Topic 5d- drosophila axis development PDF

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Kevin Friesen ...

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Topic 5d: Drosophila Axis Development Parental Influence -

Nurse and follicle cells determine egg and embryo axes o Maternal mRNA o Paracrine signals o Juxtacrine signals

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Fertilization occurs through the micropyle  hole in chorion

Fertilization and Early Cleavage -

Centrolecithal yolk

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Superficial cell cleavage = cleavage occurs at rim of the egg (1st – 10th) o Cells do not form until after nuclei divide several times o Karyokinesis occurs to allow for syncytial specification

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9th – 10th  nuclei migrate outward and surround egg o Formation of syncytial blastoderm

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13th  PM expands around nuclei to make distinct cells o Cellular blastoderm forms o Everything is in syncytium until this point

Formation of Cellular Blastoderm -

Microtubule elongation  extend and push PM down into embryo

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Actin microfilaments constrict at basal surface  separate PM from basal surface

Mid-blastula Transition -

Switch from maternal mRNA to zygotic gene transcription

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Triggers formation of cellular blastoderm o Chromatin to cytoplasm ratio increases o Smaug  maternally derived protein that degrades maternal mRNA 

Becomes concentrated as cellular blastoderm forms

o Zelda  TF that regulates zygotic genes that regulate MBT

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Opens chromatin genes  more accessible

o Cyclin regulators  slow down cell cycle so more time is allowed for gap and growth phases

Gastrulation -

Ventral furrow formed by invagination of mesoderm cells

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Endoderm forms from anterior/posterior pockets

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Expansion of ectodermal cells by convergent extension to form germ band o Germ band = collection of cells along ventral midline that form trunk of embryo

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Ventral mesoderm tube

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Invagination of endoderm

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Convergent extension of ectoderm o Neural ectoderm is internalized as convergence and invagination continue

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Migration of pole cells

Katatrepsis, Anatrepsis, and Organogenesis -

Katatrepsis = tissues reorganize so posterior end is in the correct place

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Anatrepsis = tissues extending from ventral to dorsal side

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Organogenesis = forming of new organs

Larval segments give rise to corresponding adult segments -

Not a reorganization of the segments, but a regrowth of something new in the adult phase o Larval segments are not the same as the segments in an adult fly

Imaginal Cells -

Cluster within the larva that are waiting for a signal to differentiate

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3 main types: o Imaginal discs  form cuticular structures (wings, legs, antenna, thorax) o Histoblasts  form adult abdomen o Cluster of imaginal cells in each organ that will divide and form new organs

Syncytial Specification -

Bicoid  high concentration at anterior end, low concentration at posterior end

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Caudal  high concentration at posterior end, low concentration at anterior end

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Signals are established within the egg  progression of mitotic divisions and cellularization of blastoderm traps these signals

Body Axis Formation -

Anterior-posterior and dorsal-ventral polarity begins in the egg o A/P axis determined by: 

Bicoid  anterior

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Oskar/nanos  posterior

o D/V axis determined by: 

Gurken-torpedo  expressed and transcribed by oocyte and binds to torpedo on follicle cell 

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Establishes axis in unfertilized egg

Pipe-nudel dorsal

Posterior follicle cells initiate microtubule growth from posterior end of oocyte o Par1 localizes and binds microtubules to posterior end o Bicoid mRNA binds to dynein  goes towards anterior (-) end o Oskar mRNA binds to kinesin  pulls toward posterior (+) end

Embryonic Patterns -

Cells on ventral side of embryo destine to become mesoderm (site of gastrulation)

Dorsal-Ventral Genes -

Twist  specifies mesoderm

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Snail  suppresses non-mesoderm genes o Enhanced by dorsal; also specify neural tissue

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Zerkult  specifies amniosereosal cells

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Decapentaplegic  specifies dorsal ectoderm (BMP) o Suppressed by dorsal

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Dorsal expression by ventral nuclei leads to ventral specification of cells

Gene hierarchies determine the anterior-posterior body plan -

Maternal effect genes o Bicoid and nanos 

Hunchback and caudal

o Torso -

Zygotic genes

Segmentation Genes -

Gap genes  specify broad areas of the embryo o Loss of theses genes = loss of large regions of the body

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Pair-rule genes  divide embryo into periodic units o Loss of these genes results in every other segment missing

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Segment polarity genes  establish periodicity of embryo o Mutants have defects in every segment

Gap Genes -

Broad areas represented  lose these genes = loss of entire regions

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Kruppel  encodes for establishing thoracic segments 1-3 and abdominal segments 1-4

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Activity of gap genes is regulated by reciprocal interactions

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Knirps expressed near middle of embryo

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Hunchback is expressed near the posterior

Pair-rule Genes -

Divide embryo into 14 segments

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Genes are expressed in alternate segments

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Pair rule genes are TFs that activate segment polarity genes

Segment Polarity Genes -

Gene products produce arts of Wnt and hedgehog signalling pathways

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Effects seen in every segment

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Deletion = loss of each part of a segment

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Parasegments = where genes are being expressed o Engrailed (TF)  expressed w/ ftz or eve o Wingless (paracrine factor)  repressed by ftz or eve 

Wingless turns on engrailed through β-catenin

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Hedgehog and wingless promote each other in feedback loop

o Wingless and engrailed work to establish boundaries b/w parasegments o Hedgehog and wingless as morphogens 

Morphology of denticles varies depending on concentration...