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U of M BIOLOGY 4361 - Axis Specification in Drosophila

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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 Anteriorposterior Dorsalventral 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 cellsMidGastrulation 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 Anteriorposterior and dorsalventral axes established by interactions between the developing oocyte and its surrounding follicle cells Dorsalventral patterning gradients are formed within the embryo. Segments form along the anteriorposterior 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 AP axis DV axis SegmentationOocyte Formation (AP, DV Axes) Drosophila ovariole  oogonium divides into 16 cells  1 oocyte  15 nurse cells  all interconnectedAnteriorPosterior 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 reorganize microtubule systembicoid / Oskar / nanos Nurse cells manufacture bicoid mRNA  deliver cytoplasm into oocyte bicoid binds to dynein  moves to nongrowing () 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 cellsDorsalVentral 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 genesAnteriorPosterior Body Plan Drosophila use a hierarchy of gene expression to establish the anteriorposterior 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. Pairrule 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 pairrule genes  divide embryo into 14 segmentwide units 5. Homeotic selector genes; stimulated by interactions of gap, pairrule, 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 AnteriorPosterior 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 Torsolike 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 anteriorposterior axisMaternal effect genes Gap genes Pairrule genes Segme nt polarity genes bicoid nanos hunchback huckebein evenskipped fushi tarazu engrailed wingless hedgehog patched giantfushi tarazu – pairrule 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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U of M BIOLOGY 4361 - Axis Specification in Drosophila

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