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Summary chapters Developmental Biology of Plants and Animals
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Samenvatting – Hoofdstuk 1 – EZO-223061.1.1
Before oogenesis begins – oogonium divides - 15 nurse cells and 1 oocyte precursor (at posterior
site of the egg chamber) are formed.
Anterior-posterior polarity in oocyte
Gurken - synthesized by nurse cells – transported to oocyte nucleus – translated to Gurken
protein – oocyte nucleus is very near posterior end of egg chamber – Gurken signal received
by follicle cells through torpedo gene (receptor protein) results in posteriorization of
follicle cells. following Par-1 protein (at posterior site oocyte) organises microtubules
(grow from ant. To post.).
PAR proteins are initially uniformly distributed in cytoplasm, but become concentrated in
posterior pole of oocyte.
Oskar – translated at posterior site
- recruits more par-1 protein
So microtubular rearrangement + increase in oocyte volume
increase in oocyte volume transfer of bicoid mRNA’s + nanos from nurse cells.
Dorsal-ventral Patterning in oocyte
nucleus moves anteriorly – determines dorsal position
Gurken signal has short diffusibility, only the dorsal follicle cells receive signal – dorsal morphology –
inhibits synthesis of pipe protein- pipe protein only made by ventral follicle cells.
Pipe activates Nudel protein – Nudel acgivates proteases – are products of gastrulation defective etc.
cascade reactions results in cleaved Spätzle protein, binding to Toll receptor – ventralizes cell.
1.1.2
Dorsal, the ventral morphogen
Dorsal gene – placed in oocyte by nurse cells
- when translated – found throughout the embryo
- translocated into nuclei in ventral part embryo - acts as transcription factor
- if Dorsal does not enter nucleus – the genes and all cells become dorsal cells
- in mutants in which all cells have ventral phenotype, Dorsal is found in every nucleus
Establishing a nuclear Dorsal Gradient
Dorsal produced – complexed with Cactus in cytoplasm syncytial blastoderm – if Cactus bound to it
Dorsal remains in cytoplasm – separation of Dorsal from Cactus is initiated Toll receptor – Toll is
activated by Spätzle protein, that is highest in ventral region, so Dorsal translocation also in ventral
cells – highest concentration Dorsal in ventral cell nuclei.
Effects Dorsal protein gradient
Dorsal protein specifies cells to become mesoderm in 2 ways:
1. The protein activated genes that create mesodermal phenotype
target genes: twist, snail, fgf8, fgf8 receptor, rhomboid
Twist: activates mesodermal gene, Snail: represses mesodermal genes, Rhomboid and fgf8:
activated by Dorsal, but repressed by snail and are not expressed in most ventral cells, but expressed
,adjacent to mesoderm.
Low levels of nuclear Dorsal activate transcription of Short gastrulation (sog) (Chordin in vertebrates)
– sog expressing cells become moest ventral cells . Diffusion of Sog creates gradient. Sog inhibits
Decapentaplegic (dpp) (=BMP in vertebrates) gradient of dpp from dorsal (high conc) to ventral
(low conc). Sog prevents ectoderm to become epidermis and begins process neural differentiation.
1.1.3
First zygotic genes to be expressed are Gap genes (mutations in them cause gaps in segmentation
pattern).
Gap genes – encode transcription factors
- regulate transcription of Pair-rule genes – divide embryo into periodic units striped
pattern of 7 transverse bands on anterior-posterion axis.
- proteins ecoded by pair-rue=le genes activaten segment polarity genes –
All these genes interact to regulate the homeotic selector genes – determines developmental fate of
each segment.
Maternal gradients: polarity regulation by oocyte cytoplasm
Morphogenic gradients emanating from 2 poles during cleavage – interacted to produce positional
info determining identity of each segment
Molecular model; protein gradients in early embryo
2 maternal mRNA’s : bicoid (anterior tip of unfertilized egg) and nanos (posterior tip of unfertilized
egg) – formation of anterior-posterior axis. – result of polarization of microtubule networks in oocyte
Embryos lacking bicoid could nt form a head and when bicoid mRNA was added to bicoid deficient
embryos, the place where bicoid was injected becme the head – areas around it became thorax etc
(concentration dependent)
The bicoid protein complex is transported out of the nursecells into oocyte via microtubules. Bicoid
MRNA complex attaches to dynein proteins that are maintained at microtubule organizing centre.
Nanos mRNA appears to get trapped in posterior and of oocyte by passive diffusion. Nanos becomes
bound to cyto skeleton in posterior interior of egg. Nanos absent – no abdomen. Oscar protein is
trap. Hunchback and caudal is repress by diffusion gradients of nanos and bicoid. Nanos function by
preventing hunchback translation.
. An anterior-to-posterior gradient of Bicoid protein
. An anterior-t0-posterior gradient of Hunchback protein
. A posterior-t0-anterior gradient of Nanos protein
. A posterial-t0-anterior gradient of Caudal protein
Bicoid, Hunchbak and Caudal proteins are transcription factors
Exuperantia gene is responsible for keeping Bicoid at interior of egg. Absence of exuperantia Bicoid
diffuses farther – protein gradient is less steep.
Terminal gene group
Acron – terminal portion of head
Telson – (tail) mutations in these terminals genes result in loss of Acron and Telson.
, Torso – is synthized by ovarian cells deposited in Oocyte translated after fertilization.
- Must normally be activated at ends of egg.
Distinction between anterior and posterior depends of the presence of Bicoid. If Tailless and
huckebein act alone terminal region becomes Telson. However if Bicoid is also present terminal
region forms Acron.
1.1.4
Selffaith commitment in two steps: speciation and determination
Cells undergo a transition from loosetype of commitment to irreversible determination – cell
intrinsic. Transition from speciation to determination is mediated by segmentation genes. That divide
early embryo into repeating series of segemental primordia along anterial posterial axis.
Gap genes
Activated or represt by maternal effect genes. High levels of Bicoid and Hunchback induce expression
of giant. Krüppel transcript appears over the region by Hunchback begins to decline. The end result
of these repressive interactions in the creation of overlapping gap mRNA expression patterns.
Pair-rule genes
The divide embryo into regions that are precursors of segmental body plan.
Primary pair-rule genes include hairy, even – skipped and runt each of which is expressed in seven
stripes. Different concentrations of gap proteins determine whether or not a pair-rule gene is
transcribed. Even skipped: its enhancer regulate a separate stripe or a pair of stripes. The primary
pair-rule genes also form the context that allows or inhibit expression of the later acting secondary
pair-rule genes like fushi tarazu.
Pair-rule products activate segment polarity genes.
Segment polarity genes
Once cells form interactions are between cells segment polarity genes expression of wingless and
engrailed is initiated. Engrailed is expressed in cells with high even skipped or fushi tarazu proteins.
Engrailed gene is activated in cells that will express hedgehog. Wingless activated when there is no or
little even skipped or fushi tarazu. Wingless transcribes solely in the row of cells directly anterior
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