PZ Myers. 2005 Nov 27. Mysterious Trichoplax. <http://pharyngula.org/index/weblog/mysterious_trichoplax/>. Accessed 2008 Aug 30.
Posted on M00o93H7pQ09L8X1t49cHY01Z5j4TT91fGfr on Sunday, November 27, 2005
Mysterious Trichoplax
BioEssays regularly runs a feature called "My Favorite Animal"; this month's choice is barely an animal at all, the placophoran Trichoplax adhaerens. I've written about Trichoplax before. It's a strange creature, a small flat blob that creeps amoeba-like over the substrate, that replicates by simply splitting in two, and that seems to have no distinguishing features at all—no head, no sense organs, no nervous system, no gut, just a collection of cells that hang together and slurp up algal slime. They are, however, multicellular, and their bodies contain at least four functionally distinct cell types, and the molecular evidence suggests affinities to other animal groups (they have a ProtoHox/ParaHox gene, for instance)…so they are definitely metazoans. They are just the simplest, barest kind of metazoan we can find now.
As a thin disc with no polarity other than which side sticks to the substrate and which faces the open water, Trichoplax has provocative appeal as a representative of an ancient multicellular form—a creature that has clustered cells together to grow larger, has taken the first steps towards specializing cells for digestion and protection, but hasn't even achieved pre-worm status. It's the kind of pre-Cambrian browser from which our own line might have evolved. Here's a simple and speculative (but not at all outrageous) idea of how this could have occurred:
Placula hypothesis of metazoan evolution. Flagellated protozoans unite to form a benthic-vagile, plate-like metazoan organism. The one-layered protist form (a) evolves to the two-layered "placula" (b,c). Cells of the upper layer form the ectoderm, while cells of the lower layer (orange) adopt a nutritive function and later invaginate to form the entoderm (d-g)
Trichoplax represents an intermediate grade of organism, one which has learned the trick of maintaining multiple cell types in specific roles and how to organize them into a spatially patterned relationship, but has not established any other kind of polarity—it doesn't know its head from its ass, and actually has neither one of those sophisticated features. At this point, it's an intensely mysterious organism, and there are many things we simply don't know about it. That is one of the many provocative things about this animal, things that draw scientists to it in fascination.
- We don't know how many species of Trichoplax there are. It lacks distinguishing morphological characters, obviously enough, and molecular/genetic tests of animals collected in the Red Sea and the Sea of Japan have found them to be indistinguishable. A single species seems to have a world-wide distribution, yet other analyses have found distinct differences between different strains.
- We know nothing of their ecology. Samples are collected and grown in the lab for study, but what exactly these nearly invisible animals do in the wild is unknown.
- As a developmental biologist, I'm most puzzled by their modes of reproduction. In the lab, they are known to reproduce solely by vegetative division—the animal just splits in two, and two placozoans crawl away. However, they've also been known to produce motile sperm and eggs, which fail to produce viable offspring in the lab…they just die. Do they reproduce sexually only under certain very specific conditions in the wild, or is sexual reproduction a fading vestige of a primitive condition they are in the process of abandoning?
Where I find these creatures particularly interesting, though, is how they may help answer questions about evolution and development.
There is a concept in evolution that many people find hard to grasp: that is, new genes and new functions do not evolve out of any "need" for a new property. Instead, genes that have one role may be coopted into a new role by fortuitous ancillary properties. For instance, the set of genes that are involved in early longitudinal patterning in us chordates, that is, that help define which end of an embryo should form a head and which end should form a tail, are the Hox genes. It would be a mistake to argue that these genes arose in order to specify the head end of an animal, though—the first animals to have these genes would not have had a head! That primordial Hox-like gene must have had some other function, a function which could be easily modified by chance to localize to just one end of the animal, an event which would have been crucial in defining anterior-posterior polarity and turning a blob into a worm.
What would that function have been? How can we find out?
The strategy is to look for animals that do not form heads, but do have Hox-like genes, and ask what they use them for…animals like Trichoplax. They do have a gene called Trox-2 which is in the Hox gene family, and we can examine where it is being expressed in Trichoplax. It is expressed in scattered cells around the margin of the animal, seen as the pink dots in A, below:
The study of regulatory genes reveals insights into basic and basal mechanisms of metazoan development and evolution. Most regulatory genes studied so far are expressed within or close to a small region of potentially undifferentiated cells embedded between the lower and the upper epithelium. A,B: Trichoplax whole mount in situ hybridization for the putative ProtoHox/ParaHox gene, Trox-2; note the strong and homogenous expression close to the body margin; arrows in B point to small undifferentiated—yet undescribed—cells between the lower and upper epithelium. C,D: Trichoplax whole mount in situ hybridization for the putative ProtoPax gene, TriPaxB; note the more spotted expression along the body margin; the arrow in D points to a small TriPaxB-expressing cell that is similar to cells expressing the Trox-2 gene.
The gene seems to be active in a pool of undifferentiated cells, which are not localized to one end, but are instead found in a ring. It is a kind of spatial localization; a change in gene regulation that involved a loss of expression over a part of its range would lead to an asymmetry that could be a precursor to defining an anterior-posterior axis.
It's more than just a single gene. Other genes similar to patterning genes found in more complex metazoans have also been identified: the gene Pax B is illustrated above and is also present in a ring, and genes related to Not, the T-Box class, and antp have been found. There is a whole network of regulatory gene interactions present in these marginal cells, a network that was a precursor to the more restricted networks we possess that are responsible for giving us a head and a more sophisticated form than a disc-like blob.
This represents another successful evolutionary prediction. What we expect to find in biology is that the networks of genes responsible for specific morphological features in complex organisms will also be present in simpler forms, but with broader, more general, and sometimes rather different functions. With Trichoplax, we're identifying the evolutionary foundations of some of the most basic features of our embryonic organization—features so primitive that we take them entirely for granted.
Schierwater B (2005) My favorite animal, Trichoplax adhaerens. BioEssays 27:1294-1302.
Science • EvoDevo • Organisms • 2 Trackbacks • Other weblogs • Permalink
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Fascinating! An organism that has evolved to occupy a local evolutionary maximum (a 'Hill improbable'), and whose genotype has no easy route to further development, or an organism that is an intermediate step to something more complex? (I'm not a biologist, as may be evident to those who are from my first sentence!) Presumably something like this does not fossilize, so we have no idea how far back this goes?
#: Posted by on 11/27 at 06:36 PM
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Here's a guy that cultures Trichoplax in acquaria: http://tinyurl.com/dc74y
There's a Photoshopped "artist's impression" there. The guy also talks about video he took, but there's none on the site.#: Posted by on 11/27 at 07:16 PM -
is sexual reproduction a fading vestige of a primitive condition they are in the process of abandoning?
I know I'm far too vertebro-centric for my own good, but I don't know how far back on the phylogenetic tree sperm and eggs go. I know plants reproduce sexually, but is that considered sperm and eggs? If not, where do they first show up in animals? From the squid and spider porn, I understand that inverts have them, but if this four-celled non-polar beastie is our side of the primitive condition, they must extend *way* back there. - oops--of course, that's four cell *types*, not four cells total.
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not having any taxonomic diversity within the group to help estabish a phylogeny, and without having been given any phylogenetic comparison with other groups, it is not possible to ascertain whether this is a 'primitive' or a 'degenerate' organism.
#: Posted by on 11/27 at 08:10 PM
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Perhaps the gametes originally evolved for a different, non-reproductive purpose, maybe as excretory vacuoles, and were only later co-opted into reproductive functions?
Of course, meiosis and sexual reproduction seem to predate the origin of animals, so maybe there's just a missing element in trichoplax's sexual reproduction that can't or hasn't been reproduced in the lab...has anyone just tried playing some Barry White?#: Posted by on 11/27 at 08:24 PM -
I was bad and failed to include the phylogenetic comparison. Here:
Unexpected diversity has been found in the phylum Placozoa, formerly assumed to be monotypic. Shown is a MP phylogram of placozoans based on combined SSU and LSU data. The sampling has been very limited so far and in order to unravel biodiversity of this unique phylum, more and extended studies are urgently needed.
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Placozoans are more closely related to Ctenophores than Cnidarians? I would have thought Placozoa and Cnidarians would be the closest relatives. Neato.
#: Posted by Mike the Mad Biologist on 11/27 at 09:14 PM
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another point/question:
how big are they?#: Posted by on 11/27 at 10:57 PM -
Oh wow! So there is now a real literature on the Placozoa? I've always wanted to know more about these guys, and especially about what they could tell us about the evolution of patterning and the nervous system (that they don't have). Is there a genome project for the phylum, or some central repository of sequence data outside of Genbank for these guys? Any recent reviews you could recommend? Thanks for giving them more and better press.
Well, back to grading term papers...#: Posted by on 11/27 at 11:06 PM - Actually, there is evidence to suggest that the ancestors of Placozoa were more complex, and that their simplicity is the result of reductive evolution. See here.
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This is the kind of thing that keeps me coming back here for more. Thanks, PZ!
#: Posted by on 11/28 at 01:38 AM
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Fascinating! I won't pretend to have understood some of the lingo and the genetic talk, but fascinating all the same. And I never thought of an ass as a "sophisticated feature." I'll never look at a butt the same way again! *LOL*
#: Posted by Bourgeois Nerd on 11/28 at 01:51 AM
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teach the controversy!! (sorry, just had to say that!)
here's another site that proposes 3 different phylogenetic positions.
http://www.ucmp.berkeley.edu/phyla/placozoa/placozoa.html
the evidence (latest citation is 1995) suggests that they are a "degenerate" group, possibly even a sister group to the bilaterians (their phylogeny C).
one fascinating observation deals with their attempts at sexual reproduction: they get only as far as the 64-cell stage, then the cells stop dividing while the DNA continues to do so, until the nucleus bursts! surely this is not an ancestral condition!
returning to the original posting. re: pattern of expression of the HOX ana/homo/paralog, it seems to me that there is an actual gradient of expression: top-to-bottom (fig B). the apparent concentration around the edges (Fig A) seems to be an artifact of the creature getting thinner near the edges.
ecological speculation: the site i gave earlier says that the species can be maintained in culture by feeding it chlorella and some other algae. from this and their tendency to adhere to surfaces, i suspect that they will be found on 'leaves' of large algae.#: Posted by on 11/28 at 02:12 AM -
Don't even amobeae have Hox genes? (you know, the kind which can congregate into a meiosis-capable slug in time of food shortage)
And one question just occured to me : have Hox genes even been looked for in simple, down-to-earth protozoan? -
they get only as far as the 64-cell stage, then the cells stop dividing while the DNA continues to do so, until the nucleus bursts! surely this is not an ancestral condition!
No, sounds like "intelligent design" to me.#: Posted by on 11/28 at 03:34 AM -
The article Krauze linked doesn't actually seem that bad, except for the requisite discussion of `Darwinists' in the conclusion.
Of course, none of the paper cited were written by ID advocates, so it's obviously another example of the self-correcting nature of evolutionary biology and science in general. If the mistake hadn't been found by `Darwinists', you can bet your ass that it wouldn't have been found by ID proponents as they don't do any research.
So sentences like "In neither of these cases did Darwinism raise any doubts about the result" seem a little odd.
Got that scientific theory of ID, Krauze? You know, the falsifiable one with any testable predictions?#: Posted by on 11/28 at 04:41 AM -
Abie: Ahh, the amoebozoa... yet another experiment with muticellularity. The group includes yer classic unicellular Amoeba and a couple of derived groups, the slime moulds, which includes the one you are referring to. http://en.wikipedia.org/wiki/Slime_mould
The congregant chemical is cAMP.
http://universe-review.ca/R10-18-slimemoulds.htm (whether this is somehow related to activity of the HOX-like complex i don't know.)
Ahh, the things we miss, being large vertebrates! I've only encountered them as brownish incipient stalactites on my bathroom ceiling...
p.s. please don't might mistake me for an authority on this subject... this info is all the result of 5 minutes of websearch.#: Posted by on 11/28 at 04:58 AM -
No, sounds like "intelligent design" to me.
you're onto something, Zilch!
I propose that Pharyngularians collect a database of truly "unintelligent design" for use as a counteragument to the ID camp!
Each entry must have at least one recent valid citation.
Whaddaya say PZ?#: Posted by on 11/28 at 05:09 AM -
Hi DrFrank,
Considering that PZ likens hearing IDists talk about ID to having "your knuckles smashed with a ball-peen hammer", I'd better refrain from commenting on your post. If you're interested in discussing ID with me, you're welcome on Telic Thoughts. -
Krauze is doing what is known as "framing." In the simplest form, the essay is another whine about "Darwinism wrong, therefore ID right." The complaint Krauze makes actually goes well beyond "Darwinism." Scientists feel momentum in whatever field they work with, sometimes pushing them to erroneous conclusions. For instance, early physicists trying to figure out the nature of light had much inertia in classifying it either as a particle (like most solid matter), or as a wave (like ripples in water). Why? Because most observable phenomena in those days could be visibly classified as either. Scientific momentum. There is nothing impossible in principle about light being either particulate or either a wave. But scientists proposing such models were wrong. How did they figure it out? Certainly not ID.
Therefore, intelligent design, in striving to produce a historical narrative of life, is not going to be cut down by people raising philosophical objections.
Pathetic, wishful thinking. Until IDiots propose something more concrete (i.e. testable) about their Designer(s), they will never rise above the level of in principle philosophy, and will always be cut down most readily in that fashion. This is the real irony in Krauze's article. He asserts that Darwinists were wrong, without realizing that the key point was that Darwinists was able to figure out they were wrong. What did ID do help Darwinists realize their mistake, in a non-philosophical manner?
Case in point. Will Krauze stake the claim that Placozoa are derived, and not ancestral, as an ID prediction? How does he propose to test this hypothesis, and control for evolutionary mechanisms?#: Posted by on 11/28 at 07:45 AM -
Talk science at Telic Thoughts? Hah.
See http://www.pandasthumb.org/archives/2005/08/the_reaction_to.html#comment-41083#: Posted by on 11/28 at 07:50 AM -
Oops, that was the other smack down of Krauze.
This is the one that is relevant:
http://www.pandasthumb.org/archives/2005/08/the_reaction_to.html#comment-41183#: Posted by on 11/28 at 07:53 AM -
Science is claiming the debate about whether the placozoa are derived or ancestral as valid, and if you think about it, either alternative is still an evolutionary explanation. Science also proposes methods for evaluating the hypotheses, and a number of labs are working on the problem right now.
ID isn't even part of the argument, let alone participating in the solution. -
Rather than loss of expression on some part of the ring leading to assymettry, how about invagination of the ventral 'endoderm' with a ring around the resultant oral/anal pore?
#: Posted by on 11/28 at 08:22 AM
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Thanks for the offer, Krauze, but after reading Behe and Dembski I'd really have to agree with PZ's ball-peen hammer assessment
In summary, evolutionary biologists posed a hypothesis that the flagellum evolved from the TTSS. They performed a detailed investigation, and concluded that evidence suggested that this was not the case, thus making scientific progress in our understanding of the origin of the flagellum. In light of the new evidence, more informed hypotheses will be formed and investigated in future.
This is in stark contrast to the ID proponents which, despite the enormous amount of funding the DI gets (which goes on PR), have done no experiments at all. Behe hasn't even bothered performing the one (deeply, deeply flawed) experiment that he suggested.
Odd that, isn't it?#: Posted by on 11/28 at 08:28 AM -
The website that RBH linked in the second comment includes an intriguing description of the results of dissociating the cells in a Trichplax. Apparently the cells re-associated, but the new Trichoplax were mixtures of more than one old Trichoplax. That raises the question of how the four cell types find their way back to the right place in the new Trichoplax. Or, are they capable of re-differentiating?
#: Posted by on 11/28 at 08:34 AM
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Do the cell types in this animal form really small tissues, or are they just scattered?
#: Posted by Keith Douglas on 11/28 at 08:56 AM
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This reminds me of an evolutionary question that I've had for some time. Are there any well-documented examples of unicellular organisms that descended from a multicellular ancestor?
HeLa cells could be one example. (Some have apparently proposed they be considered a new species - Helacyton gartleri. Is that a well-established view among those in the know?)
I'd be more interested in any cases that appear to have evolved without human intervention.#: Posted by on 11/28 at 09:19 AM - Myxozoa.
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Hmm. Your prior article speaks of "small motile presumptive sperm cells", and "ova" that divide without being fertilized. Have these been verified as haploid? What about X/Y (or even W/Z) sex chromosomes? For that matter, how certain are you that each individual "plate" is genetically homogenous?
I'd like to see comparisions, on the one hand to any diploid unicellular form (DUF) that produces (functional) gendered gametes, and on the other to a DUF that produces homogenous haploid spores.#: Posted by on 11/28 at 12:19 PM -
I drop in to inform PZ of some of the "self-correcting evolutionary biology and science" that he'd missed. In my comment, I didn't mention ID with a word. Not one word.
Now, Wad of ID, referring to it as a "smack down", wants me to revisit some old discussion about ID, in which he consistently failed to adress any of my points. What's the matter, Wad? Don't have any comments to the question of Placozoa phylogenetics? You know, one of the topics actually raised by PZ's post.