Axis Specification in Drosophi la Developmental Biology – Biology 4361 November 2, 2006Axis Specification in Drosophila Superficial cleavage Fertilization Gastrulation Drosophila body plan Oocyte formation Genetic control of axis specification Anteriorposterior Dorsalventral Segmentation genes Homeotic genesDrosophila Fertilization Eggs are activated prior to fertilization. oocyte nucleus has resumed meiotic division stored mRNAs begin translation Eggs have begun to specify axes by fertilization. Sperm enter at the micropyle. probably prevents polyspermy Sperm competitionSuperficial Cleavage in DrosophilaCleavage Superfici al cleavage zygotic nucleus undergoes 8 divisions (256 nuclei) nuclei migrate to periphery karyokinesis continues Cellula r blastoderm following division 13, oocyte plasma membrane folds inward partitions off each nucleus and associated cytoplasm constricts at basal end Pole c ells at the 9 th cycle ~ 5 nuclei reach the posterior pole enclosed by membranes primordial germ cell (gamete) precursorsGastrulation germ band movement ventral furrow anterior midgut invagination posterior midgut invaginationEarly Gastrulation Pole cellsMidGastrulation germ band cells: form trunk of the embryo thorax and abdomen fullest germ band extension: just prior to segmentationSegmentation germ band movement organogenesis segmentation segregation of imaginal discs nervous systemFirst Larval InstarGenes that Pattern the Body Plan Anteriorposterior and dorsalventral axes established by interactions between the developing oocyte and its surrounding follicle cells Dorsalventral patterning gradients are formed within the embryo. Segments form along the anteriorposterior axis, then become specialized. Specification of tissues depends on their position along the primary axes.Drosophila Body Plan Thoracic segments T1 – legs T2 – legs & wings T3 – legs & halteres AP axis DV axis SegmentationOocyte Formation (AP, DV Axes) Drosophila ovariole oogonium divides into 16 cells 1 oocyte 15 nurse cells all interconnectedAnteriorPosterior Axis Formation nurse cells synthesize gurken gene (FGF homologue) transported to oocyte nucleus localized between nucleus and cell membrane Torpedo Gurken receptor (FGF receptor homologue) Gurkin signal results in “posteriorization” of follicle follicles send signal to reorganize microtubule systembicoid / Oskar / nanos Nurse cells manufacture bicoid mRNA deliver cytoplasm into oocyte bicoid binds to dynein moves to nongrowing () end of microtubules oscar mRNA forms complex with kinesin I moves toward growing (+) end of microtubules Oskar binds nanos message retains nanos in posterior end “posteriorized” follicles produce polarized microtubulesGurken Dorsalizes Follicle Cells Oocyte nucleus moves anteriorly along the dorsal margin Gurkin/Torpedo interactions “dorsalize” follicle cellsDorsalVentral Polarity Gurken/Torpedo inhibits Pipe synthesis Pipe (ventral) triggers nuclear Toll receptor activity; results in Dorsal activation Dorsal determines ventral fatesDistribution of Dorsal Dorsal: large amount = mesoderm lesser amount = glial/ectodermal mesodermal cells that will invaginate to form ventral furrow Dorsal Dorsal activates genes that create mesodermal phenotype transcribed only in cells with highest Dorsal concentrations genes with low affinity enhancers (lots of Dorsal necessary) some of these gene products bind to/inhibit others Dorsal also inhibits dorsalizing genesAnteriorPosterior Body Plan Drosophila use a hierarchy of gene expression to establish the anteriorposterior body plan. 1. Maternal effect genes (e.g. bicoid, nanos) mRNAs differentially placed in eggs transcriptional or translational regulatory proteins diffuse through syncytial cytoplasm activate or repress zygotic genes 2. Gap genes: first zygotic genes expressed expressed in broad, partially overlapping domains about 3 segments wide 3. Pairrule genes; differing combinations of gap genes regulate transcription divide the embryo into periodic units results in a pattern of seven transverse bands 4. Segment polarity genes; activated by pairrule genes divide embryo into 14 segmentwide units 5. Homeotic selector genes; stimulated by interactions of gap, pairrule, and segment polarity proteins determines developmental fate of each segmentAnterior 1 Bicoid binds to caudal 3’UTR; prevents translation Caudal specifies posterior domainAnterior 2 Hunchback – anterior patterningBicoid Mutants Martin KlinglerMessin’ with BicoidPosterior 1 Nanos prevents hunchback translation nanos trap: Staufen allows oskar translation Oskar binds nanosPosterior 2Model of AnteriorPosterior Patterning Oocyte mRNAs Early cleavage embryo proteins hunchback translation repressed by Nanos caudal translation repressed by BicoidTerminal 1 Torso – transmembrane RTK Torso uniformly distributed Torso activated by Torsolike protein located only at ends of eggTerminal 2 Distinction between anterior and posterior = Bicoid Bicoid = acron formation Torso kinases inactivate an inhibitor of tailless and huckebein Tailless and Huckebein specify terminiSegmentation Genes Cell fate commitment: Phase 1 – specification Phase 2 – determination early in development cell fate depends on interactions among protein gradients specification is flexible; it can alter in response to signals from other cells eventually cells undergo transition from loose commitment to irreversible determination The transition from specification to determination in Drosophila is mediated by the segmentation genes. these divide the early embryo into a repeating series of segmental primordia along the anteriorposterior axisMaternal effect genes Gap genes Pairrule genes Segme nt polarity genes bicoid nanos hunchback huckebein evenskipped fushi tarazu engrailed wingless hedgehog patched giantfushi tarazu – pairrule gene Segments and Parasegments expression patterns in early embryos are not delineated by segmental boundaries; parasegments appear to be the fundamental units of embryonic gene expression segments and parasegments organize compartments out of phase cells of adjacent compartments do
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