Morphogen patterning

ur mom

Patterning

formation of body axes

morphogen regulation of patterns

Diffusible signals whose diffusion creates concentration gradient

>secreted proteins

drive cell differentiation

produced by specific clusters

Drosophila A-P patterning, asymmetric distribution of maternal mRNA

asymmetric distribution of maternal mRNA

zygotic expression of gap genes in response to maternal mRNA gradient

(pair rule genes, and then segment polarity and homeotic)

Early fly embryogenesis

fertilized egg, division of nuclei, movement to periphery, cellularly blastoderm

Patterning screen in drosophila

Forward screen for mutants affecting patterning,

mutagenized males with Ems then combined with temp sens allel to select mutants,

if the phenotype matches homozygotes tested too

How do you know which ones are maternal from zygotic genes

mutant flies x wt produced no viable flies

if the maternal mutant is wt, then it doesnt matter if the male is mutant

To find zygotic mutants, it requires 2 mutations to make a recessive?

Bicoid

The bicoid mutant is from maternal mRNA

and creates the anterior gradient

anchored to the anterior end

forms a gradient across blastomere

Nanos

Creates posterior gradient (from the nanos mutant)

In mutant, posterior is not formed

Otd

Weak bicoid binding site high levels of biocoid

Hb

(resulted from strong binding of bicoid and weak binding) (maternally deposited everywhere)

How does bicoid and nanos establish gradients

high levels of biocid > high levels of hunchback

bicoid represses caudal

High levels of caudal> high levels of nanos

caudal represses hb

Gap genes

gap in segmentation when the gene is mutated

varying affinity for maternal transcription factors which can affect others

(hunchback, giant, krupel, knirps, tailess)

how do gap genes affect pattern pair-rule genes

pair rule genes are activated in 7 stripes ( 14 segments)

if mutated every other will b e missing

pair rule genes regulated via activators and repressors

gene hierarchy

Maternal effect gene

Segmententation gene (gap, pair-rule genes, segment-polarity)

(homeotic gene)

4 factors regulating eve

biocid, and hb activates it, > giant represses it along with kruppel,

Maternal effect genes

Establish gradient of posterior and anterior pole of egg (bicoid, caudual)

Segmentation genes

defines broad regions in the egg, (hunchback, kruppel)

Mutation: Adjacent segments missing in major region of body

Pair-rule

7 segments (eve, odd, ) Mutated: part of pattern deleted in every other segment

segment-polarity gene

define 14 segments (goosey)

mutation: segmentation replaced by mirror images

Homeotic gene

inapproapriate structure

Drosophila D-V patterning

driven by maternal genes, and not transcription factors but reeceptor-ligand

gurken (dorsal side)

nucleus travels to the dorsal side of the embryo

transforming growth factors a family signaling ligand activates epidermal growth receptor torpedo

Gradient of EGFR activation triggers intracellular Map kinase cascade on dorsal side (inhibits the pipe)

gurken ( ventral side)

after fertilization nucleus travels

NO gurken signal to ventrall side

(only pipe ) on verntral side and makes spz, the toll ligand

D-v patterning mystery

toll receptor protein is expressed everywhere, ligand Spz ventrally

Dorsal is a transcritpion factor only activated when spz binds to toll

Caudal

expressed maternally EVERYWHERE, caudal protein only shows in the back

How did they identify eve regulatory regions

isolated enhancers of the eve skip gene, and they tested (1,5 enchancer).

Each enchancer showed expression of lac z reporter when isolated 😲

Gurken pathway

when present> torpedo active> pipe repressed> stable cactus > dorsal in cytoplasm

not present> no torpedo> pipe active> spaetzle (active)> toll active> cactus degraded > dorsal enters nucleus