PZ Myers. 2004 May 28. Bilateral symmetry in a sea anemone. <http://pharyngula.org/index/weblog/bilateral_symmetry_in_a_sea_anemone/>. Accessed 2008 Dec 04.
Posted on M00o93H7pQ09L8X1t49cHY01Z5j4TT91fGfr on Friday, May 28, 2004
Bilateral symmetry in a sea anemone
There are quite a few genes that are known to be highly conserved in both sequence and function in animals. Among these are the various Hox genes, which are expressed in an ordered pattern along the length of the organism and which define positional information along the anterior-posterior axis; and another is decapentaplegic (dpp) which is one of several conserved genes that define the dorsal-ventral axis. Together, these sets of genes establish the front-back and top-bottom axes of the animal, which in turn establishes bilaterality—this specifically laid out three-dimensional organization is a hallmark of the lineage Bilateria, to which we and 99% of all the other modern animal species belong.
There are some animals that don't belong to the Bilateria, though: members of the phylum Cnidaria, the jellyfish, hydra, sea anemones, and corals, which are typically radially symmetric. A few cnidarian species exhibit bilateral symmetry, though, and Finnerty et al. (2004) ask a simple question: have those few species secondarily reinvented a mechanism for generating bilateral symmetry (so that this would be an example of convergent evolution), or do they use homologous mechanisms, that is, the combination of Hox genes for A-P patterning and dpp for D-V patterning? The answer is that this is almost certainly an example of homology—the same genes are being used.
What Finnerty et al. did was to search for and stain for Hox and dpp genes in a cnidarian with bilateral symmetry, Nematostella vectensis. Here is their summary diagram:
Summary of Hox and TGFβ gene expression. (A) Provisional homology of Nematostella Hox genes based on phylogenetic analysis of homeodomains. Vertebrate Hox paralogs are numbered from 1 to 13. Arthropod Hox paralogs are named with Drosophila gene terminology (lab, labial; pb, proboscipedia; zen, zerknullt; Dfd, Deformed; scr, sex combs reduced; ftz, fushi tarazu; Antp, Antennapedia; Ubx, Ultrabithorax; abd-A, abdominalA; and AbdB, AbdominalB). (B) Gene expression along the oral-aboral and directive axes. The germ layer composition of Nematostella is shown in longitudinal section. To simplify the depiction of the primary body axis, the pharynx has been drawn as though everted. The mesenteries are not shown. Collectively, the five Hox expression domains span practically the entire oral-aboral axis. Anthox1a, anthox7, and anthox8 are restricted to one side of the directive axis. Likewise, both TGFβ genes, dpp and GDF5-like, exhibit asymmetric expression about the directive axis. Only the asymmetric aspects of their expression are shown. Dpp is expressed in the pharyngeal ectoderm on the side of the directive axis opposite the sector expressing anthox1a, anthox7, and anthox8. GDF5-like is expressed in the endoderm on the same side as anthox1a, anthox7, and anthox8.
Nematostella has perfectly good Hox genes that are expressed in a staggered anterior-posterior pattern. It's not quite as tidy as the vertebrate or athropod pattern—there's a lot of overlap, as you can see—but it's good enough to see the canonical Hox arrangement. Dpp and another gene in the same family, GDF5-like also show the typical metazoan asymmetry. That some members of the Cnidaria exhibit precisely the same molecular organization to their body plan suggests that the function for these molecules arose early in the origin of multicellular animals, and that the radially symmetrical cnidarians have secondarily lost them.
The data summarized here suggest that bilateral symmetry evolved before the split between Cnidaria and Bilateria. Both taxa exhibit bilateral symmetry. Both taxa exhibit staggered Hox expression domains along the primary body axis and asymmetric dpp expression along the secondary body axis. Homology is the most parsimonious explanation for the shared possession of these morphological and molecular traits. If we invoke homoplasy as an explanation, we must presume that one or both of these complex axial patterning systems evolved convergently in two independent evolutionary lineages.
Finnerty JR, Pang K, Burton P, Paulson D, Martindale MQ (2004) Origins of bilateral symmetry: Hox and dpp expression in a sea anemone. Science 304:1335-1337.
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Coooool.
But cnidaria aren't the only phylum with radial symmetry. Has anyone done the echinoderms yet?
'coz if the echinoderms also have Hox, then that means that /radial/ symmetry is the independent (and convergent) characteristic.
Doug M.#: Posted by on 05/29 at 03:37 PM - Yes -- echinoderms have Hox genes. So do molluscs.
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Then it's radial symmetry that has independently evolved?
(I won't even ask about the sponges.)
Doug M.#: Posted by on 05/29 at 10:07 PM -
Yes; as I recall from my animal physiology class, echinoderms start out with bilateral symmetry during embryological development and then undergo a transformation to obtain pentamerous (five-fold) radial symmetry.
See some of the pics on:
http://www.microscopy-uk.org.uk/mag/indexmag.html?http://www.microscopy-uk.org.uk/mag/artjul00/echino4.html
http://www.microscopy-uk.org.uk/mag/indexmag.html?http://www.microscopy-uk.org.uk/mag/art98/janstar.html
http://www.uoguelph.ca/zoology/devobio/210labs/gastrulation1.html
CK#: Posted by on 05/31 at 03:46 AM -
Wow, great site. Thanks, Conrad.
I'm still wondering, though: if radial symmetry evolved independently in echinoderms and cnidaria, are different genetic mechanisms involved?
For starfish, at least, the morphological development seems pretty funky: "swim around as a bilateral larva for a while, then grow a new, pentameral body on my butt".
-- echinoderms are deuterostomes, like us. I remember learning, way back when, that deuterostomes are all more closely related to each other than proterostomes. So echinoderms and chordates are on one branch of the clade, molluscs and arthropods and cnidarians on another. That would make us expect a different mechanism...?
No chordate has radial symmetry, though most chordates are marine, and plenty are sessile or bottom-creepers (tunicates, lancelet worms). I wonder why radial symmetry never evolved in any of these groups -- tunicates, at least, look like they're trying for it.
Doug M.#: Posted by on 05/31 at 01:29 PM -
You're a little off on the phylogeny, so let me straighten that out before we continue. Here's a simplified version of the animal 'family tree', with several major taxa omitted for clarity:
^
|----- Porifera (sponges)
|---- Cnidaria (jellyfish, etc.)
|--- Platyhelminthes (flatworms, etc.)
|-- Coelomates (see notes below)
</pre>
[I hope that looks ok and isn't too hard to interpret; basically, it shows the branching; i.e. sponges divided off first, so they are the most 'primitive'. The ^ is the origin of animals.]
The coelomates have two branches (as you correctly noted)
the Deuterostomes [which have subdivisions of (among others) Echinodermata (starfish, etc.) and Chordata (fish, etc.)]
and the Protostomes [Mollusca (squid, etc.) and {Annelida (earthworms, etc.) and Arthropoda (spiders, etc.)}]
--
Now, while many sponges are irregular or branching, some are in fact radially symmetrical; this may be an example of radial symmetry evolving independently though.
In any event, radial symmetry is the norm in cnidarians and (referencing the original article mentioned in this thread), it appears that in bilateral symmetry evolved and was subsequently passed on to other animals which ran with it (well, moved slowly at first anyway ;)
Only much later did the ancestral echinoderm abandon it (in its adult stage) for whatever reason (possibly to do with a change in feeding strategy). The underlying genetic mechanisms are probably thus significantly different (compare and contrast with vertebrate vs. arthropod legs)
As for 'why don't tunicates revert to radial symmetry?' note that bilateral symmetry is still very important for their larval form:
And even as adults (like these-
) the one-way flow of water means that they probably need to maintain bilateral symmetry.
#: Posted by on 06/01 at 02:40 PM -
Addendum: this got cut off
the Protostomes [Mollusca (squid, etc.) and [Annelida (earthworms, etc.) and Arthropoda (spiders, etc.)]]#: Posted by on 06/01 at 02:42 PM