Evolution of median fins

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Often, as I've looked at my embryonic zebrafish, I've noticed their prominent median fins. You can see them in this image, although it really doesn't do them justice—they're thin, membranous folds that make the tail paddle-shaped.

zfish_sm.jpg

These midline fins are everywhere in fish—lampreys have them, sharks have them, teleosts have them, and we've got traces of them in the fossil record. Midline fins are more common and more primitive, yet usually its the paired fins, the pelvic and pectoral fins, that get all the attention, because they are cousins to our paired limbs…and of course, we completely lack any midline fins. A story is beginning to emerge, though, that shows that midline fin development and evolution is a wonderful example of a general principle: modularity and the reuse of hierarchies of genes.

Continue reading "Evolution of median fins" (on Pharyngula)

3 Comments

Thanks, PZ, another good one.

The only teensy drawback–for this pinhead, maybe not for others!–was the labelling and coloring of the first figure. It would have been clearer–again, for me!–had the label and tissue colors been more distinctive across tissues and more consistent within them.

For example, in my (admittedly-retarded) browser, the ectoderm tissue layer shows up as a pinkish tone (and likewise the label), but the ectoderm-derivatives of the neural tube and neural crests are shown in the same orange-red as the mesoderm.

The remaining mesodermal tissue is then consistently shown in the same orange-red (ignoring the overlap with the ectodermal derivatives noted above), but only until we get out to the extraembryonic mesoderm, which shows up in yet another, lighter shade of orange.

All becomes clear as one works through the text, the succeeding graphics, and the linked graphics and animations, but the initial illustration did stop me in my tracks for a bit while I went back and forth between colors, labels, and text, sorting things out which seemingly could have been clear from the get-go with a more straightforward application of colors.

Don’t the homologies shared by the limb- (or extrusion-) forming gene-modules reach deeper into time than the fins of sharks and lampreys, at least as far back as the common ancestors of arthropods and vertebrates? That is, while the limbs/fins/gills/wings of insects and vertebrates may not be homologous themselves, hasn’t the underlying bump-extruding gene-module itself been shown to be homologous?

It’ll also be interesting to learn how this whole gene-module is triggered or suppressed at given locations in the first place. That is, if it indeed should turn out that the killerwhale dorsal fin, for example, is yet another expression or deployment of this “same” set of genes (even though the fin itself would again not be directly homologous with the midline fins of fish), then what upstream (?) regulatory elements have allowed that novel deployment in relatively recent times, but somehow suppressed it (and all other novel extrusions in novel locations) for lo these many eras.

Though, thinking back in time, there have certainly been midline fin-like “crests” and the like in some of the early reptile and dinosaur clades, as well. Hmmm.

That diagram was taken from a site linked to in the article – it broke the rules! Ectoderm is supposed to be blue or green, mesoderm red, and endoderm yellow! I let it slide, though, since it’s mainly illustrating the proximo-distal distribution of mesodermal derivatives.

They haven’t yet identified the initial trigger for the initiation of a ‘bump’. It’s probably a unique combination of transcription factors.

Or maybe it’s Jesus!

Well, I had heard of “Jumpin’ Jesus Jehosophat,” or something to that effect.

In light of your insight, it now seems likely that that was a mere dialectical variant on “Bumpin’ Jesus J.”!

I’m visualizing these heavily-modified cars in certain neighborhoods of L.A., “bumpin’” up and down, while dangling crucifixes dance and jangle beneath their rear-view mirrors.

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This page contains a single entry by PZ Myers published on August 3, 2006 6:27 AM.

Standards and the Teachers who Need Them was the previous entry in this blog.

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