Development & Evolution of Nervous Systems

1. Genes Genes Development & Evolution of Nervous Systems Galileo: You cannot teach a man anything; you can only help him discover it himself.

2. fly bristle fish neuromast Anticipate on next talk, about the fish lateral line mechanosensory system made of individual, superficial sense organs called neuromasts provides fish with a sense of touch-at-a-distance

3. fly bristle shaft shaft cell fish neuromast cupula hair cells cupula cells support cell sensory neuron glial cell • Cf Detlev Arendt in retina: evolutionary tendency to segregate different cell functions into different cell types • allows cell specialization (segregation of rod, cones and bipolar cells; segregation of hair cells and neurons) • allows amplification (many rods => one ganglion cell; many hair cells => one sensory neuron)

4. mechanosensory organs Fish Fly Cell organization support cells (=> external structure) sensory neurons glial cells mechanosensory hair cells + sensory neurons support cells (=> external structure) sensory neurons glial cells mechanosensory neurons Origin All cell types (incl. neurons) originate from a common pool of ectodermal precursor cells long-range migration All cell types (incl. neuron) originate from a common ectodermal precursor cell through fixed lineage Developmental Genetics sensory cell fate depends on proneural gene proneural gene expression leads to Delta expression Cell types sorted out through Delta-Notch interaction precursor fate depends on proneural gene proneural gene expression leads to Delta expression Cell types sorted out through Delta-Notch interaction

5. If bristles and neuromasts are truly homologous, their common features were probably present before arthropods and chordates separated

6. Another example of conservation of sensory organ normal human eye eye of aniridia heterozygote aniridia eye lighted from behind Aniridia eye illuminated from the back

7. In mouse, there is a similar mutation, Small eye (Sey) wild type Heterozygote (reduced eye)

8. Sey- / Sey- homozygotes wild type human homozygotes for the aniridia mutation also show anophtalmia (absence of eyes) together with lethal cerebral alterations

9. Paired-box codes for a DNA-binding protein motif called "paired-domain" Positional cloning of the gene affected in the Sey mutant: the gene comprises a "paired-box" (because first found in a pair-rule mutant called paired) The presence of a paired-box defines a family of genes called Pax. The Pax gene that is affected in the Sey mutant is Pax6.

10. The Pax6 paired-box is highly conserved among animals (100% between mouse and man, 95% between man and fly)

11. vertebrate eye arthropodeye cephalopodeye cephalopod eyes resemble ours, but similarity only superficial (orientation of retina is inversed) camera-type facet eye (large numberof ommatidia) 600 in the fly, thousands in bees => at least 5 and arguably as many as 30 types of eyes evolved independently

12. wild type ey- / ey- Walter Gehring the eyeless mutation wild type the eyeless gene is the fly homolog of Pax6! • What will be the gain-of-function phenotype? • forcing the expression of ey in antennae and legs (Gal4-UAS system)

13. Exactly same result is obtained using the mouse Pax6 gene

14. vertebrate eye arthropodeye cephalopodeye When you think about development, take the point of view of the cells that make it, not the point of view of the outsider who observes it

15. There are other mutations that reduce or remove the eyes: eyes absent (eya) eyes gone (eyg) sine oculis (so) eyeless-2 (ey2) twin-of-eyeless (toy) dachsous (dac) Relations with Pax-6??

17. The fly "eyeless" syntagm Not only Pax6 but many other genes of the « eyeless » network are conserved in vertebrates where they also play a role in eye development => another case of sensory organ homology across all bilaterians Olfactory system: cf Heinrich

18. Sensory systems may have been conserved, but the central nervous system looks quite different!!!

19. => inversion of dorso-ventral axis between protostomians et deuterostomians such that dorsal side of vertebrate embryo homologus to ventral side of arthropod embryo

20. => The CNS of vertebrates and arthropods do occupy homologous positions

21. I I I In the fly embryo, slit (green) is expressed at ventral midline (red: muscle cell nuclei) + plays important role in scaffolding Development deals with topology NOT with function In vertebrate CNS, slit is expressed at ventral midline (by floor plate cells)

22. URBILATERIA + mechanosensory organs + olfactory system + synaptic asembly and plasticity + learning and memory + circadian rythm... Cambrian explosion (530 - 500 Myrs ago)

23. in spite of infinitely diverse morphologies development of all present-day animals based on the same very old genetic systems Why? Since they share the same old genetic systems How do they form infinitely diverse morphologies? nice: the conservation of genes and gene networks makes developmental genetics universal In this "new" biology, nematode, fly, zebrafish, and man, are just different expressions of a general programme for development (provides a conceptual basis for the "model systems" approach) also raises several (at least 3) serious questions:

24. x x x x 500Myrs 30Myrs 500Myrs Urbilat One had to wait until the advent of a complex species (in terms of structure and of developmental genetics) before there could be any "evolution" (long-lived diversification) Why? We don't know but... can we try to form a theory ?

25. Note: 610 - 550 myrs, Ediacaran (pre-cambrian ) fauna - England, Canada, Australia… Dickinsonia

26. Mawsonites Parvancorina - protoarthropod???

27. Tribrachidium (sym. 3) cnidarian-related? Charniodiscus (England, Australia, Russia, Canada…) What happened to those? We have no idea…

28. Why has basic structure of developmental programme been so strongly conserved to produce morphologies (and even functions) so drastically different? What did developmental genetics tell us? Conclusions of the work on fly bristles : 1. Developmental processes result from the concatenation of discrete, qualitatively different steps 2. Each step depends on a small network of interacting genes (a syntagm)

29. developmental genes embedded in complex network of interactions • any change will first be screened for its compatibility with network: • coherence must be maintained • as more interactions, number of degrees of freedom smaller: • changes will be more and more constrained • given the complexity of the developmental program, • the only changes that can be accepted are those that change nothing 1st rule : rule of conservative changes

30. genetic inertia of syntagms few developmental of developmental operations innovations functional equivalence Conservation of interactions of orthologous genes (pax6, otd, sc, Hox)

31. rule of conservative changes valid for all biological systems Any change in a structure is screened for its compatibility with the other elements the absolute requirement is to keep the coherence ex: homeobox, the most conserved residues are those that ensure the structural stability of the motif ex: number of possible proteins = n20 for n ac. amino acids. Not true! internal constraints (foldability, stability...) ex: psychology: new ideas or news are first screened for compatibility screen can become too rigid - autism ex: brain: any change is screened for its compatibility with existing connectivity (ex: auditory projection vs lateral line projection) Because of the nervous system's complexity, requirement for internal coherence may have been most pronounced in this system

32. Problem: if developmental program so resistant to change, how can it evolve? (cf Ian's wild variations of nervous systems: range of 106 in numbers of neurons, of 103 in synapses/neurons, yet each of them perfectly suited to its owner's needs, and ensures long-term survival)

33. By changing the connectivity between syntagms interactions within a syntagm: many and often bidirectional interactions between syntagms: limited and usually unidirectional Examples reiteration : many examples in wiring of embryonic CNS temporal change (heterochrony, neoteny) spatial change : pigmentation patterns

35. Spatial change may have important developmental consequences

36. By changing the connectivity between syntagms By new use of old syntagm: use of Hox genes in the limbs of vertebrates Hox genes re-expressed during limb formation

37. 1. Hox code may help define the five digits Human syndactily associated to mutations in Hoxd11, (normally repressed by Hoxa13) Mouse syndactily in Hoxa13 mutant (defective apoptosis)

38. Hexadactyly: never six different fingers (always 5 fingers + 1 duplicate)

39. 2. Hox code makes different sections of limb genetically different

40. once the limb program has become very robust (because relies on the Hox system?) can withstand wide morphological variations but necessarily based on a five fingers pattern!

41. 1st rule: only conservative changes are tolerated any addition that does not alter a process will be accepted (redundant genes, redundant mechanisms, redundant factors) • example: both pre- and post-synaptic elements can initiate synapse formation (Bill Harris) • L1 and L2 pathway mediate redundant or cooperative response depending on light conditions (Ian) • => long term tendency to stabilize any developmental process • because only compatible « plug-ins » can accumulate and further stabilize the process • example: many more proteins involved in synapse stabilization than in transmission! (Bill Harris) « canalization » (Waddington) Once a developmental process has become extremely robust, can withstand wide variations without collapsing example: the five digits of the hand, common to all tetrapods, possibly related to Hox gene coding 2d rule : stability breeds variability only robust pattern-generating system can withstand large variation without collapsing

42. x x x x 500Myrs 30Myrs 500Myrs Urbilat

43. SUMMARY SO FAR • Developmental genes embedded in complex network of interactions • their function depends on this network • the only changes that can be accepted are those that do not change anything • (1st rule : rule of conservative changes) • => genetic inertia (cannot modify the building blocks) Once a developmental process has become extremely robust, can withstand wide variations without collapsing only those developmental programs that are very stable can vary (2d rule : stability breeds variability) => evolvability(if not stable enough, cannot change)

44. paradox: within a few generations any species will adapt to selection (dogs, horses, flies) therefore any species optimally adapted to its environment (Heinrich Reichert: the shark's brain is exactly what you need if you are a shark) => departure from "normal" (species-specific) pattern will be detrimental conclusion: speciation will necessarily proceed through less adapted individuals will depend not on adaptation to the environment but on tolerance of the environment 3d rule: evolution depends on the survival of the misfits (tolerance) classical view : evolution depends on selection of the fittest developmental view: selection allows the optimization of a species tolerance allows the formation of new species

45. Tolerance allows individuals to move away from the stable state to explore the neighbouring "evolutionary space" If there is some tolerance, a less efficient variant can be maintained in the population (at least for a while) may get associated to another variation that may somewhat compensate => succession of transient, unstable forms that accumulate compensatory changes to recover lost coherence Once a subpopulation has derived too far away from the starting, stable solution (species) succession of unstable, short-lived, transitional types (short-lived "species") until hopefully hits a new coherent, stable state = a new species Ex: human evolution standard lifespan of a species: 4.106 years divergence of Homo with closest primate : 2-4. 106 ans

46. series of hominid species, all quite short-lived, 105 à 4.105 yrs only one remaining at the moment, sapiens, appeared about 2.105 yrs ago strongly suggests a number of poorly coherent short-lived species live just long enough to generate next attempt until eventually new equilibrium will be reached, or attempt will have failed

47. Massive extinctions that wipe out 80 - 95% of existing species accompanied by wide expansion of new species (e.g. the expansion of mammals related to the extinction of dinosaurs) (??? Cambrian explosion related to extinction of ediacaran fauna???) Possibly because tolerance much increased allows more risky explorations of evolutionary space with less efficient transitional types Not meaning that Darwin was wrong!!! (he was concerned about the problem of the Cambrian explosion) He did not know what we know today about genes and development did not appreciate how development is evolving

48. The end

49. (beginning) • Course about development and evoln of NS - • know a lot about NS, genes: excitability (channels), transmission (synapses, transmitters), signaling (plasticity) • A lot about dev, steps, gene networks - v little about evol (on principle act on genes; besides that: v little) • Oh yes comparative neuroanat, comparative embryol: end result of evo - tells nothing about evo itself • Cannot teach you anything - not too bad bec Galileo said - What I will do is to review with you some of the things we learned so far about evoln and genes and dev • And try to see if we can form hypotheses, or at least find a way to look at these problems that makes sense • (end) • I know that you are going to applaud - and I think you are right • but I also think that you do not know why - I 'll tell you • In the old times, 2500 years ago, greeks were playing tragedies - often wore masks, smiling of crying • same as African dancers, and also in South-America ritual dancing • that is because they were not themselves anymore, they were possessed - fate, gods, fortune, death • at the end the public would clap their hands to get them out of their trance • clapping the hands meant "come back now, you who are possessed, come back to us" • I think that when we teach something similar happens - less intense, of course • we are not ourselves, we are just servants, tools, interpreters • So it is very well that you call me back among you now, please, clap your hands Galileo, again: All truths are easy to understand once they are discovered; the point is to discover them

50. Mike, Heinrich, Ian, Matthews Matthews (Indian) feel exactly what I say about "possessed" even students seem to undersatnd it, at least some of them - clapping their hands high next to head Mike wonders how it is that I always get something highly unusual but true to say (last time: Easter's curve of attention - Vijay took a copy of it, remembers it so vividly! Coffee after talk, Mike: "I won't say anything about your talk because you do not want to listen anyway" "I always feel privileged to listen to you" Heinrich: it all makes so much sense, I saw the succession of human species in textbooks many times but never made sense of it! Next day: about marine station near Bangalore, Mike: "I always wanted to look at some transparent marine animal" Me: "why didn't you do it?" he: "I got distracted" me: "that would be a good epitaph to put on our tombstones" He: right, "sorry, I got distracted!"