The Hox code


The Hox genes are a set of transcription factors that exhibit an unusual property: they provide a glimpse of one way that gene expression is translated into metazoan morphology. For the most part, the genome seems to be a welter of various genes scattered about almost randomly, with no order present in their arrangement on a chromosome — the order only becomes apparent in their expression through the process of development. The Hox genes, on the other hand, seem like an island of comprehensible structure. These are all genes that specify segment identity — whether a segment of the embryo should form part of the head, thorax, or abdomen, for instance — and they're all clustered together in one (usually) tidy spot.

How do these genes work together to regulate axial patterning in chordates?

Continue reading "The Hox code" (on Pharyngula)


PZ, no, chordates do not have four clusters, only vertebrates do as far as I know. Other chordates like amphioxus have a single cluster, right?


Nice post. We can always count on you to keep us up to date on the most ground-breaking evo/devo research.

It seems to me that the redundancy in the vertebrate HOX system might produce a sort of evolutionary inertia that would make it more difficult for different types of body plans to evolve in vertbrate as compared to arthropod lineages. That might partially account for the vast diversity of arthropod body plans versus the more conservative vertebrate plans. Of course, the increased complexity of the genetic regulatory mechanisms might make it possible for more complex structures and pathways to eventually arise in vertebrates. Or maybe I’m just bieng way too simplistic here. IN any event, this is really interesting stuff.

At a guess, the internal skeletons of vertebrates might also make major layout changes more difficult, and slower when they do occur, since major systems may be more tied to particular parts of the skeletal structure than they are in something with an external skeleton.


That’s better

Enjoyed the summery. I wrote my undergrad thesis on Hox genes. I would also like to point out that, yes, mammals at least have four clusters (and, if my memory serves, amphioxus has only one), but there is evidence that bony fish (after the split with tetrapods of course) may have undergone an additional hox cluster duplication and have eight clusters, followed by the loss of function of numerous genes…my thesis and copies of all my cited papers are currently in a box in my parent’s garage so I can’t cite the particular study right now. It is also important to remember that Hox is only the most studied of developmental gene clusters…there are more and we are only beginning to understand them. With the gain and loss of regulatory gene function, in addition to copying and co-option of other genes, there is plenty of material for variation aside from traditional frame-shift, point, and other mutations that affect the coding regions.

D’oh…i saved my thesis on my computer, so i had the digitalized version. Most of it read like crap. Typos througout. Wish I had a couple extra days to work on it (eh, I would have wasted them anyway). Parts of it did read good though…retaught myself on many fine points. Thanks PZ for reminding me what I got into biology for in the first place.

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