Embryology Biology 441 Spring 2013 Albert Harris
Evolutionary Developmental Biology
A little history:
in the early 1800s, Karl Ernst Von Baer discovered surprisingly close anatomical similarities between embryos of fish, amphibians, reptiles, birds & mammals.
Among crustaceans, there are even more dramatic patterns, which I will over-simplify by saying that larval crabs look like shrimp. Somewhere, I have seen a really beautiful diagram of the whole "family tree" of arthropods, emphasizing parallels between evolutionary branching and embryonic anatomy of different groups.
Ernst Haeckel invented the over-simplified slogan "Ontogeny Recapitulates Phylogeny".
That means that the changes undergone during embryonic development (That's "Ontogeny") supposedly repeat an extremely accelerated version (That's "Recapitulate" )
The sequence of anatomical shapes that occurred during evolutionary origin of each kind of animal. (That's "Phylogeny")
To over-simplify even further: tadpoles look like fish because frogs evolved from fish, Or the reason that mammals, birds etc. develop gill slits when embryos is because we all evolved from fish, that had real gills.
So there is considerable truth in the old slogan.
But many of Haeckel's specific ideas turned out to be wrong: He thought that cartilage evolved into bone, and that is why our skeleton begins by being almost entirely made out of cartilage.
Really, later evidence is that bone evolved before cartilage, and the reason embryonic skeletons start out being made of cartilage is because it can grow by internal expansion, whereas bone can only get bigger by having more bone deposited on its surfaces.
"Recapitulation" allowed Embryology to climb on the band-wagon of Darwinian Evolution (1859).
Darwin was right; Haeckel over-interpreted, but became very popular.
From around 1870 up beyond 1910, embryology teaching and hiring of embryology professors was overly-dominated by Haeckel's ideas. People studied embryos mostly looking for clues about the evolutionary origin of each kind of animal.
H. V. Wilson established his very good reputation, that caused UNC to hire him, mostly by a book-length study of embryonic development of a teleost fish, the Black Sea Bass.
He did this research at the Woods Hole Fisheries Laboratory (not to be confused with the Marine Biological Laboratory (which is across the street). (also not to be confused with the Oceanographic Laboratory)
Wilson himself was quite doubtful and sarcastic about recapitulation and other then-popular ideas.
In recent years, lots of people have convinced themselves that "birds are dinosaurs." Prof. Alan Feduccia, my friend and former chairman of this department, has led the opposition to this idea.
Notice contrasts and similarities to disputes about recapitulation
"Evo-devo" = Evolutionary Developmental Biology
Which sometimes seems to mean "Transcription recapitulates phylogeny"
Many surprising discoveries:
Especially that the same (closely "homologous") genes are used to stimulate equivalent organs, even when those organs are not homologous in the evolutionary sense..
Transcription factors that apparently control the development of eyes are closely similar in base sequence in vertebrates and in Drosophila. Please notice the paradox that vertebrate eyes are NOT evolutionarily homologous to the eyes of insects and other arthropods. Also, insect compound eyes are very different from vertebrate camera eyes.
As you know, their physics of compound eyes versus camera eyes are very different. Their embryological development is also very different. Yet their development is (at least in part) controlled by transcription factors with very similar amino-acid sequences (as if homologous!)
What can this mean??
On the final exam, please be ready to propose one or more possible explanations.
Notice that the word homologous (as applied to a gene) means "similar because of common evolutionary ancestry", but in the evo-devo literature is often used to mean only "similar in base sequence" (i.e. independent of the reason for the similarity, which might be convergent evolution, or who knows what!
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Some interesting research done recently in this department, regarding the evolution of birds from dinosaur-like reptiles
Bird toes are 2, 3, 4 (5) Dinosaur toes are 1, 2, 3
Ancestral reptiles have 5 fingers and 5 toes.
This number was reduced to three in dinosaurs and birds; but are they the same three?
All sides agree that the fossil record of dinosaurs shows that dinosaurs lost toes #4 and #5.
In collaboration with Anne Burke (who used to be a professor in this department) and Julie Nowicki (who was a Teaching Assistant in this course for two years) Alan Feduccia studied limb bud development in Ostriches (because they have very big eggs).
They proved that the ostrich's three toes are #2,#3 and #4.
This is based not just on morphology but also on in situ hybridization staining for mRNAs of certain transcription factors.
The Dinosaurs-are-birds theorists accepted to truth of the research by Feduccia, Nowicki and Burke,
but they invented a way to avoid giving up their dinosaur theory:
They invented a "frame shift" according to which gene expression shifted by one segment.
This is a good example of "And then a miracle occurred!"
Such "save the theory by adding extra postulates to a theory" are called "Ad Hoc hypotheses".
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Colinearity (spatial correlation between locations in the body where how genes are transcribed versus the relative locations where how genes are located on the chromosomes (in the sense of genetic mapping)
A quote from the (very well done!) Wikipedia article about hox genes.
"The reason for this colinearity is not yet completely understood."
Translation: Nobody knows how it works;
Neither the chromosome-level mechanism by which gene transcription is stimulated;
Nor does anyone really understand the animal-level mechanism by which how gene expression helps control body anatomy.
Furthermore, nobody really knows what function is served by colinearity. Probably how gene expression serves to control (in part) the development of each part of the body. But germ layers and cell differentiation had already been already determined by cell rearrangement phenomena, including gastrulation, neurulation, somite formation, placode formation, and folding of lobes of the brain, before the times when hox genes begin to be transcribed in their co-linear geometric pattern in the developing body.
This is one paradox.
Another paradox is that how gene expression patterns vary (in the posterior direction) as gradients of gradual decrease, which look very much as if they were diffusion gradients, even though the how gene proteins (and their messenger RNAs are too large to diffuse through plasma membranes). Therefore those can't be diffusion gradients.
Also, because the circulatory system is functioning at the time of formation of these patterns, any freely diffusing molecules would quickly be swept away by blood flow.
Furthermore, why are the anterior borders (of the parts of the body where Hox genes are expressed) very sharp and sudden (no gradient)?
Some researchers (Slack) have write that they don't think that colinearity is important! Others would sell their souls to discover the mechanisms.
Elements of the puzzle: (I repeat)
Hox gene proteins are too big to diffuse through plasma membranes, from one cell to another; Yet they form concentration gradients.
Those can't be diffusion gradients; not unless either the proteins or the m-RNAs can diffuse through plasma membranes.
Also, if they could diffuse through membranes, then why wouldn't they be swept away by the circulatory system?
You should try to think of other paradoxes or puzzles related to how gene expression patterns.
Questions on the exam will include your own conclusions and hypotheses about co-linearity.
These questions will include suggestions of experiments by which you could reasonably hope to prove or disprove your theory.
Methods for testing your theories can include in situ hybridization, bioassays, viruses in which how genes have been inserted into the virus genome, tissue culture, and any other techniques you have learned about in this course and its prerequisites.
Please carefully read the corresponding chapter in Wolpert's textbook (chapter 15) You will be as much responsible for the contents of that textbook chapter as you have been held responsible for this and previous lecture notes.
Be prepared for thought questions that ask you to relate this material to subjects taught in previous parts of the course.