Chapter 21. Development of Multicellular Organisms Sydney Brenner, 1960

Mutation vs. variation Mutation Variation Mutation

Types of mutation 1. No phenotype -Amorph 2. Loss of function -Null -Hypomorph 3. Gain of function -Hypermorph -Antimorph (dominant negative)

1. Robust model for the behavior of individual and identified cells 1000 somatic + 1000-2000 germ cells 2. Convenient model for genetics  Single heterozygote worm can produce homozygous progeny 3. Cell fates and lineages are almost perfectly predictable

Mechanisms for C. elegans development Polarity formation by maternal effectors Cell-cell interactions to make complex patterns Heterochronic genes Apoptosis

1. Maternal effect genes mRNA from mother is asymmetrically distributed along anteroposterior axis Par (partitioning defective) genes bring P granules to posterior pole One cell having P granule give rise to germ cells

2. Cell-cell interactions to make complex patterns

2. Cell-cell interactions to make complex patterns P2-EMS interaction: 1. Mom mutants without gut -Mom gene (Wnt) expressed in P2 cell -Frizzled gene (Wnt receptor) expressed in EMS cells 2. Pop mutants with extraguts -Pop genes encode LEF-1/TCF homolog -Reduced pop activity  gut -Increased Pop activity  muscle

Cells change over time in their responsiveness to signals At four cells Anterior cell specification  depend on Notch signals At 12-cell stage, both Aba and Abp progenitors exposed to Notch signals  Granddaughter of Aba cells induce pharynx  Granddaughter of Abp cells unresponsive to Notch

Heterochronic genes control the timing of development Heterochronic phenotypes: The cells in a larva of one stage behave as though they belong to a larva of a different stages, or cells in the adult carry on dividing as though they belonged to a larva lin-4 for the transition larval stage1  3 let-7 for the transition late larva  adult

Apoptosis 1030-131 Cell death abnormal gene -ced-3, ced-4, egl-1 (caspase, Apaf-1, BAD homolog)  cause cell death -ced-9 (Bcl-2 homolog)  repress cell death

The Nobel Prize in Physiology or Medicine 2002 "for their discoveries concerning 'genetic regulation of organ development and programmed cell death'"                              Sydney Brenner H. Robert Horvitz John E. Sulston     1/3 of the prize United Kingdom USA The Molecular Sciences Institute Berkeley, CA, USA Massachusetts Institute of Technology (MIT) Cambridge, MA, USA The Wellcome Trust Sanger Institute Cambridge, United Kingdom b. 1927 (in Union of South Africa) b. 1947 b. 1942

Drosophila and the molecular genetics of pattern formation: Genesis of the body plan Seymor Benzer

Overall procedures

Overall procedures

Syncytial Specification Pole cells

Maternal effects Egg polarity determination A-P and D-V axis Cytoplasmic bridges

Egg polarity genes (Maternal effectors)

Egg polarity genes (Maternal effectors)

Dorsoventral axis Dorsal protein (NF-kB): -Dorsally, the protein is present in the cytoplasm and absent from the nuclei; ventrally, it is depleted in the cytoplasm and concentrated in the nuclei. -Toll gene controls the redistribution

Dorsoventral specification Dorsal protein concentration High  activate twist, repress dpp (decapentaplegic) Intermediate  sog (short gastrulation)

Dorsolventral specification Fate map

Distribution of twist in mesodermal cells

Anteroposterior specification Maternal genes, bicoid, nanos Segment genes refine the pattern Zygotic genes Six gap genes: Coarse subdivision Pair rule genes: Segment alteration Segment-polarity genes: Homeotic selector genes

Anteroposterior specification Krupel lacks 8 segments (T1-A5) Even-skipped (eve) lacks odd- numbered parasegments Fushi tarazu (ftz) lacks even-numbered parasegments Gooseberry posterior half is the mirror image of anterior half

Pair rule genes Formation of parasegments Expression pattern of ftz (brown) and eve (gray)

Segment polarity genes Forms parasegments polarity Involves cell-cell interactions Associated with two signaling pathways Wnt Hedgehog

Regulatory hierarchy

The Nobel Prize in Physiology or Medicine 1995 "for their discoveries concerning the genetic control of early embryonic development"                              Edward B. Lewis Christiane Nüsslein-Volhard Eric F. Wieschaus     1/3 of the prize USA Federal Republic of Germany California Institute of Technology Pasadena, CA, USA Max-Planck-Institut für Entwicklungsbiologie Tübingen, Federal Republic of Germany Princeton University Princeton, NJ, USA b. 1918 d. 2004 b. 1942 b. 1947

Homeotic mutations

Homeotic selector genes Hox gene complex Antennapedia complex Bithorax complex Contain homeodomain 60 amino acids DNA binding region Regulate positional information

Hox gene complex

Expression of hox gene complex

Hox gene complex

Comparison of hox gene expression

Comparison of hox gene expression

Organogenesis and patterning of appendage

Methods in fly genetics: Somatic mutations Mosaic

Methods in fly genetics: Enhancer trap

Imaginal discs Groups of cells set aside undifferentiated 19 discs (9 pairs + 1 genital) Develop into organs such as leg, wing, eyes etc.

Specific ID genes define organs Distal-less  expressed in appendages Pax-6  expression in eyes

Wing formation Sector formation in wing disc Four compartments foms engrailed, apterous genes are involved

Wing formation

Wing formation (A) The shapes of marked clones in the Drosophila wing reveal the existence of a compartment boundary. The border of each marked clone is straight where it abuts the boundary. Even when a marked clone has been genetically altered so that it grows more rapidly than the rest of the wing and is therefore very large, it respects the boundary in the same way (drawing on right). Note that the compartment boundary does not coincide with the central wing vein. (B) The pattern of expression of the engrailed gene in the wing, revealed by the same technique as for the adult fly shown in Figure 21–40. The compartment boundary coincides with the boundary of engrailed gene expression.

Limb formation

Bristle formation achaete, scute genes -HLH -Proneural genes Scute expression in wing disc

Lateral inhibition Notch-delta

Lateral inhibition Notch somatic mutation Loss of lateral inhibition Bristle patches

Bristle formation Numb gene -Block Notch gene activity

Bristle formation Planar polarity genes Orienting bristle backward position frizzled proteins  control planar polarity dishevelled  downstream of frizzled

Bristle formation