Patterning Drosophila Embryo

Question 1

Patterning

Answer 1

Differences between cells getting established

Question 2

Drosophila early embryo basic

Answer 2

Egg formed in mother ovary
Fertilised
Laid
First just a single nucleus
Then just nuclear division
Within 60 minutes a pop of nuclei has formed
Then by 90 mins they have migrated out to the edge
Then at 150 mins - the syncytial blastoderm with the nuclei lining the outer edge of the syncytium

Then the beginning of cell formation by infolding of the membrane

Pole cells towards posterior form first
Go on to form germ cells

Cell layer forms around the surface
Opaque yolk in the centre

Different cells along ap axis form diff segments

Question 3

What part of embryo gastrula yes

Answer 3

Strip on the ventral side gastrula yes and forms the inner mesoderm
Most of rest makes ectoderm
Except for gut ectoderm at each end (A and P)

Strip along dorsal makes serosa

Segment identities are already established by 3hrs

Question 4

AP axis patterning mutants

Answer 4

2 diff classes of mutants

Zygotic effects
Mutant embryo is abnormal
Genes involved are needed to be functional in the embryos cells

Maternal effect genes
Needed functional in mother to produce normal embryo
Mother with mutation makes affected embryo even if embryo has functional copy
Gene function needed during oogenesis

Question 5

Classes of maternal effect gene mutations

Answer 5

Anterior end affected
Posterior end affected

Question 6

OSKAR and NANOS mutants

Answer 6

Mother with these mutations
Make egg that gives embryo with no posterior patterning

Needed for posterior patterning in embryo

Question 7

BICOID mutant

Answer 7

Bicoid mutant mother gives embryo with no anterior patterning

Bicoid functional needed from mother to pattern anterior patterning

Question 8

Patterning cells in drosophila oogenesis

Answer 8

Ovary in mothers abdomen
Consists of parallel ovariole tubes
Each ovariole is independent of neighbouring ones

In tube oocyte safe produced sequentially
Mature as they move along tube and are laid when mature

Oocyte is not alone
It is accompanied by other cells
-stem cells at proximal end
-produce cells called egg chambers
-each egg chamber consists of large oocyte surrounded by layer of much smaller follicle cells
-at proximal end of egg chamber there is 15 supporting nurse cells
-as egg natures follicle cells produce shell and nurse cells die

Question 9

Patterning of the oocyte

Answer 9

Occurs by interaction of adjacent egg chambers in tube
Produced sequentially so each chamber has an older sister in front of it
At particular stage the older sister produces a signal
Received by follicle cells on nearest side of younger sister (the egg oocyte end of chamber)
Changes their expression
At later stage a reciprocal signal comes from these follicular cells
This signalling causes MTs to be arranged correctly in the oocyte

These two phases of signalling are necessary for MT arrangement to happen in oocyte
AP axis patterning is dependent in it

Question 10

Differences between the three cell pops in the egg chamber

Answer 10

Oocyte is big
Stuffed w yolk and maternal gene products
Oocyte is inert and doesn’t transcribe it’s own genes just receives material produced by cells around it

Follicle cells put yolk in the oocyte

Nurse cells are polyploid and polythene - for producing gene products to be put into oocyte

Follicle and nurse cells contribute to oocyte development and die off

Question 11

MT polarisation in oocyte and it’s patterning contributions

Answer 11

Plus end (where additional subunits are added) is oriented away from nurse cells - pointed towards older sister direction
Minus ends towards nurse cells
Parallel orientation

Kinesics travel to plus end
Dyenins to minus

maternal Gene products given to the oocyte travel along them

OSKAR RNA is transcribed in nurse cell
Carried to plus end by kinesin and builds up at this future POSTERIOR end of the oocyte

Question 12

OSKAR RNA localisation consequences

Answer 12

Comes into oocyte early
Carried by kinesin to plus end
OSKAR protein translated
Binds NANOS RNA when it comes in
Sets up future posterior end of embryo

Question 13

BICOID RNA in oocyte

Answer 13

After OSKAR stuff
BICOID RNA from nurse cell binds dyenin
Transported to and enriched at plus end
So is shunted back to minus end nearest nurse cells
Sets up future development of anterior end

Question 14

Patterning AP of early embryo - ANTERIOR

Answer 14

BICOID rna is localised to this end
It is tethered by cytoskeleton components
Then at about 60mins after laid
Begins to be translated into bicoid protein
RNA tightly tethered but protein is not
Protein produced locally but can diffuse through syncytium common cytoplasm (has only been nuclear division)

This sets up a gradient of bicoid protein conc across ap axis
High conc at ant
Fades into background by about halfway along axis
-because bicoid protein is unstable
- it is produced locally at anterior
-diffusing away
-gradually breaks down as it moves away
-so get stable gradient of bicoid as go along axis

Question 15

Bicoid properties

Answer 15

Protein is a TF
Controls set of genes
Maternal product TF

Need a lot of Bicoid to turn on Otd
So gradient causes Otd to turn on in anterior half of embryo

The. At lower bicoid conc hunchback is turned on partway through axis? Still in anterior half?

Is still a syncytium so these zygotic products can diffuse out and create gradients in common cytoplasm

Bicoid had consequences on spatial localisation of products
Turns on many targets along axis
This is beginnings of process that subdivides the ap axis of segment identity

Question 16

Bicoid protein interaction w maternal RNA (caudal eg)

Answer 16

Is able to bind maternal RNA molecules

Maternal Caudal TF RNA binds to bicoid protein where bicoid protein is
And is degraded when it binds
So bicoid protein presence causes degradation of maternal caudal rna

So caudal can’t be present anteriorly
Only posterior?

Question 17

Patterning embryo posterior - OSKAR and NANOS

Answer 17

OSKAR RNA localised to post of egg
OSKAR protein binds NANOS RNA
Localised NANOS RNA to the posterior

About an hour where NANOS protein is translated and diffuses from anterior
Binds posterior maternal hunchback and causes it’s degradation

Hunchback RNA IS DISTRIBUTED UNIFORMLY
But since NANOS protein in posterior
It is degraded there

Question 18

Two hunchback RNA stages

Answer 18

Maternal hunchback RNA comes into oocyte and is degraded in posterior by NANOS protein
Then is turned on in anterior (zygotically) by Bicoid protein activity

Question 19

Gradients in patterning - syncytium vs multicellular environment

Answer 19

In the syncytium
Eg with bicoid
Have bicoid maternal rna localised in anterior
Then protein is only produced in that localised area
Is unstable so diffuses out and breaks down as it moves ooht
Sets up a gradient in the anterior of the oocyte
Has consequences in transcription along the axis

Typically it is different in multicellular environments
Secretion of signal
Causes gradient in extracellular space across the cell surfaces
Different response thresholds in diff cells
So cells respond differently across the gradient

Question 20

AP axis divisions

Answer 20

Segments
Repeated anatomical units, developmental modules

Simple way of building body complexity
Segments in larva not massively different
But in adult there are many

Question 21

Segment grouping

Answer 21

Tagmata
Eg
Head
Thorax
Abdomen

Head segments fuse together in drosophila adult and lose a lot of distinct look

Question 22

when is drosophila body plan determined?

Answer 22

in development of egg
already halfway through embryogenesis (~10hrs) embryo looks v segmented
so body plan already somewhat determined before this

Question 23

adult structures origin basic

Answer 23

adult doesnt form directly from larva (ie larva doesnt just sprout legs/wings and many larval segments dont correspond to adult ones

adult built somewhat from scratch
larval

larval skin becomes pupa outside
parts of larval tissue in each segment bud off and retain fate becoming the imaginal discs
stay proliferating in larva until it pupates and become adult structures within the pupa

adult segments are given their identity during this stage (stage 13 11hrs in)

Question 24

gap gene regulation basic

Answer 24

done by morphogen gradients

Question 25

protein gradients in early embryo revision

Answer 25

bicoid rna anterior side of oocyte
posterior - nanos RNA
hunchback and caudal generally distributed

bicoid and nanos mRNA translated into protein at respective ends
proteins form gradients across AP axis

maternal caudal mRNA translation inhibited by bicoid protein in anterior - so gradient of nanos and caudal from posterior
other way around with hunchback - mRNA translation inhibited by nanos in posteior - forms gradient from anterior

these gradients give the nuclei in the syncytial blastoderm info about their position along the AP axis
act as morphogens and affect zygotic expression of nuclei differently across the AP axis

Question 26

bicoid maternal gradients to zygotic gene activation revision

Answer 26

bicoid (in anterior) activates Otd and Hunchback zygotic expression (different from maternal hunchback from earlier)

Otd has a higher threshold so is expressed in nuclei only in the VERY anterior end
Hunchback has a lower threshold for bicoid levels so is activated further along the AP axis from anterior

now there is already a difference between very anterior and further less anterior (Otd+Hb and just Hb further down)

Question 27

gap genes basic

Answer 27

their expression results in patterning of the embryo
from A to P
knirps
hb
giant
kruppel
knirps
giant
hb
tailless

gap gene expression in cells of embryo reflect their AP position “positional info”

Question 28

gap gene mutants

Answer 28

mutating a gap gene causes a “gap” in the body plan along the AP axis - the gap gene domain is missing

Question 29

kruppel necessity

Answer 29

is required for thorax segments
mutant gives gap in thorax - head stuck onto abdomen

Question 30

knirps necessity

Answer 30

required for but further posterior - mutant gives large chunk of abdomen missing

Question 31

Kruppel and hunchback interaction

Answer 31

kruppel inhibited at high Hb conc
is activated at a lower one
hence its expression in future thorax region

Question 32

Knirps gap gene regulation

Answer 32

is activated by caudal coming from posterior
inhibited by hunchback from anterior
is active just behind kruppel

cant activate further posterior even tho caudal is higher there as - Tail-less inhibits its expression there

so only activates just where caudal is high enough but its repressors from posterior are low enough

Question 33

gap gene refinement - cross-regulation

Answer 33

early on a lot of the gap gene domains overlap
but they cross regulate - try to inhibit each other

ends up with dynamic change from fuzzy broad domains to v precise domains of expression

Question 34

gap gene summary basic

Answer 34

divide ap axis into domains
next step - gap gene products are TF factors that regulate
-segment formation
-segment identity

Question 35

what genes do gap genes regulate?

Answer 35

pair-rule genes

Question 36

pair-rule gene expression patterns

Answer 36

each pair-rule gene is expressed in 7 stripes corresponding to alternate PARASEGMENTS (hence 14 parasegments in total)
parasegments are forerunners of the final segments
-even-skipped (eve) is expressed every ODD parasegment
-fushi tarazu (ftz) expressed in even parasegments

-other pair-rule genes (hairy, paired, runt, sloppy paired, odd-skipped, odd-paired etc…) are expressed in the parasegments but are out of sync with them

Question 37

7 stripe expression pattern of pair rule genes necessity

Answer 37

are important for development

if eve is missing - gives phenotype - just even number parasegments stuck together

ftz mutant just gives the odd number parasegments stuck together

mutations in pair-rule genes affect cells in every alternate parasegment

Question 38

eve gene regulation

Answer 38

regulation is modular
each stripe regulated independently by a separate enhancer
-put a reporter on one of these enhancers and it will show just ONE parasegement

nuclei in different stripes activate the same eve gene with a DIFF enhancer

Question 39

eve gene regulation - stripe 2 enhancer

Answer 39

eve expression stripe 2 (parasegment 3 ig)
the enhancer for this one location is regulated by combiantion of maternal gene products and zygotic gap gene expression
hunchback and bicoid (bicoid is maternal) (anterior) gradients activate it
Giant gap gene product inhibits stripe 2 on anterior side
Krupppel inhibits expression on posterior side

mutate giant(or the enhancetr’s giant binding site) - stripe 2 enhancer activation spreads much further anterior than it should

same for posterior but kruppel mutation

mutate bicoid binding site on the enhancer - no stripe 2 at all)

Question 40

hair an runt pair-rule genes (and eve)

Answer 40

are primary pair-rule genes
similar regulation as eve for all 3 of these

the rest of the pair-rule genes (incl Ftz) dont respond to gap genes
but instead to the primary pair rule genes

called secondary pair rule genes

Question 41

secondary pair-rule gene regulation

Answer 41

eve in the odd stripes represses ftz expression - so ftz is only active in the even stripes

lots of dynamic interactions between the secondary pair-rule genes similarly to what happens with the gap gene domains cleans up the expression boundaries

Question 42

order of things affecting ap patterning so far

Answer 42

maternal gradients
gap genes
pair rule genes

Question 43

next steps after pair-rule gene division into 14 parasegments

Answer 43

1. activation of genes that defin parasegmental borders and then patterning detail WITHIN segments
2. assignment of segmental identity so that each develops with its characteristic anatomy

Question 44

segmentation genes

Answer 44

define the parasegmental borders
and then pattern within the segments

pair-rule genes degine the POSITIONS of the parasegments
expression of pair-rule genes is transient (unstable proteins)
so the next genes they activate (segmentation genes) establish the segments permanently

Question 45

pair-rule genes and Engrailed

Answer 45

engrailed is the key segmentation gene
en initiation is controlled by pair-rule gene combinations

have 14 stripes of engrailed
because ftz and eve combine to activate engrailed
ftz in even no. segments
eve in odd no. segments

engrailed is only activated in certain region within the segments die to other pair rule genes whose 7 stripes are on slightly out of phase with eve and ftz - offset by a few cells

opa (odd paired) prevents activation by ftz in even segments
causes engrailed to be activated only in the anterior of the parasegments
same in odd numbered segments where runt prevents eve activating en except in just anterior due to few cell offset

Question 46

future body segments relation to parasegments

Answer 46

parasegments are defined by anterior en expression

but future body segments are defined by posterior expression of en
so parasegment and segment borders dint line up

Question 47

important change at stage where segment genes are activated

Answer 47

after 3.5hrs cell membranes form and CELLULAR blastoderm is formed
TFs can no longer diffuse directly

nuclei in cells need to communicate via secreted signals
eg Hedgehog, Wingless

Question 48

what does engrailed expression define in segment

Answer 48

the POSTERIOR COMPARTMENT
TF expresed in posterior of each segment
activates signals which stabilise the boundary
posterior and anterior compartment cells differ in adhesion so A and P cells do not mix together and stay separate

Question 49

stabilising the parasegment boundary

Answer 49

engrailed in the posterior compartment of segment
activates expression of the Hedgehog secreted signalling molecule

hedgehog then diffuses forward (anteriorly) to stimulate
hedgehog activates wingless expression in these cells
hedgehog and wingless form positive feedback loop (wingless activates En?? expression by diffusing to these more posterior cells too)

this keeps expression of En, Hh, and Wg stable even after the pair-rule genes stop their expression - which was necessary for the set up but are no longer needed)

Question 50

Patterning within segments

Answer 50

cascade of further segmentation genes

Wg expression in the posterior compartment prevents production of Denticles - an anterior feature of future segments

Serrate signalling molecule is activated in the gaps between Hh and Wg signals

where diffused Hh and Serrate mix - they activate Rho
Serrate +Rho mixing casues formation of denticles

Wg suppresses this elsewhere - naked cuticle

diagram in notes

Question 51

engrailed and looking ahead to adult segments

Answer 51

adult segments also have A and P compartments regulated by Engrailed

to regulate this - Engrailed is expressed in the imaginal discs

Question 52

Pt2 - assignment of segment identity

Answer 52

Done by Hox genes

segmetns initially all similar in embryo
follow different developmental programmes to attain different segmetnal anatomies and to form tagmata
-eg thorax = legs, abdomem - no legs

pair rule genes (individual segment AP) and gap genes (overall segments) information combines to give Hox gene activation to give segment identity based on segment location A-P

Question 53

larval segment identity:

Answer 53

somewhat similar early on
later on begin to look diff (head, thorax, abdomen)

later even - segment differences
-head relatively small
-thorax segments have different muscle layout to abdomen - also has vestigial legs

Question 54

Hox gene complexes:

Answer 54

sets of genes related to each other in complexes
in drosophila the Hox gene complex has been split in two
but can be looked at as one

order of genes on chromosome responds directly to the order of the segemetns on AP axis to which they give identity

fewer genes in the abdominal segment as they are v similar to each other so need less to differentiate them

Question 55

posterior dominance

Answer 55

expression domains of Hox genes overlap
the most posterior gene’s function dominates

eg antennapedia function is repressed by Ubx presence

Question 56

3 most posterior hox gene mutant

Answer 56

Ubx, abd-A, Abd-B triple mutant

Head, T1, and T2 are all fine
but segmetns behind that ALL have T2 identity - even habe the vestigial leg indicating thoracic identity
A1-A8 segments transformed
A9 is fine somehow tho

in the absence of these posterior Hox genes
they still express Antennipedia
but now the more posterior ones arent there to dominate over it
antennipedia ends up defining the identity of these segmetns too

Question 57

homeotic genes

Answer 57

Hox genes are examples

homeotic mutations definmes by characteristic phenotypes
something being transformed to be like another feature

v dofferent to gap gene mutations where mutated part is missing instead of transformed

Question 58

embryo to adult segment identity basic

Answer 58

larval segments arise from hox gene expression

the identities of the adult structures come from the imaginal discs
wings come from imaginal discs set aside in T2
T3 haltere imaginal discs
etc

Question 59

adult segment features

Answer 59

head segments seen cleareer in biting mouthpart insects eg locusts
diff segments have diff mouthparts ig

head anf thorax segments have appendages forming diverse structures (legs, mouthparts, etc)

drosophila is dipteran so abdominal segments generally lack appendage

Question 60

Ubx, Abd-A mutant adult

Answer 60

this would not make it to adult stage
but prediction

internally in larva there will be many wing and leg imaginal discs in segments past T2
fine up to T1
then T2 onwards is transformed to T2 - A9-10 are fine

Question 61

Hox gene mutations giving viable adult flies

Answer 61

regulatory mutations tend to more likely give viable adult flies
affect the expression patterns of Hox genes

Question 62

Ubx regulatory mutant

Answer 62

prevents its expression in T3
is just expressed in the abdomen
so it cant dominate over antennipedia in T3 as it normally would

so antennipedia gives T3 T2 identity
T3 now trasnformed to habve T2 wings and legs instead of T3 legs and Halteres

Question 63

Regulatory mutation in antennipedia

Answer 63

gain of function where antennipedia is mutated in an enhancer allowing it to be expressed firther anteriorly
gives T2 identity yo head segments
- transforms antennae into legs
weak expression so not full transform but thoracic identitiy partially given to head

Question 64

identity of the 3 thoracic segments

Answer 64

the combos of Hox genes expressed in thoracic segments gives them their different identities

Scr (sex combs reduced) + Antp -> T1
Antp alone -> T2
Antp+Ubx -> T3

Scr mutant - sex combs are T1 leg identity - Scr mutated - T1 legs become more T2 like - so sex combs are hence reduced

Question 65

evolution of Hox genes

Answer 65

v high conserved amongst bilateria
v ancient function in AP axis patterning (so it ancestrally in all bilateria - some have lost this tho but still ancestral to them)

Hox gene complexes arose by ancient gene duplication events and divergence
-new gene duplicates
-new duplicate diverges - helps define AP axis detail even more
-repeat this to make AP axis of more and more complex body plans

Question 66

how did diff body parts arise in diff species

Answer 66

thought before that diff hox genes arising by duplication in diff lineages gave diff body plans

but this is wrong

arthropods have diverse body plans
but their hox genes dont differ much at all
same hox gene product between two species is more similar to diff hox gene product within the same species????????????? (in arthropods at least)

differences in body plans arises from how the segments RESPOND to hox genes

animal ancestor inferred to have complete Hox gene set alreadt - due to animals sharing Hox stuff

Question 67

mouse Hox genes

Answer 67

4 gene complexes
due to whole genome duplication event during vertebrate evolution
position on chromosome corresponds to position of patterning on AP axis

Question 68

mouse hox genes and AP diversity of spinal column

Answer 68

mesodermal somites
vertebrae arise from somites
diff identities of vertebrae due to hox fgene patterning

mutate lumbar identity Hox genes - causes them to gain thoracic identity and heve ribs

Question 69

imaginal discs beginnings

Answer 69

start as small groups of ectodermal cells
bud off embryonic ectoderm (mostly head and thorax)
remain undifferentiated epithelial cell and proliferate
gain their segment identity from wherever they are formed in the embryo
one for each structure
-leg
-wing
-haltere
-mouthpart
-…

Question 70

patterning the wing imaginal disc

Answer 70

buds off in mid embryo from T2 segment
cells proliferate and grow during larval development to about 50,000 cells
wing disc goes on to form 1/2 of thorax (top and part of side) and the wing:
-grows and folds out to form wing - coming together of dorsal and ventral surfaces to form the wing blade
-has DV and AP axes
-can see that within fate map of imaginal disc
-concentrate on AP

Question 71

AP tissue patterning in the wing

Answer 71

characteristic wing vein pattern from A to P
distinct A and P compartments
P compartment expresses Engrailed - gives post identity
no vein along The p border

Question 72

Engrailed expression set up in wing imaginal disc

Answer 72

imaginal disc inherits this compartment identity by virtue of where it arises in the embryonic segments - their origin in the embryo
When imaginal disc cells bud off they bud off from cells at the PARASEGMENT boundary (offset from segment one remember)
so this gives the difference in En expression in A and P of ID

En is a SELECTOR gene for posterior identity

Question 73

engrailed mutant wing (en as posterior selector)

Answer 73

anterior features (vein 1,2,3) not affected
posterior compartment partially transformed to anterior compartment identity (formation of vein 1,2,3 somewhat)

en allows the posterior cells to gain posterior identity instead of the anterior DEFAULT

Question 74

En second role in wing patterning

Answer 74

sets up gradient for patterning wihtin the A and P compartments
almost the same as what happens in the embryo segments

-boundary of En/No En acts as part of set up of Hedgehog and Decapentaplegic (Dpp)
-En activates Hh expression
-Hh diffuses a short distance to anterior cells
-activates dpp expression in anterior cells at the AP compartment boundary
-the AP boundary now becomes an ORGANISER: dpp diffuses to form concentration gradients that pattern the details of the A and P compartments (eg wing veins)

get dpp morphogen gradients on either side
diffuses out in both directions forming pattering gradients from the boundary
diff organiser to the embryo

Question 75

dpp target activation

Answer 75

dpp is long range organising signal
target genes ativated according to dpp conc

> Spalt at high conc - closest to AP border
Omb responds to lower levels so further from border in wider domain
Brinker only expressed if they are not exposed to a moderate level of dpp - normally inhibited by it in cells nearer dpp boundary

completely symmetrical pattern in A and P directions

Rho gene expressed at boundaries of these above genes
Rho expression corresponds to vein formation on wing
in ant - wing vein 3 formed at highest dpp
1 at furthest lowest dpp

same for posterior side BUT it is interpreted differently due to posterior identity given by presence of engrailed

Question 76

Dpp mutant wing

Answer 76

loss of function
would predict no dpp patterning gradient so veinless wing
INSTEAD end up with v reduced wing (possible loss of veins but hard to tell)
whole wing stricture is changed

so Dpp doesnt just activate wing vein formation genes but it also regulates growth of the wings by activating Vestigial (Vg) gene

Question 77

Dpp gain of function

Answer 77

cant see wing veins on Dpp loss of function
instead need to to gain of function

constitutively activate dpp in cline of cells in A compartment
sets up new patterning gradient - dpp constitutive cells act as new organiser
get whole new wing structure formed

can interpret dpp function:
>Dpp forms new organiser in anterior
>diffuses out in both directions from clone cells
>activates vg in cells nearby - get outgrowth of wing
>but also get the patterning elements in that new part of the wing
>-it is summetrical - because there is no engrailed expression to differentiate posterior identity - just anterior mirror
>-only get the further elements - veins 1 and 2 as expression of dpp in clone cells is weaker so the gradients are lower in conc so dont get the closer elements from the highest conc

evidence for both dpp functions

Question 77

Dpp and Vg mechanism

Answer 77

Dpp activates Vg in wing blade cells in the wing pouch
this promotes proliferation in wing pouch to give enough cells to form wing blade

so dpp has 2 functions
growth of wing
patternign of wing veins

Question 78

Haltere imaginal disc

Answer 78

a modified wing
ancestral insect form has 2 pairs of sinmilar wings - on T2 and T3
in diptera T3 hindwing highly modifies to haltere structure

starts same as wing within embryo development
identical at early stage

but in larval stage where they proliferate - dramatic difference in size of discs appears - halteres much smaller
but has the same positional patterning components - en, hh, dpp (from the parasegmental border in the embryo)
engrailed
regulating hh
regulating dpp
same as wing but they go down diff pathway because of differing hox genes in the T2 and T3 segments

Question 79

wing vs haltere patternign differences

Answer 79

Ubx present in T3
but not T2
so same positional values and compartments but different development because of diff hox context
see from ubx mutant

Question 80

Ubx effect on haltere development

Answer 80

modifies haltere disc growth
Dpp activation of Vg is repressed by Ubx
end up with little expansion of this area
haltere remains small
allows haltere development instead

so hox genes alloew for differences in fore and hindwings in insects
see in other orders too
bees have smaller hindwing
beetles have elytra as forewing

Question 81

Leg imaginal disc

Answer 81

has AP and Proximo-distal axes
forms by outgrowth of the middle part of concentric ring looking pattern to form the distal part of leg structure

different pattern elements for different leg parts

Question 82

leg patterning AP axis

Answer 82

similar to wing and haltere -some differences

en in post
hh diffuses towards ant
activates BOTH dpp and wingless (in the ventral half of AP compartment border with dpp in dorsal half of the border)
these signals regulate ap axis patterning

diagram in notes

Question 83

PD axis leg patterning

Answer 83

Dpp and Wg signalling involved but not as gradients
the signals are initial trigger for a sequential gene regulatory cascade
three key TFs (sometimes known as - Leg Gap Genes)
-Dll - distalless
-hth - homothorax
-dac - Dachshund
their combinatorial action leads to 5 domains along the PD axis

Question 84

intrinsic temporal gene regulatory sequence resulting in spatial sequencing of leg gap genes

Answer 84

wg+dpp acitvate Dll and inhibit hth and Dac
as cells proliferate onef from the centre get pushed out
pushed out cells leave the dpp+wg overlap region
cells from the middle that have had dll expression activated have not yet turned it off - but since they have left the wg+dpp region - dac is no linger inhibited so Dll can activate it
-then after another period of time they move out more dac begins to inhibit dll
(hth expressed furthest out idk but think it must be expressed once dac represses dll - so dac no longer activated - so hth can come on maybe?? not on the regulatory chart)

so much diff method of patterning - the wg+dll for setting up initial expression - then intrinsic temporal gene regulatory sequence results in spatial patterning

Question 85

Dll and Dac mutations

Answer 85

partial loss of function Dll:
loss of distal part of leg

Dac loss of function - loss of middle part of leg

Question 86

eye-antennal imaginal disc

Answer 86

one for each eye and antenna
two discs in one form eye and antenna

antennal disc - similar to leg disc in patterning
A and P compartment
Dpp in ventral part
wg in dorsal
gives rise to dll, dac, hth
antenna and legs both modified from more general appendage - different due to hox gene context