Download On the Origin of Language

Robustness and genetic networks
Eörs Szathmáry
Collegium Budapest
Eötvös University
Selected members of five gene
classes
• Only one segment
polarity gene is shown
• Most of the proteins
are transcriptional
activators
Some regulatory interactions that
esablishing gap gene expression
• Krüppel is expressed
in the grey region
• Bicoid is an activator
• Hunchback and giant
are repressors
• Inelegant mechanism!
Cooperativity in biomolecule
activity
• The degree of cooperativity can be measured by
the Hill coefficient: the slope at inflection point
• Common in transcriptional regulation and ligandreceptor interactions
Expression of segment-polarity
genes in the embryo
• Pair rule gene expression patterns regulate engrailed and
wingless expression
• 14 parasegments are thus formed at en/wg boundaries
• True segment boundaries are displaced one cell to the right
of the parasegment boundaries
A model for the maintenance of
segment polarity gene expression
• Gene expression patterns (black in a)
• Part of the segment polarity network (lowercase:
genes, uppercase: gene products)
• Each cell has one such network
Space of functional segment polarity
networks
• 22 parameters out of 48
are shown
• Gray polygons yield
functional networks
• Kxy: concentration of X at
which activity of y is half
maximal, etc..
• Mean and standard
variation for polygons are
shown
Some insights
• Extremely robust to variation in individual
parameters
• Also against change in netwok topology
• Robust against change in concentration also
A theoretical treatment
Si (t       jwijSj (t ))
• Let us borrow a model
from neural networks
• Only two states: on
and off
• Activators and
inhibitors influence
the weight
• Switch-like
cooperativity
• 60% of random
changes gives the
same expression
pattern
• One can select for
networks with even
higher robustness
Dorsal view of the Drosophila wing
• Heat-shock treatment induces the disappearance of
cross-veins
• One can select for or against this sensitivity
• Genetic assimilation: some show the phenotype
without the inducer!
• Does not work with inbred lines
Incidence of Hsp90-induced eye
deformities
• Hsp90 associates with
signal-transduction
molecules
• Lines can be selected
for tolerance of
mutation of Hsp90
Many ways of building the same
body: the sea urchin
Larvae can be very different
• Feeding pluteus (a)
• A non-feeding larva
(b)
• Direct development
(c)
Evolution of developmental modes
• Black bars: direct
development
• Most adult forms are
conserved
• Ecological correlates:
direct development 
no dispersal, large
eggs