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Anterior-posterior patterning in Drosophila The fly body plan: Each segment has a unique identity and produces distinct 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 Wild-type Bicoid promotes anterior fates and inhibits posterior fates. 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 Transplantation of egg cytoplasm An organizer of the anterior-posterior pattern is located at the anterior pole wt abdomen only polarity normal wt rescue of head in the center pattern polarity reversal wt thorax at posterior pole polarity reversal bicoid mRNA induces head and thorax bicoid mRNA bicoid (bcd) gene encodes a homeo-domain transcription factor Injection of bicoid mRNA: anterior (head) structures at site of injection & reorganization of polarity no head Bicoid protein: transcriptional and translational regulator zygotic target genes (promotes anterior fates) maternal target mRNA (inhibits posterior fates) Transplantation of egg cytoplasm Posterior cytoplasm also has polarizing activity wt abdomen only polarity normal wt rescue of head in the center pattern polarity reversal wt thorax at posterior pole polarity reversal wt double abdomen polarity reversal 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 Nanos is the maternal effector of the posterior system mutant rescued rescued rescue of all posterior-system mutants by injection of nanos mRNA Embryonic polarity genes Anterior-Posterior pattern formation in flies The Bcd gradient is converted into domains of gene expression Bcd protein binds differentially to enhancers of target genes Different thresholds of Bcd concentration are required to turn on different genes low affinity high affinity target genes are zygotically expressed Gap genes Bcd gradient and expression domains of target genes bcd mRNA Bcd protein target genes Bicoid and Nanos regulate Gap gene expression Expression patterns of proteins encoded by Gap genes Gap gene mutants lack different body regions Gap gene mutants lack different body regions Wild type Krüppel hunchback knirps The gap genes regulate each other and form domains with distinct combinations of gene expression. Hunchback Krüppel Anterior-Posterior pattern formation in flies Pair-rule mutants Wild type fushi tarazu mutant Even-skipped expression pattern Modularity of the Drosophila even-skipped promoter 08_18_reporter.gene.jpg Regulation of expression stripe no. 2 of Even-skipped (eve) hunchback giant Krüppel eve stripe #2 activator repressor parasegment 1 repressor 2 3 4 5 multiple binding sites in enhancer of eve repressors activators Regulation of the Second Stripe of Transcription from the even-skipped Gene Regulation of the even-skipped gene Refinement of expression domains over time early Fushi tarazu expression late Eve, Ftz expression Refined expression domains in distinct cell rows Anterior-Posterior pattern formation in flies Segment polarity mutants Segment polarity mutants Wingless signaling specifies cell fates in the ventral epidermis Anterior cells make Hair Posterior cells make Naked cuticle Wild type arm mutant Segment polarity genes – 14 stripes A L ap fg 13 12 11 10 9 hg 0 1 2 3 4 5 6 Expression of segment polarity gene wingless 7 8 Segments and Parasegments The Even-skipped and Fushi tarazu pair-rule transcription factors activate the segmentpolarity gene Engrailed Intercellular feedback maintains pair-rule gene expression states Intercellular feedback maintains pair-rule gene expression states =Wnt Wnt signaling pathway - + - + - + + - - + - + Gradients of Wingless and Hedgehog pattern each segment Anterior-Posterior pattern formation in flies
