Monday, February 20th: Morphogens; pre-patterns vs. positional information
Two very different meanings of the invented word "Morphogen"Invented by Turing to refer to the pre-pattern substances (what I called A & B)
Borrowed by Lewis Wolpert to mean chemicals that signal proportional differences.
Nobody uses the word morphogen as a name for a kind of gene.
"Pre-pattern" explanations
in contrast to
"Positional Information"
Chemical signals that control what organ develops where.
Signals that communicate X, Y & Z coordinates
Analogy to Latitude, Longitude, Altitude.
What evidence supports positional information, in this sense?
It's what I prefer to call "The Driesch Phenomenon"
Regulative development: parts made smaller in proportion to size of whole.
Wolpert's hypothesis is that diffusion gradients serve this adjustment function.
Other evidence: Some very similar control proteins in insects as in vertebrates.
Why is that evidence?
Please consider phenotypic effects of mutations that alter morphogen gradients.
Consider the question whether or not changes could evolve in morphogen gradients.
Hint: what if mutations in certain genes have lethal effects, even when heterozygous.
Do you see why lethal genes are effectively invisible to genetic studies?
(How ironic that the great importance of some genes makes them invisible)
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The Parable of the Nearsighted Conformists, and the Bald Professor.
Why not every gradient has to be a diffusion gradient.
Any variable that equilibrates repeatedly will create gradients of amounts.
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The Parable of the Cells that Protruded Spikes.
#1) Imagine that each cell pokes its nearest neighbors with some number of pseudopodia.
#2) Each cell counts the total number of these spikes that are poking it.
#3) Each cell protrudes the same total number of these spiky pseudopodia as are poking it.
#4) These protruded spikes are evenly distributed across each cell's surface.
#5) All the cells along one side continue to protrude a maximum number of spikes, & don't decrease this number.
#6) Cells on the opposite side protrude no spikes, or some non-varying low number.
Concentration gradients of non-permeable proteins sometimes develop. How to explain them?
Also gradients of pigment granules, gradients of yolk granule size.
Many biologists believe that diffusion gradients are the only possible way to create gradients.
Please think about possible mechanisms of the "colinearity" gradients of hox gene protein concentrations.
Please try to invent a hypothesis capable of producing colinearity based on cells detecting whether individual cells transcribe those hox genes that are closest (in position along a chromosome) to whichever hox genes are being transcribed the most by the cells nearest to them (that is, closest in the body).
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"Biomechanical" explanations of embryonic processes and shapes.
D'Arcy Thompson's ideas are early examples of biomechanical explanations.