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Drosophila dorsal/ventral axis detemination How are different tissue types specified at distinct positions on the embryonic dorsalventral axis? Cell fate specification at the blastoderm stage amnioserosa dorsal dorsal ectoderm neuroectoderm mesoderm ventral fate map mesoderm formation Dorsal-Ventral fate map Genes identified in a famous screen for Drosophila mutants with embryo patterning defects Torpedo Gurken Localized maternal mRNA sets up anterior and posterior poles Gurken protein specifies the Anterior-Posterior axis of the Drosophila embryo during oogenesis (Similar to EGF) Gurken also signals dorsal pole formation during oogenesis oocyte nucleus 1-6 A + 7 8 D migration of nucleus 10A D - P anterior posterior V V follicle cells microtubules 8 10A 1-6 + gurken expression in the oocyte gurken expression in the oocyte Expression of the Gurken Message and Protein Between the Oocyte Nucleus and the Dorsal Anterior Cell Membrane Gurken signaling inhibits production of an extracellular signal (Spätzle) by follicle cells follicle cells Torpedo = EGF receptor (in follicle cells) Gurken = Epidermal Growth Factor (EGF) Oocyte pipe expression X Ventral follicle cell Pipe (Golgi?) X Wind (ER?) Nucleus X modified from van Eeden & St.Johnston Maternal effect mutations in dorso-ventral patterning Somatic (follicle cells) nudel, pipe, wbl ndl, pipe, wbl dorsal RNA gd, snk, ea - serine proteases Spätzle Toll protein spz - ligand Spätzle protein Germline (nurse cells) Toll Dorsal protein Tl - membrane receptor Dorsal protein tube - cytoplasmic protein pelle - ser/thr protein kinase cactus amnio serosa cact - cytoplasmic inhibitor dorsal ectoderm of Dorsal nuclear translocation dorsal neuroectoderm dl - transcription factor (morphogen) mesoderm Dl nuclear protein dorsal and cactus mutants (maternal germline effect) dorsal Wild type dorsal cactus mutant mutant A7 A8 A6 A5 A4 A3 A2 T3 A1 T2 T1 ventral dorsalized ventralized Translocation of Dorsal protein into ventral nuclei but not lateral or dorsal nuclei Wild type toll mutant cactus mutant Generation of Dorsal-Ventral Polarity in Drosophila Generation of Dorsal-Ventral Polarity in Drosophila The Toll pathway in dorso-ventral pattern formation Inject wild-type cytoplasm into toll mutant eggs toll mutant ventral dorsalized local rescue Wild type dorsal ectoderm dorsal neuro-ectoderm (denticle belts) mesoderm polarity reversal Conserved pathway for regulating nuclear transport of transcription factors in Drosophila and mammals Cells with highest nuclear Dorsal levels become mesoderm Zygotically expressed genes Action of Dorsal protein in ventral cells Action of Dorsal protein in ventral cells High affinity for promoter, Not much Dorsal needed to activate Action of Dorsal protein in ventral cells Lower affinity for promoter, More Dorsal needed to activate Zygotically expressed genes Action of Dorsal protein in ventral cells Snail repression of rhomboid creates domains with distinct gene expression patterns Dorso-ventral pattern formation: summary nudel, pipe, windbeutel dorsal RNA Toll protein Spätzle protein Dorsal protein Dorsal protein dpp twist oocyte nucleus dorsal > repression of ventral fate in dorsal follicle cells ventral production of ligand > activation of Toll receptor > graded nuclear uptake of Dorsal morphogen > regulation of zygotic target gene expression > cell fates along DV axis Use of a similar regulatory system to pattern insects and vertebrates Patterns ectoderm in Drosophila Patterns mesoderm in vertebrates Gastrulation in Drosophila Schematic representation of gastrulation in Drosophila Anterior-posterior patterning in Drosophila The fly body plan: each segment has a unique identity and produces distinctive structures 3 head 3 thorax 8 abdomen Mutations affecting the antero-posterior axis 3 independent maternal systems: anterior, posterior, terminal fate map larva active systems triple mutants active systems A P T wild-type single mutants anterior bicoid posterior oskar terminal torso - - - double mutants - P T - P - A - T - - T A P - A - additive phenotypes Maternal effect mutations Zygotic effect mutations bicoid mutant phenotype Embryo from wild-type mother Embryo from bicoid mother Anterior: bicoid is required for head and thorax Wild type bicoid mutant blastoderm fate map head + thorax abdomen abdomen Bicoid mRNA localization in embryo (tethered to microtubules) Nuclei divide without cell division in Drosophila to produce a syncytial blastoderm embryo Fig. 9.1 Bicoid protein gradient in syncytial blastoderm embryo - diffuses after translation from localized mRNA - protein unstable
