How does order in biological systems arise from molecular processes which are intrinsically noisy?

2. How does order in biological systems arise from molecular processes which are intrinsically noisy? or How to form spatial pattern? Noise, precision, diffusion and scaling in biological pattern formation

3. Life cycle of Drosophila melanogaster Life cycle

4. Early Embryonic Development (first 3 hours) First 3 hours fertilization 13 rounds of nuclear divisions syncytium, no cells yet ~6000 cells form after 2h morphologic cell shape changes after 3h (gastrulation)

5. Synchronous mitotic divisions during syncytial stages

6. Gastrulation Gastrulation Identical cells 15 minutes later Cells display distinct behaviors

7. Maternal gradients determine early embryonic gene activity Dorsal Nanos Bicoid Torso Torso ~6000 nuclei need to know their identity from 4 maternal gradients

8. Bcd-hb activation Bicoid RNA deposited at anterior end of egg is translated into Bicoid protein, which is thought to diffuse along the length of the egg, establishing a gradient Bicoid RNA Bicoid protein Bicoid protein activates Hunchback gene expression

9. Bicoid protein is a “transcription factor” that enters the nucleus, binds to and activates transcription of other genes One example is the “Hunchback” gene

10. Bicoid protein activatesgap genes Threshold readout Final distribution reflects simple physical parameters (diffusion, degradation) Bicoid protein Activation ofHunchbackandKruppeloccurs wherever Bicoid protein is above acritical threshold Information is Quantitative Response depends on concentration levels Hunchback HunchbackKrüppel

11. Intro genes maternal gradient bicoid Gap genes hunchback Pair rule genes runt Segment polarity genes wingless

12. Final observed precision is at the level of one nuclear spacing Even-skipped Runt Head fold initiates in single row of yellow cells

13. Brief Summary of what we have so far…

14. larva Embryo (~3h) Larva (~24h)

15. Need more quantitative approach to address these questions… bicoid

16. Model based on 3 assumptions: • Constant source at head pole • Diffusion • Uniform degradation D = diffusion constant t = protein lifetime Concentration Steady state: 0% 100% Position(m) Hunchback (Hb) Activation of hb occurs wherever Bcd is above a certain threshold Bicoid protein activates hunchback gene Bicoid turns on hunchback Bicoid protein (Bcd)

17. Need Input/Output relations

18. What happens if the size of the box changes?

19. Dipteran eggs vary greatly in size Calliphora (blow fly) ~1500mm Drosophila melanogaster ~480mm Lucilia ~1200mm Drosophila busckii ~320mm Musca (housefly) ~1200mm

20. questions How are gradients established?How do morphogen molecules move? Can simple physical mechanisms produce the required accuracy? How do cells “read” small differences in concentrations?How reproducible are these decisions Are responses always on/off?

22. Nuclear Stainings Cycle 9 Cycle 11 Cycle 13 Cycle 14

23. Image Analysis – ‘Spot Recognition’

24. The response to the Bicoid gradient (hunchback trans-cription) is precise and invariant despite Bicoid variability Bicoid Hunchback B. Houchmandzadeh, E. Wieschaus, S. Leibler, Nature, 2002