower metazoans will clearly benefit, as will the bioinformatics and comparative genomics community. Prospects for future funding of functional annotation, microarrays, and evolutionary genomic studies are excellent, and available biological resources include established laboratory cultures, high-quality genomic DNA, arrayed fosmid and full-length cDNA libraries. A Trichoplax genome will make it possible to determine the gene and proteome content of the simplest known animals and provide the first genomics platform in a simple multicellular system.
You can also get a more detailed summary from the Trichoplax sequencing proposal (pdf).
Ender A, Schierwater B (2003) Placozoa Are Not Derived Cnidarians: Evidence from Molecular Morphology. Mol. Biol. Evol. 20(1):130-134.
Miller DJ, Ball EE (2005) Animal Evolution: The Enigmatic Phylum Placozoa Revisited. Curr. Biol. 15(1):26-28.
Science • Organisms • 2 Trackbacks • Other weblogs • Permalink
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I wonder what insights ID Theory will have, into this creature.
#: Posted by on 06/26 at 09:54 AM
- OK, Timmah, ever wonder what it feels like to have your knuckles smashed with a ball-peen hammer? Invite those morons back, and you might find out.
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I was about to start screaming incoherently about front loading myself. It's all front loading, FRONT LOADING I TELL YOU!
Because aliens have nothing better to do than make things as inefficient as possible. Everything needs to evolve for several millions years before it can get anywhere rather than being designed efficiently from the beginning.
It's so OBVIOUS.
In any event, these are really interesting little organisms and can provide us with many clues about the origins of higher metazoans. It's just a shame a few more of them haven't stuck around.#: Posted by Joseph ODonnell on 06/26 at 10:09 AM - hmm...clearly a case of Reducible Simplicity.
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PZ
Thanks for the science. ID and politics are fun, but, its enlightening and enjoyable to read some real science. This attracts be to your site.#: Posted by on 06/26 at 12:34 PM -
What is the "Shiny Sphere" in the diagram?
#: Posted by on 06/26 at 01:26 PM
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Oh, man, what have you done? What can of worms have you opened? A lovely, gleaming scientific gem like this, and the first comment has to do with Intelligent Design!
There ought to be some sort of filter that snatches up all religious/political comments cropping up on science posts, and dumps 'em in some sort of steaming, roach-infested Internet hell. (Or, you know, a perpetual flame thread. Keep all the rubbish in one place.)
(Oh. Great post, by the way.) -
"...ever wonder what it feels like to have your knuckles smashed with a ball-peen hammer?"
What, have you abandoned hitting people with Gould's, massive, book?
Great post - someone should nominate it for Smarter-Than-I.#: Posted by on 06/26 at 04:40 PM -
I could spend a happy year watching this hairy blob. I find it fascinating that Tricoplex is a free living animal grazing on algae, like a blind cow on pasture. Unlike parasitic and/or miniaturized animals, free-living ones do not tend to extreme simplification. Maybe once it was a parasite or has a sexual stage (yet undiscovered) that is parasitic. How can it go on for billenia without sex? How is that the Eilat (Red Sea) and the East Pacific Tricoplex are the same species?
On the other hand, having no brain nor nerve cells, could it be that Tricoplex belongs in the IDer phyllum?#: Posted by on 06/27 at 08:36 AM -
What can this animal teach us about the central mystery of evolutionary theory (and it's ultimate downfall)? How are there MIDGETS and DWARVES?
#: Posted by on 06/27 at 08:48 AM
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John H wrote:
What is the "Shiny Sphere" in the diagram?
I'm guessing that it is one of the birefringent granules in the photos. That said, what would be the purpose of these granules and how are they formed? Have they something to do with the Parahox Trox-2 gene?#: Posted by on 06/27 at 08:53 AM -
Here's what Miller and Ball say about it:
The presence in this layer of birefringent granules (Figure 1F) of what appears to be calcitic calcium carbonate also suggests similarities with the cnidarian ectoderm, which for example produces the calcium carbonate skeleton in corals. However, it is difficult to relate this apparent similarity to the overall structure of Trichoplax.
In other words, we don't know what they're for, but they may be a relic of their ancestry. -
What? First mesozoans, then cnidaria and other pinko jellies, now placozoans. A sorry shower of degenerates, living proof of the second law of thermodynamics?
No. On the contrary.
Placozoans are different. They're special, and remarkable for their modesty and chastity. That is, they can, but they choose not to.
The complete absence of any traces before the 19th century suggests that they may indeed be a totally new kind of animal. One theory is that they were created during Louis Pasteur's famous experiments so as to punish him for his effrontery. Unfortunately things don't always go according to design: the scientist overlooked them and flushed them down the drain. Some rapidly colonised the oceans and fishtanks of the world. Others grew to a prodigious size and enjoyed a brief fame in "B" movies of the 1950s [run for your lives, it's Treptoplax reptans! Aaaaargh! Aaaaaargh! Aaaaaaaaaaaaaaaaaaargh!], afterwards becoming disillusioned and retiring into obscurity.
Their DNA was assembled from a junkyard of odds and ends from unfortunate existing creatures that had been killed by the pasteurisation process. I predict that it will therefore be found to have some similarities, in parts, to
other animals.#: Posted by on 06/27 at 11:57 AM -
The "shiny spheres" are not the birefringent granules. Instead, these spheres have been described as degenerate cells and it is known that they contain lipids. Vicki Pearse suggested that they might contain toxins as she as observed that at least some grazing animals (snails, etc.) pull back upon encountering placozoans. But basically no one knows much about the shiny spheres, or much else about the basic biology of Trichoplax.
As for why the same lineage of placozoans is found in localities half way around the world from eachother, it is possible that these little guys just get around easily. Some people argue that for little organisms, "everything is everywhere."#: Posted by Allen Collins on 07/04 at 03:47 AM -
In Cnidarians, the presence of shiny lipid-like droplets is related to sexual reproduction, not to defensive apparati (?) or toxic/repellent substances. Trichoplax may be presumed of being possessed by sex no less than the rest of animalia, so they must be doing their thing when no one watches them. Of course, I know nothing, that's why I am so sure.
#: Posted by on 07/04 at 06:26 AM