1. Robustness and genetic networks
Eörs Szathmáry
Collegium Budapest Eötvös University

2. Selected members of five gene classes
Only one segment polarity gene is shown
Most of the proteins are transcriptional activators

3. 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!

4. 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 ligand-receptor interactions

5. 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

6. 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

7. 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

8. Some insights Extremely robust to variation in individual parameters
Also against change in netwok topology
Robust against change in concentration also

9. A theoretical treatment
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

10. 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

11. Incidence of Hsp90-induced eye deformities
Hsp90 associates with signal-transduction molecules
Lines can be selected for tolerance of mutation of Hsp90

12. Many ways of building the same body: the sea urchin

13. Larvae can be very different
Feeding pluteus (a)
A non-feeding larva (b)
Direct development (c)

14. Evolution of developmental modes
Black bars: direct development
Most adult forms are conserved
Ecological correlates: direct development  no dispersal, large eggs