Transcript Maternal Effect Genes

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
Ventral Views
Dorsal View
Cephalic
Fold
Ventral
Furrow
Pole
Cells
Germ Band Extension & Retraction
Full Extension Lateral Views
Note
Segmentation
Mandibular
Maxillary
Labial
A8
T1
A7
T2
T3
A6
A1
A5
A1
A3
A4
Retracted
1st Instar Larva
Cuticle
Denticle Bands
Telson
Morphogenesis
Segmentation in Larva & Adult
Thorax
Hierarchy of Gene Expression
Maternal Effect Genes
Localization of Proteins
Maternal Effect Genes - Bicoid
Pair-rule Genes - Fushi tarazu
Gap Genes Hunchback (orange),
Krüppel (green)
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 acron
w/o bcd, tll & hkb specify telson