Drosophila melanogaster
Genetic studiesMicrosurgical manipulationOne of the best understood developmental systems
13,600 genes
Axis determinationSignaling pathwayTranscriptional regulation
P48-52
4 stages: embryo, larva, pupa, adult
Rapid division9 mins/division9 divisions
13 divisions
Single cell
Transgenic flies
One single epithelial layer –all tissuesMesoderm—muscle, connective tissuesEndoderm---midgut (foregut and hidgut-Ectoderm)Ectoderm---nervous tissue and epidermis
gastrulation
Larva hatch 24 hrsAcron: associated with headTelson:posterior terminal structure3 thorcic and 8 abdominal segments—specialization in cuticle(denticle belts and cuticular structure)
GenitaliaSex combPigmentationSmall wing
p. 421-431
Sex determining signal--- Sex-lethal (X chromosome)
Transformer-spliced + transformer 2
2X higher numerator Repression by autosome
Dosage compensation
Barr bodyXist-non-coding RNA
Male specific geneRepressed by Sxl
Primordial germ cell -special cytoplasmGerm plasm-polar granules, pole plasm
Oskar—organization and assembly of the pole plasmmRNA-posterior pole—3’ untranslated region
Polarization of the body axes during oogenesis
Cyst formation:
16 cell cyst enter a long S phase
only one (Oocyte) continues meiosis
Oocyte—4 ring canals
15 cells become nurse cells
after germarium nurse cells left meiotic cycle, grow rapidly without division, and form polytene chromosomes
A/P during oogenesisThe oocyte move towards one end in contact with follicle cellsBoth the oocyte and the posterior follicle cells express high levels of the E-cadherin
If E-cadherin is removed, the oocyte is randomly positioned.Then the oocyte induces surrounding follicle cell to adopt posterior fate.
Microtubule cytoskeleton reorganization is essential for localization of bicoid and oskar mRNA
Maternal effect mutations---reguired for pole plasm assemblyLack polar granules: grandchildless mutation (homozygote female— Progeny—sterile)
Central role: Oskar, Vasa, and TudorPole cell number = amount of oskar RNAEctopic pole cells: oskar RNA at the anterior pole
MtlrRNA (mitochondrically encoded large ribosomal RNA)+ gcl RNA for pole cell formation
antisense reduce pole cells mtlrRNA rescue UV-irradation
Vasa: DEAD-box RNA helicase—translational regulator
Germ cell—extragonadal origin, migrate to
reach the somatic gonada. posterior endb. gastrulation c. migrate dorsally through the wall of the
posterior midgutd. associate with the somatic gonadal prec
ursorse. GC align with somatic gonadal mesoder
mf. coalesce to form the embryonic gonad
Germ cell migration
PGC migration----Genes and mechanisms
Genetic screen—somatically expressed genesGuidance (cues):Wunen: repulsive signal (exclude migrating pole cells from wrong places)Misexpression wunen: transform a tissue permissive to PGC to repulsive onePhosphatidic acid phosphatase 2 (transmembrane protein) Columbus: factor (gonadal mesoderm) attracts pole cellsMisexpression Columbus—attract PGCs to tissues other than gonadal mesoderm3-hydroxy-3-methylglutaryl coenzymeA reductase (cholesterol biosynthesis in human, but fly does not make cholesterol) nanos, pumilio mutants stall at the outer gut surfacedifferentiate prematurely---act as complete migration to the somatic gonadsnanos target: RNA binding protein Sex lethal (Sxl)---splicing and translational regulation
also depend on specific germ plasm components, e.g polar granule component (Pgc)
Patterning of the fly embryo
Localized mRNA and ProteinsTranslated after fertilization—
Positional information to activate zygotic genes
parasegment
Pattern in the segment
Segment identities
Temporal sequence
Appendages:imaginal discs—pattern formationEctoderm invagination-epithelium(20-40 cells—larva 1000X)Specification occurs –segment being patterned-according to it
p.350-358
A/P and D/V compartment
Wing blade
Ventral fold under dorsal-double layers of epithelium
Signal region and the compartment
Maintain compartment boundaries—communication between compartmentsHh—10 cells, induces expression of Dpp through activation of Ci
The hedgehog signaling pathway
Without signal—Ci is processed as a repressor into nucleusWith signal---full length Ci acts as an activator inthe nucleus
Intercellular signaling set up PS boundary
Wg distributed asymmetrically—less in posterior (endocytosis and degradation)
TGFb , Activin: R-Smad 2,3BMPs: R-Smad 1, 5, 8Common Smad4Inhibitory Smads: I-Smad6, 7—recruting Smurf (ubiquitin ligase to receptor) Cell, 95,737,1998
Smad= Sma + MadSma-C. elegansMad-Fly
Dpp-secreted into both compartment
Long range signal—expression of spalt
Patterning the A/P axis of the wing disc
Dpp-morphogenLow level—ombHigh level—spalt1. Clones can’t respond to Dpp— no spalt and omb2. Ectopic hh-Dpp—localized activation of spalt and omb around the hh clones3. Ts mutant of dpp—reduction in the region with expression of lowThreshold genes—omb4. Clones expression low or highDpp—distinquish these two types of genes
Ectopic expression of Hh and Dpp
L4—compartment boundaryL3– HhL2 ---adjacent to cells expressing spalt
Ectopic Hh in posterior—no effectsIn anterior—mirrow-symmetric repeated pattern
Hh--Dpp
Expression of Wingless (green) and vestigial
Homeotic selector gene—apterous (Lmx-1)induces fringe and Serrate, then Notch receptor activation –Leading to Wingless expressionWg—achaete, distal-less, vestigial
Wingless (green)Vestigial (red)D/V boundaryDpp, Wg morphogenGFP-dpp active transportation—EndocytosisRegulate their receptorsDpp inhibits receptor—thick veinsDpp high--receptor low, and dpp lowReceptor high—1, prevent spreading2,cells reach threshold at low Dpp
Notch –transmembrane proteinDSL family—Delta, Serrate, Lag-2
Kuzbanian—cleave Notch ECDPresenilin—cleave Notch ICD
nervous system: selection of a single neuroblast (lateral inhibition)
Leg disc extension
Jointed tubes of epidermis—secrete the hard cuticle (exoskeleton), inside:Muscles, nerves
Fate map of the leg imaginal disc
Proximo-distal segmentCenter—distal end
Signaling centers in A/P compartment
Dpp, wg meets—Dll (distal end) homothorax (proximal)
Regional subdivision
Dpp, and Wg induce Dll and inhibit homothoraxActivates dachshund between Dll and hth
Butterfly wing pattern
Eyespot center—distal-less
Segmental identity of imaginal discHomeotic selector genesSimilar signal into different structures—Different interpretation—controlled by Hox genes
Antennapedia—PS4 and 5– 2 pairs of legsIf in head, antennae into legs (clones) –which part of the leg—depends on their position along the P/D axis (positional values are similar)
Hth (proximal) and Dll (distal)—in antennae and legIn combination as selector to specify antennaNo Hth, antenna into legIn leg: antennapedia prevents Hth and Dll acting togetherDominant antennapedia mutant (gene on)—blocks Hth and Dll in antennae disc, so leg forms
Imaginal discs and adult thoracic appendages
Bithorax mutation—Ubx misexpressed T3 into T2 –anterior haltere into Anterior wing
Postbithorax muation (pbx)—Regulatory region of the Ubx—Posterior of the haltere into wing
If both mutations—effect is additive—Four wings
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