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

GastrulationVentral Views Dorsal View

Ventral Furrow

Cephalic Fold

Pole Cells

Germ Band Extension & RetractionFull Extension RetractedLateral Views

Note Segmentation

Mandibular

Maxillary

Labial

T1

T2

T3

A1

A1

A8

A7

A6

A5

A4A3

1st Instar LarvaCuticle

Denticle Bands

Telson

Morphogenesis

Segmentation in Larva & Adult

Thorax

Hierarchy of Gene Expression

Maternal Effect Genes

Localization of Proteins

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

Pair-rule Genes - Fushi tarazu Segment Polarity Genes - Engrailed

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)

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

Bicoid Anterior Mutations

Bicoid Anterior Mutations

Three A-P Morphogens

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

Pathways of A-P Patterning

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

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

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

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)

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

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

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

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 acronw/o bcd, tll & hkb specify telson