Neurulation & Organogenesis

Neurulation Neural cells originate from the neurectoderm Clusters of proneural cells are specified by the proneural genes Single neuroblasts are specified within the proneural cluster via lateral inhibition Proneural and neurogenic genes, respectively, regulate these specification events

Neurogenesis

Proneural & Neurogenic Genes Proneural: Achaete-Scute complex of genes (AS-C) –Achaete (ac), scute (sc), lethal of scute (lsc), asense (as) Neurogenic: Notch-Delta pathway –Notch (N), Delta (Dl), Enhancer of split (E(spl)), Hairless (H), Suppressor of Hairless (Su(H)), brainiac (brn), mastermind (mam), neurlalized (neu), kuzbanian (kuz), big brain (bib)

Acheate Expression en = purpleacheate = black stain

Regulation of Proneural Genes hairy (h), gooseberry (gb), Extramacrochaete (emc) –Emc & h are inhibitory bHLH txn factors –Gb has a homeobox ventral nervous system defective (vnd), engrailed (en), wingless (wg), iroquois (iro)

Neurogenic Genes Juxtacrine signaling molecules Delta (Dl) - ligand Notch (N) – receptor kuzbanian (kuz) – protease kuz Su(H) E(spl)

Delta-Notch Signaling by kuz

Lateral Inhibition Summary Delta and Notch are expressed in every cell in the proneural cluster Dl is usually slightly more highly expressed than N Dl activates N N activates Su(H) Su(H) displaces H and activates E(spl) E(spl) inhibits AS-C genes & indirectly, Dl Without AS-C gene products, neural differentiation is blocked AS-C normally activates Dl and other proneural genes

Drosophila Neurogenesis & Vertebrate Neurogenesis Occur Via Homologous Pathways Mash – mouse acheate-scute homologue Dll-1 – delta-like 1

Delta-Notch Signaling in Vertebrates

Drosophila Peripheral Nervous System proneural clusters form in imaginal discs proneural clusters differentiate into SOPs and ultimately into sensory bristles

Differentiation of Sensory Bristles numb & Notch mutations SMC (sensory mother cells) also called SOP (sensory organ precursor) no SMC

Organogenesis Imaginal Disc

Imaginal Discs

Initial Specification of Imaginal Discs Intersection of wg stripes with Dpp expressing cells specifies discs Leg Disc Primordia Wing/Haltere Disc Primordia

Patterning of Wing Discs Anterior-Posterior specification established by en, & hh with Dpp at the border

Dorsal-Ventral Polarity in Wing Disc Apterous protein (green) dorsal wing Vestigial protein (red) ventral wing Wing Disc

Patterning of Wing Discs dorsal-ventral patterning is established by apterous & vestigial expression with wg establishing the border

Patterning of Wing Discs Vestigial (green), apterous (red), wingless (purple), cubitus interuptus (blue)

Patterning of Leg Discs

Eversion of Leg Imaginal Disc

Proximal-distal Axis in Leg Disc Expression of txn factors in regions of leg disc specifies proximal-distal polarity distalless (dll)(red), dachshund (dac) (green), homothorax (htx) (purple)

Eye Disc Specification Twin of eyeless (Pax6) Eyeless (Pax6) Eyes absent (eya1, 2, 3) Sine oculus (Six3) dachshund

Ectopic Expression of ey

Patterning of the Eye Disc

Differentiation in the Eye Disc A P Morphogenic Furrow

Progress of Morphogenetic Furrow & Cell Specification in Its Wake

Rhabdomere Specification

Drosophila Compound Eye Ommatidium 20+ cells; 8 photoreceptors 12 accessory cells 800 Ommatidia

Drosophila ommatidium

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Analysis of R7 Differentiation through Genetic Mosaics sev only needed in R7 for R7 differentiation sev functions cell autonomously