PZ Myers. 2004 Dec 13. Deep homologies in the pharyngeal arches. <http://pharyngula.org/index/weblog/deep_homologies_in_the_pharyngeal_arches/>. Accessed 2008 Aug 20.
Posted on M00o93H7pQ09L8X1t49cHY01Z5j4TT91fGfr on Monday, December 13, 2004
Deep homologies in the pharyngeal arches
PvM at the Panda's Thumb has already written a bit about this issue in the article "Human Gland Probably Evolved From Gills", but I'm not going to let the fact that I'm late to the party stop me from having fun with it. This is just such a darned pretty story that reveals how deeply vertebrate similarities run, using multiple lines of evidence.
Here's the start of the situation: fish have a problem. Like most animals, they need to maintain a specific internal salt concentration, but they are immersed in a solution that is much more dilute they they are (for freshwater fish) or much more concentrated (for saltwater fish). To make matters worse, fish have large respiratory membranes, the gills, which expose a huge amount of surface area to the watery medium. Their answer to the problem is to do much of their regulation of various ions at the gill surface, using sensors and salt pumps to constantly work, extracting excess salts from one side and pumping them to the other. Gills are therefore more than just the organ fish use to breathe—it's also where they regulate salt balance.
Us terrestrial tetrapods have a different problem. We can't pump salts out of or into the air, so instead we maintain internal stores of salts (calcium, for instance, is packed into bone) and use hormones to regulate them, by telling cells to either sequester the ions, or to release them into the bloodstream. We don't do this with gills, obviously—instead, we have several glands that monitor and control blood salt levels.
One of the most important sets of glands for this function are the thyroid and parathyroid glands, which regulate calcium ion balance. When the amount of calcium salts dissolved in the blood drops below a certain level, receptors called CasRs (Calcium sensing Receptors) detect the change, and trigger the parathyroid gland to release PTH (Parathyroid Hormone) into the blood, which tells cells in the bone to gnaw away and free up calcium (which can lead to osteoporosis, for instance), and it also signals the kidneys to retain calcium. The thyroid, which I'm not going to say much about, has a complementary role in managing the situation when blood calcium levels are too high.
So fish have gills to regulate calcium, and we have parathyroid/thyroid glands to regulate calcium. Do they have anything else in common? One minor thing seems to be location: gills are in the fish's 'neck', and the parathyroid glands are located at roughly the same place, in your neck—and that's interestingly coincidental, since there's no particular need for these glands to be in that particular location. They can do their job just as well anywhere. And they happen to be located in a particularly fascinating place for those of us curious about vertebrate evo-devo; all kinds of action occurs in this pharyngeal region in early development.
For one thing, many features of the face and neck are assembled from those curious and fateful tissues, the branchial arches, well known for their homology with the gill arches of fish. Here's a diagram of a 5-week-old embryo on the left, with the arches color-coded, and an infant on the right, showing what connective tissues develop from each arch.
The first arch contributes to the jaw, while the second through fourth are going to form various cartilages in the neck—the cartilages near where the thyroid/parathyroid will form. What an interesting coincidence! Could it be that the parathyroid is also derived from a branchial arch, and is therefore homologous with the gills of fish?
One way to find out is to use an early molecular marker. The parathyroid is distinguished by the expression of a unique transcription factor, Gcm-2. Gcm-2 is expressed only in the parathyroid, and is absolutely critical to its formation: knocking out the gene causes the parathyroid to fail to form. Another useful marker is to stain for CasR, the receptor protein, or PTH, the hormone itself. The top row of pictures (A-D) below show that these are effective markers for the thyroid gland in an older chick embryo. The lower series (E-I) are in younger embryos, at a stage when they have clear pharyngeal arches, stained with Gcm-2.
Expression of Gcm-2 in the parathyroid gland and the pharyngeal pouches in the chick. (A#D) Whole-mount in situ hybridization of chick E11 thyroid (T) and parathyroid (P) glands for the following probes: Gcm-2 (A), PTH (B), CasR (C), and TTF-1 (D). Gcm-2 can be seen to be expressed in two round masses, the parathyroid glands, which are adjacent to the thyroid, which expresses TTF-1. The parathyroid glands additionally express PTH and CasR. (E-I) Expression of Gcm-2 in chicken embryos, staged as described in ref. 16. In these micrographs, anterior is to the left and ventral is to the bottom. OV, otic vesicle; pp, pharyngeal pouch; II-IV, pharyngeal arches. Expression of Gcm-2 starts in the third pharyngeal pouch at stage 18 (E), and then, as development proceeds, expression is also evident in the fourth pharyngeal pouch and additionally weakly in the second pouch (F). At stage 24, expression in the third and fourth pouches is concentrated in a region dorsal of the pharyngeal pouches and is lost from the second pouch (G). In Vibratome sections of stage-22 embryos (H), it is clear that Gcm-2 expression is localized to the pharyngeal endoderm and that, by stage 24, the region of the pharyngeal endoderm expressing Gcm-2 has thickened and given rise to round masses that are the parathyroid gland rudiments of the third and fourth pouches (I).
What you see is that this marker appears early in the third and fourth pharyngeal arches, showing that the parathyroid is derived from the same tissues as fish gills.
This is cool enough, but Okabe and Graham take it a step further. Gcm-2 is a marker for the parathyroid in tetrapods. Fish lack a parathryoid gland, but is Gcm-2 expressed anywhere in them? I think you can guess where Gcm-2 is expressed in fish:
Phylogenetic analysis of the distribution of Gcm-2 and its expression in teleost (zebrafish) and chondrichthyan (dogfish) species. (C-F) Gcm-2 expression in zebrafish embryos. In these micrographs, anterior is to the left and ventral is to the bottom. Gcm-2 initiates expression in the second pharyngeal pouch in early 3-day-old larval fish (indicated by arrowhead in C). Subsequently, Gcm-2 is expressed sequentially in the more posterior pouches (D), and, by day 4, Gcm-2 is expressed in all of the pouches (E). It is also apparent by day 4 that Gcm-2 is expressed in the developing internal gill buds emerging from the pharyngeal pouches (F). (G and H) Gcm-2 expression in dogfish embryos. This gene is expressed in the internal gill buds protruding from the pharyngeal pouches in stage-27 dogfish embryos (17). The pharyngeal arches are numbered II-VI.
These photographs show where Gcm-2 is expressed in zebrafish and dogfish embryos: the pharyngeal arches! These are beautifully symmetrical results showing that the tetrapod parathyroid is derived from the pharyngeal arches, and that the pharyngeal arches of fish also express a parathyroid marker. In addition, the zebrafish pharyngeal arches also express PTH and CasR. And the researchers take it a step further still.
Remember that Gcm-2 is essential for parathyroid formation: mutate it, and the animal doesn't form a parathyroid. What if we knock out Gcm-2 in a fish? It doesn't have a parathyroid, after all…all it has are gills.
Gcm-2 is required for the elaboration of the internal gill buds from the pharyngeal pouches in zebrafish. Zebrafish embryos were injected at the one-cell stage with either control or antisense Gcm-2 MOs. The embryos were then analyzed at day 5 for the presence of internal gill buds. (A-C) Five-day-old zebrafish larva injected with control MO. (A) Nomarski viewof the pharyngeal region of a day-5 embryo injected with the control MO. The internal gill buds protruding fromthe pharyngeal pouches are clearly evident (arrowheads). (B) Embryo injected with control MO hybridized for Gcm-2. Gcm-2-expressing internal gill buds can be clearly seen protruding from the pharyngeal pouches. (C) Embryo injected with control MO, showing normal pharyngeal pouch formation as judged by Pax-9a expression. Each pharyngeal pouch is indicated by an arrowhead. (D-F) Five-day-old zebrafish larva injected with Gcm-2 antisense MO. (D) Nomarski view of the pharyngeal region of a E5 embryo injected with the antisense Gcm-2 MO. There are no internal gill buds protruding fromthe pharyngeal pouches. (E) Embryo injected with the antisense Gcm-2 MO hybridized for Gcm-2. There are no Gcm-2-expressing internal gill buds protruding from the pharyngeal pouches. (F) Embryo injected with the antisense Gcm-2 MO, showing normal pharyngeal pouch formation as judged by Pax-9a expression. Each pharyngeal pouch is indicated by an arrowhead. EY, eye; YK, yolk. Anterior is to left and ventral is to the bottom.
In zebrafish injected with a morpholino that blocks the Gcm-2 transcription factor, poof, gills fail to form.
So, what we have here are multiple lines of evidence—location, function, several molecular markers, and developmental origins and processes—that converge to show that parathyroid glands and the gills of fishes have a common evolutionary origin.
Okabe M, Graham A (2004) The origin of the parathyroid gland. PNAS 101(51).
Science • EvoDevo • 4 Trackbacks • Other weblogs • Permalink
-
Once again, thanks for telling 'the rest of the story'.
#: Posted by on 12/13 at 10:35 PM
-
I hadn't heard it over on Panda's.
This work is truly stunning, and beautiful.
Gracias, PZ (and Okabe/Graham), for completing more of the picture!
#: Posted by on 12/14 at 12:18 AM - Scientific work like this is as breathtakingly beuatiful as any piece of art!
-
Wonderfully clear yet again. You are one of the most lucid and interesting expositors of science I have read. Is there any chance of you collecting some of these essays and publishing them in book form?
Tom Curtis#: Posted by on 12/14 at 05:39 AM -
And yet one more elegant chunk of information demonstrating how evolution theory works well in application.
How long will it be before creationists and IDers will deny that fish have gills?#: Posted by on 12/14 at 06:32 AM -
What a cool post. It has been so long that I can't remember: is stuff like this taught in K-12 science? It is elegant and powerful proof of evolution, and when presented in this way, it is fun to learn, too!
#: Posted by on 12/14 at 09:19 AM
- Jes' one more reader adding to the deserved accolades. Marvellously well-written.
- No, this isn't taught in K-12. It's probably a bit much all in one lump for most kids. I think the basics could be taught -- what is a parathyroid gland, what is a branchial arch, what does it mean to say a gene is expressed, etc. -- but we have a lot of people focused on stripping out even the most elementary concepts from our kids' science education. The fundagelical freakshow often distracts from teaching the really good stuff, I think.
-
PZ, I think this could be taught in K-12. It's such a good example and gives so much insight into the evolutionary process that it should be. It's not necessary for students to be familiar with all the concepts - I'm certainly not - but the underlying principle is clear enough: we got gills!
Sorry. Did anyone catch the reference to the comic strip BC, which regularly spouts religious nonsense?#: Posted by on 12/14 at 09:53 AM -
Hey PZ fans, don't miss
http://www.talkorigins.org/faqs/wells/haeckel.html
But then there's
http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=pubmed&dopt=Abstract&list_uids=12475051
and then fit this in:
http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=pubmed&dopt=Abstract&list_uids=12610301
.#: Posted by on 12/14 at 10:45 AM -
::Worships::
I second Tom's idea: have you ever thought about making something of a coffe table book with lots of glossy pictures and intriguing articles like this? Looking at my childhood, the best things I remember include looking at science and literature with great artistry.
Kudos. Things like this remind me why I love science.#: Posted by on 12/14 at 05:54 PM -
In high school, I just assumed that evolution was "how things were."
Now, this was without knowing very much direct empirical evidence for it. For some reason, I just don't think I would have appreciated the true ramifications of this research.
Now that I have seen and understood a lot of evidence for evolution, this type of work just astounds me even more.
When you are just starting a puzzle, getting the first few pieces is not all that exciting.
But, once you are really getting the big picture, each additional discovery makes it all the more beautiful.
#: Posted by on 12/14 at 07:01 PM -
For me the most appealing aspect of PZ’s little essay lies in its offering more evidence for the validity of evolution than all the books, papers, press releases, and diatribes published by the ID promoters offer for the validity of ID.
And as a one time editor and writer of K-12 textbooks for the sciences, I do think that it could be adapted for a high school text---maybe. One large hurdle would be getting the major buyers of textbooks, e.g., Texas, California, New York, etc., to include the topic in their syllabi or state curricula. Without that all the good writing in the world will be like pissing in the wind. A second hurdle will be describing this particular group of homologies at a level accessible to high school freshman or sophomores, given the inclusion of some terms and topics (e.g., transcription factor, pharyngeal, etc.) that are either uncommon or probably absent in most current high school biology texts (I'm speaking from recollection at the moment). One also has to cope with the many teachers who lack significant training in biology, which is probably the majority. Don’t get me wrong, there are some wonderful high school biology teachers in this country, but they are the exception, not the rule. Too often they’re the coach or driver's ed instructor filling in because the school can’t find someone with the proper education. Maybe the No Child Left Behind act will cure that, but I doubt it since little money has been offered by the Feds and there's even less likelihood of it in the future.#: Posted by on 12/14 at 08:39 PM -
A little elementary embryology would be a good thing in a high school biology class (it could even be introduced without mentioning evolution, although this would be shortchanging it). In my HS biology, we had to memorize such useful facts as the tissues of wood (xylem and phloem), and as far as I can recall, we did not have to memorize ectoderm, mesoderm, and endoderm, or learn what pharyngeal arches are -- which seems strange in retrospect. Development can seem weird and daunting to study, but it definitely has enough intrinsic interest to get the attention of beginning students.
#: Posted by on 12/14 at 09:31 PM
-
What a splendid, lucid presentation. Your students may or may not realize it, PZ, but they are very fortunate indeed.
#: Posted by Steve Bates on 12/14 at 11:58 PM
-
Paul, this is a brilliant post. I have never thought of the PT glands as homologous to fish gills. That demonstration of "a-gillism" when Gcm-2 is knocked out was neat. Thanks.
#: Posted by on 12/15 at 08:42 AM
-
Are these glands found in the neck of most mammals, reptiles, birds and amphibians? Or is this something that humans (and probably other apes) share with fish?
#: Posted by on 12/15 at 12:08 PM
- Hmmm, amphibians - not sure about parathyroids. Anyone? They definitely have thyroid glands, as the hormone is essential for metamorphosis. Reptiles, birds and mammals all have both of these glands, always in the neck. Parathyroid is usually a separate gland (as well as another gland in birds and reptiles - ultimobranchial), but in some groups of mammals, including us, parathyroid gland has become anatomically a part of thyroid gland. You can still see under a microscope which portions of the thyroid are really embedded parathyroid.
- These glands are found in all tetrapods, so yes to your first question.
- Yes, Rana's got parathyroids. It's a gland common to all tetrapods.
-
Thanks, I thought so, but the article focused on us.
#: Posted by on 12/15 at 12:30 PM
-
PZ, if you ever find yourself with a bunch of free time and you decide to write a book filled with stuff like the subject of this post, let us know. I'll buy the first copy for my kids -- they're 10 and 12 now -- and I think they'd enjoy it at the same time they're learning. That, I suspect, is the key to effective education (whether science, or history, or math, or whatever): get 'em while they're young, and make it fun. A book like this would do both.
#: Posted by on 12/15 at 01:48 PM