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
Maternal Effect Genes - Bicoid
Gap Genes Hunchback (orange), Krüppel (green)
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