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The problem of development
Eörs Szathmáry (Alpbach 2005)
Collegium Budapest
Eötvös University Budapest
What’s this?
Let’s take another look!
Aggregation of the slime mold


The amoebae gather into a focus
Individual cells become bipolar
Propagation of the cAMP signal
and chemotaxis


Central cell emits a
dose of cAMP then it
rests for a while
Neighbouring cells
creep toward the
signal while they also
release a dose of
cAMP
Formation of Dictyostelium
fruiting body


In the slug pre-stalk
cells are first
Finally pre-spore cells
creep to the front
Why is the problem of
development so difficult?

We do not build developing machines:
physiology is „easy”, embryology is
„difficult”
Preformation versus epigenesis


Homonculus in the
sperm
Epigenetics:
complexity truly
increases
Self-assembly of the virus capsid

Subunits are composed of proteins
The process of aggregation
Formation of
the anteroposterior axis
of Drosophila
Do the genes programme
ontogenesis?
In a sense yes, BUT
 A bag of elephant proteins is not an
elephant
 Structure that self-assemble
 Virus particles
 The formation of sponges
 Limitations of the jigsaw puzzle principle

Wave-like properties of development
Wave forms are largely independent of
chemical composition
 Animal coat patterns (zebra, tiger, etc.)
 BUT altered genes can alter the shape of the
elephant
 Somehow molecules determine form even if
they do not self-assemble
 BUT HOW?

The lesser know work of Alan
Turing





English mathematician
Breaking of the german
code
Creator of the „universal
Turing machine”
His relevant paper:
Philosophical
Transactions of the Royal
Society (1952)
What is behind animal
coat patterns?
Theory of a chemical standing wave


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
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Two compounds: an
activator and an inhibitor
The activator helps its
own formation
Inhibitor inhibits the
formation of the activator
Both compounds degrade
Inhibitor diffuses much
faster
A kémiai állóhullám


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
In a well-stirred medium both concentrations reach
equilibrium
If the solution is not stirred, then a positive fluctuation in
activator concentration will enhance itself
Fluctuation is due to chance
Inhibitor concentration also rises, but its higher diffusion
rate decrease the amplitude
The coat of a fish


Growth as well as REGENARATION seems to
follow a Turing mechanism
Reaction-diffusion systems
Wolpert: a „French flag”
A simple model of
pattern formation
Positional information and
pattern formation


Each person has a
specific seat in the
stadium
In each seat a specific
poster must be shown
Hair patterns in Oncopeltus as
indicator of polarity


Hairs point backwards on the cuticule
Removal of segment boundary reverses the
direction of hair growth
Development of the chick limb

The zone of polarizing activity (ZPA) directs the
formation of the order of digits
The effect of ZPA
Prepattern and consequence

Morphogen is the sonic hedgehog protein
Again the axis
of Drosophila
The expression of maternal
bicoid mRNA

Both mRNA and
protein can be
visualized by
appropriate
compounds
Determination of the 2nd evenskipped (eve) stripe



Bicoid and hunchback
proteins activate
Giant and Krüppel
inhibit
Not too elegant!
Gene expression and pattern

Combinatorial
determination
Vizsgaidőpontok
Január 7, 14, 21, 28 KEDDEK, Szabó
József terem 10-12 óra
 Január 9, 16, 23 CSÜTÖRTÖKÖK, Szabó
József terem
