PZ Myers posted Entry 2296 on May 22, 2006 03:01 PM.
Trackback URL: http://www.pandasthumb.org/cgi-bin/mt/mt-tb.fcgi/2291

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I'm going to briefly summarize an interesting new article on cnidarian Hox genes…unfortunately, it requires a bit of background to put it in context, so bear with me for a moment.

First you need to understand what Hox genes are. They are transcription factors that use a particular DNA binding motif (called a homeobox), and they are found in clusters and expressed colinearly. What that means is that you find the Hox genes that are essential for specifying positional information along the length of the body in a group on a chromosome, and they are organized in order on the chromosome in the same order that they are turned on from front to back along the body axis. Hox genes are not the only genes that are important in this process, of course; animals also use another class of regulatory genes, the Wnt genes, to regulate development, for instance.

A gene can only be called a Hox gene sensu stricto if it has a homeobox sequence, is homologous to other known Hox genes, and is organized in a colinear cluster. If such a gene is not in a cluster, it is demoted and called simply a Hox-like gene.

Hox genes originated early in animal evolution. Genes containing a homeobox are older still, and are found in plants and animals, but the particular genes of the Hox system are unique to multicellular animals, and that key organization arrangement of the set of Hox genes in a cluster is more unique still. The question is exactly when the clusters arose, shortly after or sometime before the diversification of animals.

If you take a look at animal phylogeny, an important group are the diploblastic phyla, the cnidarians and ctenophores. They branched off early from the metazoan lineage, and they possess some sophisticated patterns of differentiation along the body axis. We know they have homeobox containing genes that are related to the ones used in patterning the bodies of us vertebrates, but are they organized in the same way? Did the cnidaria have Hox clusters, suggesting that the clustered Hox genes were a very early event in evolution, or do they lack them and therefore evolved an independent set of mechanisms for specifying positional information along the body axis?

Continue reading "Jellyfish lack true Hox genes!" (on Pharyngula)

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Comment #101791

Posted by Todd on May 22, 2006 5:13 PM (e)

Gene rearrangement is relatively common, why couldn’t the development of the Hox system evolve before the split but, for whatever reason, get re-organized in the cnidarian phylum? Perhaps they “found” a way around whatever made the system so critical for everyone else, or maybe their basic body stucture made it irrelevant, or maybe something random happened that meant they didn’t need it.

This relate back to the recent entry regarding the evolution of the three domains. It was assumed that the more complex and more sophisticated eukaryotes evolved later, the whole “progression of evolution” idea that we know isn’t really valid. This seems to have the same basic flaw, it appears to be assuming that evolution progressed towards the Hox gene system. Why are they so sure this is the case? Why is it so impossible for an organism to lose the Hox gene system? Once the requirement of having the basic organizational structure central to the Hox gene system is gone, then normal chrosome re-ordering would take over and you would end up with the variety of different gene order seen in the article.

Of course my knowledge of the Hox gene system is pretty superficial. It may very well not be an assumption, it may very well be that it is impossible to lose the Hox gene system once it is established, but this seems to be an unstated assumption in the article so if it is certain it should be addressed.

Comment #101797

Posted by Torbjörn Larsson on May 22, 2006 5:52 PM (e)

Todd,

My knowledge about Hox is what I snapped up on Pharyngula lately. But it seems to me the article handles the difference of loosing the Hox cluster structure and not having developed it fully.

It says:
“The situation in cnidarians is therefore very different to that even in very derived members of the Bilateria. For example, whereas in urochordates the ancestral Hox cluster has fragmented, the individual genes show high levels of sequence identity and similar (A/P-restricted) patterns of expression to their orthologs in other bilaterians. In cnidarians, not only are the genes dispersed, but also there are no clear relationships in terms of expression patterns or sequence identity.”

OTOH in the post http://scienceblogs.com/pharyngula/2006/05/hox_genesis.php
the first figure seems to indicate loss between sponges and cnidarians. But instead it goes on to discuss animals that have the Hox clusters broken because they develop so fast that they have to use it differently. It refers to http://pharyngula.org/index/weblog/comments/hox_cluster_disintegration/
There they find that while a cluster breaks up it still keeps the expression patterns. It is the temporal ordering that is lost.

Perhaps your idea is valid too, but it would mean a more radical and different change than the one usually seen. At least that is what I think these posts mean.

Comment #101805

Posted by Steviepinhead on May 22, 2006 6:46 PM (e)

I have very much enjoyed PZ’s series of posts about the HOX gene complex.

That being said, I think both Todd’s question and Torbjorn’s response are thought-provoking. Perhaps when he gets a chance, PZ will favor us with his view.

Or maybe, as occasionally happens here, we’ll be lucky enough to have one of the paper’s actual authors will chime in…!

Comment #101812

Posted by the pro from dover on May 22, 2006 7:21 PM (e)

I was under the opinion that ctenophores differed from cniderians because of the presence of a third layer; perhaps not a true mesoderm but not diploblastic in the same sense that cniderians are. Is this a “black pearl?”

Comment #103112

Posted by Henry J on May 30, 2006 10:43 AM (e)

But it’s still just a jellyfish!