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A Fly by Any Other Name …

Drosophila melanogaster. A Fly by Any Other Name …. Developmental Genetics. Drosophila is one of the best understood developmental model systems Genome sequenced – 1999 100s of characterized mutant strains Easily maintained & propagated Large, easily manipulated embryos. Gastrulation.

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A Fly by Any Other Name …

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  1. Drosophila melanogaster A Fly by Any Other Name …

  2. Developmental Genetics • Drosophila is one of the best understood developmental model systems • Genome sequenced – 1999 • 100s of characterized mutant strains • Easily maintained & propagated • Large, easily manipulated embryos

  3. Gastrulation Ventral Views Dorsal View Cephalic Fold Ventral Furrow Pole Cells

  4. Germ Band Extension & Retraction Lateral Views Full Extension Retracted Note Segmentation Mandibular Maxillary Labial A8 T1 T2 T3 A7 A6 A1 A5 A1 A4 A3

  5. 1st Instar Larva Cuticle Denticle Bands Telson

  6. Morphogenesis

  7. Segmentation in Larva & Adult Thorax

  8. Hierarchy of Gene Expression Maternal Effect Genes

  9. Localization of Proteins Gap Genes Hunchback (orange), Krüppel (green) Maternal Effect Genes - Bicoid Pair-rule Genes - Fushi tarazu Segment Polarity Genes - Engrailed

  10. Maternal Effect Genes • Maternally transcribed genes • mRNA transported and localized within oocyte • Anterior Group • Bicoid (bcd), Hunchback (hb) • Posterior Group • Nanos (nos), caudal (cad) • Termini Group • Torso (tor), torso-like (tlk)

  11. Identification of Three Polarity Gradients in Drosophila • 1980 • Christiane Nüsslein-Volhard • Eric Wieschaus • Initiated mutagenic screen for developmental regulatory genes • One of 1st genes identified caused an anterior defect • Went on to identify mutations causing posterior & termini defects as well as dorsal-ventral polarity defects

  12. Bicoid Anterior Mutations

  13. Bicoid Anterior Mutations

  14. Three A-P Morphogens

  15. Protein Gradients from Maternal Effect Genes • Bcd mRNA localized to anterior, nos mRNA localized to posterior, cad & hb mRNAs unlocalized • Bcd blocks translation of cad in ant compartment • Nos blocks translation of hbin posterior compartment • Localized translation of bcd & nos causes secondary gradients of hb & cad proteins

  16. Pathways of A-P Patterning

  17. Initial Establishment of A-P Axis by Gurken Signaling • Posterior pole is 1st region to be specified • Polarized organization of microtubules • Localization of maternal mRNAs and proteins • Oocyte nucleus 1st positioned at posterior pole • Gurken (grk) protein translated in perinuclear regions • grk protein accumulates in posterior and activates the RTK torpedo in follicle cells • Gurken is an EGF homolog • Torpedo is an EGF Receptor homolog

  18. Posterior specification by grk/tor signaling Activated tor in posterior follicle cells alters cell adhesion between follicle cells and oocyte in turn repolarizing microtubules in oocyte which move nucleus to the future dorsal side On dorsal side, grk/tor signal sets off dorsal specification pathway

  19. Localization of Maternal Posterior Determinants • Grk induced repolarization of microtubules results in scaffolding for oskar localization • Localization of oskar directs localization of tudor, vasa, and valois • These proteins in turn bind and localize nanos mRNA • Nos mRNA remains untranslated until fertilization due to masking by the protein smaug

  20. Function of Posterior Morphogen Nanos • Translational Repressor • Represses translation of bcd & hb • Requires pumilio (pum) as co-factor • Pum binds bcd and hb 3’ UTRs • Alone pum promotes pA addition • Pum recruits nos to 3’ UTR which then promotes deadenylation of bcd and hb preventing translation • Nos & osk are also required for formation of pole plasm (germ plasm)

  21. Localization of Maternal Anterior Determinants • Bicoid is localized by exuperantia (exu), exuperantia-like (exl), and swallow (swa) proteins • Exl binds bcd mRNA • Exu binds exl & bcd mRNA • Swa binds the bcd-exl-exu mRNP • Swa binds microtubules thus anchoring the entire complex of mRNA and proteins

  22. Function of Anterior Morphogen Bicoid • Transcriptional Activator & Repressor • Activates transcription of hunchback and some gap genes • Represses transcription of some gap genes, pair-rule genes, and segment polarity genes • Translational Repressor • Repressed translation of caudal mRNA

  23. Specification of the Terminal Segments • Acron – anterior most segment • Telson – posterior most segment • Both regions are specified with via the same signaling pathway • Torso – RTK expressed ubiquitously in oocyte • Torso-like – ligand expressed only in posterior and anterior follicle cells

  24. Specification of Termini groucho represses transcription of tailless and huckbein. Tor signal pathway inhibits groucho allowing tll & hkb to function These proteins specify both termini. bcd + hkb + tll defines acron w/o bcd, tll & hkb specify telson

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