How an ancient vertebrate uses familiar tools to build a strange-looking head

September 14, 2014
The morphology of an adult sea lamprey (top); the ventral view of the unique oral disc, adapted for a parasitic lifestyle (bottom left); and a transient transgenic lamprey embryo (bottom right). Credit: Krumlauf Lab, Stowers Institute for Medical Research and Bronner Lab, California Institute of Technology

If you never understood what "ontogeny recapitulates phylogeny" meant in high school, don't worry: biologists no longer think that an animal's "ontogeny", that is, its embryonic development, replays its entire evolutionary history. Instead, the new way to figure out how animals evolved is to compare regulatory networks that control gene expression patterns, particularly embryonic ones, across species. An elegant study published in the September 14, 2014 advance online issue of Nature from the Stowers Institute for Medical Research shows just how humbling and exhilarating that pursuit can be.

In the study, Investigator and Scientific Director Robb Krumlauf, Ph.D. and colleagues show that the sea lamprey Petromyzon marinus, a survivor of ancient jawless vertebrates, exhibits a pattern of that is reminiscent of its jawed cousins, who evolved much, much later. Those genes, called Hox genes, function like a molecular ruler, determining where along the anterior-posterior (AP) axis an animal will place a particular feature or appendage. The new study means that that the genetic program used by jawed vertebrates, including fish, mice, and us, was up and running ages before a vertebrate ever possessed a recognizable face.

"Hox genes regulate a tissue's character or shape, as in head or facial features. Our work in the past has addressed how factors make unique structures, for example, what makes an arm different from a leg," says Krumlauf. "Now, we are excited by the common role that similar sets of genes play in creating a basic structural plan."

The team at Stowers, collaborating with Marianne Bronner, Ph.D., professor of biology at Caltech, focused on the because the fossil record shows that its ancestors emerged from Cambrian silt approximately 500 million years ago, 100 million years before jawed fish ever swam onto the scene. The question was, could the hindbrain that constructs the "modern" vertebrate head have originated in animals that lack those structures?

To answer it, the researchers created so-called "reporter" genes from stretches of regulatory DNA flanking a specific Hox gene in zebrafish or mice and linked them to fluorescent tags. When inserted into an experimental animal these types of reporters glow in tissues where the gene is activated, or "expressed". The researchers chose this particular battery of Hox reporters because when inserted in embryos of a jawed fish they fluoresce in adjacent rainbow stripes up and down the embryonic hindbrain.

The paper's startling finding came when they inserted the very same reporters into lamprey embryos using a technique developed by Hugo Parker, Ph.D., a postdoctoral fellow in the Krumlauf lab and the study's first author: the lamprey embryos displayed the same rainbow pattern of Hox reporters as did jawed fish, in exactly the same order along the AP axis of the hindbrain.

"We were surprised to see any reporter expression in lamprey, much less a pattern that resembles the pattern in a mouse or fish," says Parker, who pioneered the lamprey reporter approach as a graduate student at London's Queen Mary University. "That means that the gene regulatory network that governs segmental patterning of the hindbrain likely evolved prior to divergence of ."

Researchers knew that in mouse and zebrafish short stretches of DNA in one Hox reporter (Hoxb3) formed a landing pad recognized by a DNA-binding protein that flips on the gene. As you would expect, when inserted into zebrafish embryos, reporters mutant in those sequences were inactive (they didn't glow) in the hindbrain. Remarkably, the mutant reporter was inactive in lamprey embryos also, meaning that this control switch has been around for a very long time.

"These results suggest that regulatory circuits controlling hindbrain patterning were likely 'fixed in place' in ancient vertebrates," says Bronner. Some may find this surprising, as adult mammals (like us!) bear absolutely no resemblance to lampreys. "However, embryos of lamprey and other vertebrates show many striking similarities, so it makes sense that there are features common to all."

Explore further: Study of skates and sharks questions assumptions about 'essential' genes

More information: A Hox regulatory network of hindbrain segmentation is conserved to the base of vertebrates, Nature, DOI: 10.1038/nature13723

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7 comments

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RhoidSlayer
1 / 5 (1) Sep 14, 2014
ontogeny recapitulates ontology
little men still arguing strawmen
"it's entire evolutionary history"

as if you can build math , language , or mind without the required precursor logic
Torbjorn_Larsson_OM
5 / 5 (1) Sep 15, 2014
Nice to see yet another deep time phylogeny, testifying to how evolution forms the basis of modern biology.

[Also nice to see the creationist trolls flailing harder and harder as they too perceive that the gaps to push their magic into is _gone_, here to impute magic action ('design') where there is none.

As if there are any peer reviewed results whatsoever showing that math, language or minds are based on some form of 'precursor logic', to put against the many, many thousands of results showing the exact opposite. Say how Gödel shows that in math you have to choose between consistency or incompleteness for each and every system _you_ construct out of its usefulness in the current science and technology environment. "Put up or shut up."]
bruce36b
not rated yet Sep 15, 2014
I am surprised that the scientist are surprised about the traces of earlier DNA/RNA/Switches etc. If they remember the basics of evolution, all life forms are even still evolving today. This is because the basic A-T and G-C bonds are not that strong and can easily break with just other molecular attractions, particle radiation from space and earth objects. Also there is re-bonding with slightly different molecules,etc. Just because there are these continuing changes, does not mean immediate results for survival and how organisms look. For humans, it looks like a 2 million year gap where nothing of significance seemed to be happening to our ancestors, but in that time, minute changes allowed our brains to transfer heat to the outside, allowing for more complexity inside. When we were single cells duplicating ourselves, we couldn't keep the new copies attached because we could not make collagen. There was only 4 % O2 around and we needed to wait another 400,000 years for 19% O2.
collideronline
not rated yet Sep 15, 2014
I'm curious if anyone knows if there is a name for vertical symmetry in genetics. I'm not a biologist but have often thought there has to be some "same gene" or pattern that causes hands and feet, legs and arms to have similar characteristics. I know that mother nature is very efficient so it seems only natural that perhaps the same basic blueprints are used for both.
Taking this a step further even, I'd thought that also must mean that "other" parts along on opposite sides of a vertical fold are the same. Just out of curiosity I thought I'd type in similarity of testicles and brain tissue and to my surprise they are!
I'd love to know if there is more about this and the tissue similarity of other parts.
Torbjorn_Larsson_OM
not rated yet Sep 17, 2014
@collideronline: Great questions!

- "Bilateral" symmetry. Yes, it is a deep symmetry in our lineage, that is why they call us Bilaterians as opposed to Ctenophorans (comb jellies), Porifera (sponges) et cetera.

- In most groups (but not Ctenophora and Porifera) the so called "body plan" is managed by Hox genes, that are repeats with the same type of chemical control of development. (It is really simple too, one set of Hox genes makes the first segment in arthropods, the next the next et cetera.) Arms and legs have the same basic development process, the head differs (see above) and funnily in mammals you can see something like it in the penis/vulva area.

- Other groups have convergent (comb jellies, many larvae stages are bilateral), different (Cnidarians, say) or no symmetries (sponges). FWIW, our body has to distinguish between left and right (left heart, say) and does that by our cellular cilia rotating in one direction only, which can be used to set up left/right direction.
antialias_physorg
not rated yet Sep 17, 2014
In most groups (but not Ctenophora and Porifera) the so called "body plan" is managed by Hox genes, that are repeats with the same type of chemical control of development. (It is really simple too, one set of Hox genes makes the first segment in arthropods, the next the next et cetera.)

And one set of Hox genes tells the body when to stop making more. The fun thing starts when that Hox gene doesn't work. (E.g. in snakes that is 'by design'. A snake is not much more than a lizard that just "keeps making vertebrae with ribs" without shutting the process down.)
RhoidSlayer
1 / 5 (1) Sep 18, 2014
Nice to see yet another deep time phylogeny, testifying to how evolution forms the basis of modern biology.

[Also nice to see the creationist trolls flailing harder and harder as they too perceive that the gaps to push their magic into is _gone_, here to impute magic action ('design') where there is none.

As if there are any peer reviewed results whatsoever showing that math, language or minds are based on some form of 'precursor logic', to put against the many, many thousands of results showing the exact opposite. Say how Gödel shows that in math you have to choose between consistency or incompleteness for each and every system _you_ construct out of its usefulness in the current science and technology environment. "Put up or shut up."]


confusing a repudiation of a strawman argument against recapitulation theory for a reference to creationism justifying that diatribe is a waste of your time making it up and my time pointing it out .

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