Axis Specification in Dro sophila Developmental Biology – Biology 4361 July 9, 2008Drosophila Development Overview 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 the point of fertilization. Sperm compete with each other! Romano Dallai Drosophila bifurca Sperm enter at the micropyle. probably prevents polyspermySuperficial Cleavage Syncytial blastoderm stage zygotic nuclei undergo 8 divisions nuclei migrate to periphery karyokinesis continues Cellular blastoderm stage following division 13, oocyte plasma membrane folds inward partitions off each nucleus and associated cytoplasm constricts at basal endSuperficial Cleavage in Drosophila cellular blastodermGastrulation germ band movement ventral furrow (primordial germ cells) anterior midgut invagination posterior midgut invaginationGastrulation Ventral FurrowEarly Gastrulation Ventral Dorsal Pole cellsMidGastrulation fullest germ band extension: just prior to segmentation germ band cells: form trunk of the embryo thorax and abdomenLate Gastrulation / Segmentation organogenesis segmentation segregation of imaginal discs nervous system development germ band mo veme ntEstablishing the Drosophila Body Plan Segments form along the anteriorposterior axis, then become specialized. Specification of tissues depends on their position along the primary axes. A/P and D/V axes established by interactions between the developing oocyte and its surrounding follicle cells Head segment Thoracic segments T1 – legs T2 – legs & wings T3 – legs & halteresDrosophila Body Plan Egg Stage Translation leads to formations of patterning protein (e.g. morphogen) gradient within the embryo. Body axes are determined in the egg by distribution of maternal mRNAs and proteins. How are asymmetric distributions of messages and proteins established in the egg?Oocyte Formation (AP, DV Axes) modified from Kalthoff, K. 2001. Analysis of Biological Development. Drosophila ovariole oogonium divides into 16 cells 1 oocyte 15 nurse cells all interconnected nurse cells contribute mRNA, proteins cytoplasmAnteriorPosterior Axis Formation Torpedo (Gurken receptor) present on follicular cells Gurkin binding results in “posteriorization” of follicles posteriorized follicles reorganize egg microtubules; () = anterior Gurkin protein localized between nucleus and cell membrane Note – Gurken diffuses only a short distance Nurse cells synthesize g urken message gurken mRNA transported toward oocyte nucleus (in posterior region)Microtubules Tubulin a b dimer protofilament (+) () microtubule (+) () Andreas Merdes disassembly growth (+) () (+) () kinesin dyneinAP Axis: bicoid / Oskar / nanos Nurse cells manufacture bicoid and nanos 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 mRNA retains nanos in posterior end “posteriorized” follicles produce organized (+/) microtubulesDV Axis – Further Gurken Effects The oocyte nucleus (with associated gurken) moves anteriorly along the dorsal margin Gurkin/Torpedo interactions “dorsalize” follicle cells results in Dorsal activation Oocyte Pipe (ventral cells) eventually triggers nuclear Toll receptor activity; Dorsal determines ventral fates Syncitial blastoderm Gurken/Torpedo inhibits Pipe synthesis in dorsal cells DV PolarityDistribution of Dorsal Dorsal activates genes that create mesodermal phenotype transcribed only in cells with highest Dorsal concentrations Dorsal also inhibits dorsalizing genes these genes have low affinity enhancers (lots of Dorsal necessary) Dorsal: large amount = mesoderm lesser amount = glial/ectodermal mesodermal cells that will invaginate to form ventral furrow DorsalZygotic Patterning Genes Dorsal (TF) – expressed ventrally; establishes diffusion gradient dorsally decapentaplaegic (dpp), zerknüllt (zen), tolloid are dorsal patterning genes repressed by Dorsal High Dorsal – activates twi st and snail (low affinity enhancers) mesoderm determinants Intermediate dorsal activates rhomboid (no Twist or snail) determines neural ectoderm rhomboid + twist = glial cells Intermediate dorsal activates fgf8 fgf8 repressed by snail promotes mesodermal ingression dorsalAnteriorPosterior Body Plan Drosophila use a hierarchy of gene expression to establish the anteriorposterior body plan. 1. Maternal
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