Drosophila melanogaster development

How do Drosophila embryos develop?

How can one use genetics to find genes that regulate embryo development?

DROSOPHILA

LIFE CYCLE

4 days

4 days

1 day

1 day

2 days

Life cycle of Drosophila

egg

larva1st instar

larva2nd instarlarva

3rd instar

pupaembryogenesis

female

OOGENESIS IN DROSOPHILA

Germarium

ring canal

Germline Cyst Formation

Cystoblast

Pro-Oocyte(undergoes meiosis)

germline stem cells follicle

stem cells

germline: stem cell > cystoblast > 1 oocyte + 15 nurse cells

OOGENESIS IN DROSOPHILA

Germarium Vitellariumnurse cell oocyte

stalk

follicle cells

border cells

germline stem cells follicle

stem cells

oocyte

oocyte + nurse cells surrounded by (somatic) follicle cells

Drosophila oocyte and supporting cells

Nurse cells Ring canals Oocyte Follicle cells

(from Gonzalez-Reyes and St Johnston (1994) Science 266: 639-642.)

Drosophila oocyte and supporting cells

Nurse cell nuclei Follicle cell nuclei

(from Gonzalez-Reyes and St Johnston (1994) Science 266: 639-642.)

Nuclear divisions start without cell division in Drosophila (superficial cleavage)

Fig. 9.1Zygotic gene expression begins

egg blastoderm fate map larva

T

A

Larvae already have substantial patterning

acron

head

thorax

abdomen

telson

T1

T2T3A1A2A3

A4A5A6A7A8

(cuticle)

“stripy” expressionof segmentation gene

fushi tarazu (ftz)

anterior

posterior

ventral dorsal

epithelium(6,000 cells)

The fruit fly body plan is assembled in 24 hours: How?

Christiane Nüsslein-Volhard and Eric Wieschaus used genetics to identify proteins that

set up the embryonic body plan

Wieschaus and Nüsslein-Volhard

looked for mutants that affect the fly body plan

wildtype

Genes identified in a famous screen for Drosophila mutants with embryo patterning defects

Screen for developmental mutants (Drosophila)Lethal hits = 100% (essential genes - ca. 5.000)

(efficiency of mutagenesis = number of hits per gene)

embryonal-lethal mutantszygotic mutants

25 %with morphological defects 3 %- segmentation defects (AP) 0.5 %- tissue types defective (DV) 0.5 %

female-sterile mutants 8 %

100 %

with effects on embryogenesis 2 %(= maternal-effect mutants)

- antero-posterior pattern 0.4 %- dorso-ventral pattern 0.3 %

ca. 2% of all genes involved in embryo pattern formation(male-sterile mutants)

(ca. 100 of >15.000 protein-encoding genes, only 5.000 essential genes)

Maternal-effect mutationsGenes expressed during oogenesis (= before fertilization)

or genes expressed in maternal cells (follicle) All progeny of heterozygous mother are normal.All progeny are affected only if mother is homozygous mutant

Zygotic mutationsGenes expressed during embryogenesis (= after fertilization)Only genetically mutant embryos are affected.(25% of progeny of heterozygous mother are affected.)

Drosophila axis detemination; dorsal/ventral polarity

How does the embryonic dorsal-ventral axis get translated into differentiation of different tissue

types?

amnio-serosa dorsal

ectoderm

neuro-ectoderm

mesoderm

Cell fate specification at the blastoderm stage

mesoderm formationfate map

dorsal

ventral

Dorsal-Ventral fate map

Gurken protein specifies the Anterior-Posterior axis of the Drosophila embryo during oogenesis

(Similar to EGF)

Localized maternal mRNA sets up anterior and posterior poles

Gurken also signals dorsal pole formation during oogenesis

follicle cellsanterior posterior

A P

V

D D

V

-+-

microtubules

71-6 810A

gurken expression in the oocyte

10A

gurken expression in the

oocyte

1-6

migration of nucleus

+-

-

8

oocyte nucleus

Expression of the Gurken Message and Protein Between the Oocyte Nucleus and the Dorsal Anterior Cell Membrane

DORSO-VENTRAL PATTERN FORMATIONfollicle cells

Oocyte

pipe expression

Ventral follicle cell

Pipe (Golgi?)

X

NucleusWind (ER?)

X

X

mod. from van Eeden & St.Johnston

Gurken = Epidermal Growth Factor (EGF)

Torpedo = EGF receptor(in follicle cells)

Toll Tl - membrane receptor

cactus cact - cytoplasmic inhibitor of Dorsal nuclear translocation

dorsal dl - transcription factor (morphogen)

Zygotic mutations tube - cytoplasmic proteinpelle - ser/thr protein kinase

Maternal effect mutations

ndl, pipe, wblgd, snk, ea - serine proteasesspz - ligand

Dorsal protein

dorsal RNA

Toll proteinSpätzle proteinDorsal protein

nudel, pipe, wbl

amnio serosa

dorsal ectoderm

neuro-ectoderm

mesodermDl

nuclear protein

Dorso-ventral pattern formation

dorsal

Wild type

ventralized

dorsal mutant

cactus mutant

dorsalized T1 T2 T3 A1 A2 A3 A4 A5 A6 A7 A8

Dorso-ventral pattern formation

dorsal

ventral

Wild type toll mutant cactus mutant

Translocation of Dorsal protein into ventral nuclei but not lateral or dorsal nuclei

Generation of Dorsal-Ventral Polarity in Drosophila

Generation of Dorsal-Ventral Polarity in Drosophila

Wild type

toll mutant

Inject wild-type cytoplasm

mesodermneuro-ectoderm

(denticle belts)

dorsal ectoderm

Dorso-ventral pattern formation pivotal role of Toll pathway

into toll mutant eggs

dorsalized

local rescue

ventral

dorsal

polarity reversal

Conserved pathway for regulating nuclear transport of transcription factors in Drosophila and mammals

Cells with highest nuclear Dorsal levels become mesoderm

Zygotically expressed genes

Action of Dorsal protein in ventral cells

Action of Dorsal protein in ventral cellsHigh affinity for promoter,Not much Dorsal needed to activate

Action of Dorsal protein in ventral cellsLower affinity for promoter,More Dorsal needed to activate

twist

dpp

Dorsal protein

dorsal RNA

Toll proteinSpätzle protein

Dorsal protein

nudel, pipe, windbeutel

Dorso-ventral pattern formation: summaryoocyte nucleus dorsal> repression of ventral fate

in dorsal follicle cells

ventral production of ligand> activation of Toll receptor

> graded nuclear uptakeof Dorsal morphogen

> regulation of zygotictarget gene expression

> cell fates along DV axis

Use of a similar regulatory system to pattern insects and vertebrates

Patterns mesoderm in vertebrates

Patterns ectoderm in Drosophila

Gastrulation in Drosophila

Schematic representation of gastrulation in

Drosophila