Drosophila dorsal/ventral axis detemination How are different tissue types specified at distinct positions on the embryonic dorsal- ventral axis?

amnio- serosa dorsal ectoderm neuro- ectoderm mesoderm Cell fate specification at the blastoderm stage mesoderm formation fate map dorsal ventral

Dorsal-Ventral fate map

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

Localized maternal mRNA sets up anterior and posterior poles

Gurken protein specifies the Anterior-Posterior axis of the Drosophila embryo during oogenesis (Similar to EGF)

Gurken also signals dorsal pole formation during oogenesis follicle cells anteriorposterior AP V D D V microtubules A gurken expression in the oocyte 10A gurken expression in the oocyte 1-6 migration of nucleus oocyte nucleus

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

Gurken signaling inhibits production of an extracellular signal (Spätzle) by follicle cells follicle cells Oocyte pipe expression Ventral follicle cell Pipe (Golgi?) X X Nucleus Wind (ER?) X X X X modified 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) tube - cytoplasmic protein pelle - ser/thr protein kinase Somatic (follicle cells) ndl, pipe, wbl gd, snk, ea - serine proteases Spätzle spz - ligand Germline (nurse cells) Dorsal protein dorsal RNA Toll protein Spätzle protein Dorsal protein nudel, pipe, wbl amnio serosa dorsal ectoderm neuro- ectoderm mesoderm Dl nuclear protein Maternal effect mutations in dorso-ventral patterning

Wild type ventralized dorsal mutant cactus mutant dorsalized T1 T2 T3 A1A2 A3A4A5 A6 A7 A8 dorsal and cactus mutants (maternal germline effect) 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

Wild type toll mutant Inject wild-type cytoplasm mesoderm neuro-ectoderm (denticle belts) dorsal ectoderm The Toll pathway in dorso-ventral pattern formation 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

High affinity for promoter, Not much Dorsal needed to activate

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

Zygotically expressed genes

Action of Dorsal protein in ventral cells Snail repression of rhomboid creates domains with distinct gene expression patterns

twist dpp Dorsal protein dorsal RNA Toll protein Spätzle protein Dorsal protein nudel, pipe, windbeutel Dorso-ventral pattern formation: summary oocyte nucleus dorsal > repression of ventral fate in dorsal follicle cells ventral production of ligand > activation of Toll receptor > graded nuclear uptake of Dorsal morphogen > regulation of zygotic target 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

Anterior-posterior patterning in Drosophila

The fly body plan: each segment has a unique identity and produces distinctive structures 3 head 3 thorax 8 abdomen

fate maplarva wild-type anteriorbicoid posterioroskar terminaltorso Mutations affecting the antero-posterior axis 3 independent maternal systems: anterior, posterior, terminal single mutants double mutants triple mutants additive phenotypes active systems A P T - P T A - T A P P T A - - active systems

Maternal effect mutations

Zygotic effect mutations

Embryo from wild-type mother Embryo from bicoid mother bicoid mutant phenotype

Wild type blastoderm fate map bicoid mutant Anterior: bicoid is required for head and thorax abdomen head + thorax

Bicoid mRNA localization in embryo (tethered to microtubules)

Nuclei divide without cell division in Drosophila to produce a syncytial blastoderm embryo Fig. 9.1

Bicoid protein gradient in syncytial blastoderm embryo - diffuses after translation from localized mRNA - protein unstable