What Characterizes Homeotic Genes?

• Specify the characteristic structures of each segment (segment identity) • In antennapedia and bithorax regions of chromosome 3 • Arranged in AP order Expression Homeotic Genes

• Expression pattern is established by gap genes and pair-rule genes – Later by segment polarity and other homeotics • Encode regulators • DNA binding region: “homeodomain” – 60 amino acid region makes base pair-specific contacts in DNA

Homeotic Mutants (one structure is replaced by another)

T3 has identity of T2 (ultrabithorax mutant) Homeotic Mutants (one structure is replaced by another)

antennae converted to legs (antennapedia mutant) Homeodomain

• Found in many regulators including homeotic complex, pair-rule, maternal and many organisms • Adjacent regions also contribute to recognition specificity • Well conserved in

Dorsal-Ventral Polarity

fate map • Familiar Themes – Maternal-effect genes – Gradients – Transcription factors – Cascade of events – Signaling with follicle cells

• New Themes – Proteases – Selective nuclear localization

Genetics

• Maternal-effect genes required for D-V polarity – Class I mutants have “dorsalized” (ie. no ventral structures) • e.g. dorsal, pipe, nudel, snake, easter, spatzle, Toll, pelle, tube – Class II mutants have “ventralized” phenotype (ie. no dorsal structures) • e.g. cactus, gurken, torpedo • Goal is to get dorsal into ventral nuclei Where is Dorsal In Mutants?

wild type dorsalized ventralized During Formation

• Begins with a D-V polarity imposed on follicle cells by the developing oocyte – Oocyte nucleus (having specified anterior) moves to anterior-dorsal position – Dorsalizing effect on nearby follicle cells – gurken and torpedo genes are required for this effect – Mutations cause ventralized embryos (and absence of dorsalized follicle cells) Where do Gurken and Torpedo Act?

• Set up artificial situation where follicle cells and oocyte have different : one is wt, the other is top- • Generate germ line chimeras by interchanging pole cells

wt wt

top- top- Where do Gurken and Torpedo Act?

wt wt normal

top- top- ventralized Site of Action

• torpedo is required in follicle cells – Is in all follicle cells – encodes EGF-like

• gurken is required in oocyte – gurken mRNA localized as oocyte nucleus moves to anterior dorsal – encodes a secreted EGF-like protein – anterior-posterior gradient of Gurken protein along dorsal surface

Gurken during Oocyte Formation

– anterior-posterior gradient of Gurken protein

What Does Gurken Do?

• Gurken signals nearby follicle cells – To take on dorsal morphology – To prevent expression of “ventralizing activity” (genes such as pipe and nudel) – Ventral follicle cells look different and do express “ventralizing activity” • Pipe activates Nudel to start protease cascade

Summary

Localized by nucleus Perivitelline Space Full of Inactive Proteases

• Encoded by gastrulation defective, snake, easter • Serine proteases secreted in inactive forms Protease Cascade Activated Ventrally

• Ventralizing activity (pipe, nudel) of ventral follicle cells initiates cascade • Gd -> snake -> easter • Each protease activated by cleavage

Protease Cascade

oocyte • Final cleaved substrate is spatzle – signal peptide • Binds to Toll receptor distributed uniformly on oocyte membrane • Since spatzle is only on ventral side, signal transduction only at ventral side Signal Transduction

• Toll activates pelle when presented by tube • Pelle kinase phosphorylates Cactus • Cactus releases its hold on dorsal (is degraded) • Dorsal enters nucleus • Net effect of signaling system is to translocate dorsal to ventral nuclei (graded) What Does Dorsal Protein Do?

• Transcription regulator – Activates expression of ventral-specific genes – Represses expression of dorsal-specific genes • Ventral target genes – Snail – Twist – Rhomboid • Dorsal target genes – Tolloid – DPP – Zerknullt

Network Interactions

• Twist: activation of meso genes • Snail: repression of non-meso genes • Rhomboid: neural ectoderm • Snail inhibits rhomboid • Dorsal inhibition of dorsalmost genes Summary

• of maternal and zygotic origins • Temporal order of • Boundaries by interactions of transcription factors • Translational regulations • Signal transduction pathways • Successive specification of fates