Lecture 7

Question 1

What is necessary to define each cell geographically in the blastoderm?

Answer 1

Two coordinates set up by two different signalling systems, one for the anterior-posterior axis and one for the dorsal ventral axis

Question 2

How is the drosophila a segmented organism?

Answer 2

It is broken down into segments along its anterior-posterior axis with the head having 6-7 segments, thorax 3 and abdomen 8
There are also the specialized unsegmented acron and telson at each end of the body

Question 3

Why is it hard to determine precise numbers of segments for different body parts of the drosophila?

Answer 3

Portions of the head and abdomen fuse during development

Question 4

What is the fundamental unit of embryonic gene expression in the drosophila?

Answer 4

The parasegment

Question 5

What is the parasegment in drosophila?

Answer 5

Each adult segment has an anterior and posterior compartment
A parasegment is made up of the posterior compartment of the segment closest to the head and the anterior compartment of the next segment

Question 6

What is the heirachy of the three classes of gene involved in generating the anterior-posterior axis in drosophila?

Answer 6

The egg-polarity genes which control the segmentation genes which control the segment polarity genes

Question 7

How is polarity established in the egg?

Answer 7

The anterior, posterior and unsegmented terminal regions are specified by organizing centers established by the egg-polarity genes

Question 8

How is the drosophila body segmented?

Answer 8

The organising centres establish a periodic pattern of segments involving the segmentation genes

Question 9

How are the segments formed in drosophila able to identify what structure they are to become?

Answer 9

The influence of the homeotic genes

Question 10

What are the egg polarity genes?

Answer 10

Genes whose products specify egg polarity and the spatial coordinates of the egg and future embryo, the first elements in generating the anterior-posterior axis

Question 11

Where do the egg-polarity genes in drosophila come from?

Answer 11

They are transcribed from the maternal genome during oogenesis by nurse cells and transferred into the egg via microtubule transport

Question 12

What are the major maternal effect genes, egg-polarity genes in drosophila?

Answer 12

Bicoid and hunchback (maternal) in the head and thoracic regions
Nanos and caudal in the abdominal region

Question 13

How does the bicoid gradient form in the drosphila embryo?

Answer 13

Bicoid mRNA is anchored to the anterior of the egg and after fertilization it is translated and its protein will travel via diffusion down the cell creating a concentration gradient where the highest concentration is at the anterior end

Question 14

what is the effect of bicoid protein in drosphila?

Answer 14

The bicoid protein is a transcription factor which has a homeodomain which binds to the promoter for zygotic hunchback activating its transcription creating a zygotic hunchback gradient which is the same as the bicoid gradient

Question 15

What is the effect of the maternal hunchback gene in drosophila?

Answer 15

Maternal hunchback mRNA is initially spread evenly throughout the egg but posterior expression is inhibited by a combination of the nanos and pumilio protein this results in a maternal hunchback expression pattern that mimicks the zygotic one with a concentration gradient forming which is highest at the anterior end

Question 16

What is the relationship between nanos and hunchback in drosphila?

Answer 16

Where nanos is high, hunchback is low

Question 17

How can bicoid function as a repressor in drosophila?

Answer 17

It represses the translation of the uniformly distributed caudal gene creating an opposing concentration gradient where caudal is high at the posterior end of the embryo and it can activate posterior terminal genes

Question 18

What egg polarity genes are high in the posterior end of the drosophila embryo?

Answer 18

Nanos and caudal

Question 19

What egg polarity genes are high in the anterior end of the drosophila embryo?

Answer 19

Huncback and bicoid

Question 20

What occurs in the offspring a female drosophila who is a mutant in the bicoid gene?

Answer 20

There is no bicoid present in the embryo, as this is the anterior morphogen there is no anterior fate and telson-abdomen-abdomen-telson embryo develops
If bicoid is injected a head will grow at the site of injection

Question 21

What occurs in the offspring of a drosophila who is a mutant in the nanos gene?

Answer 21

There is no nanos present in the embryo, as this is the posterior morphogen the resulting offspring will have no abdomen

Question 22

How does nanos get into the drosophila embryo?

Answer 22

It is produced by nurse cells and some of it is trapped in the posterior pole of the oocyte

Question 23

How does nanos become trapped in the posterior pole of the oocyte?

Answer 23

oksar mRNA and staufen protein are transported via kinesin to the posterior of the egg and bind to actin
Staufen then allows oskar mRNA to be translated and the oskar protein binds to nanos

Question 24

Why isn’t all of the nanos trapped at the posterior pole of the drosphila embryo?

Answer 24

Some of the nanos can diffuse by the cytoplasm in a smaug and cup complex
Smaug binds to the 3’-UTR of nanos mRNA and recruits cup togehter these prevent access to ribosome
Oskar can disrupt this smaug/cup complex allowing translation at the posterior end only thus a concentration gradient is formed

Question 25

What is the role of the gurken-torpedo signalling pathway in the drosophila embryo?

Answer 25

nucleus sits at the posterior end of the developing oocyte the nucleus is associated gurken mRNA, which diffuses to the torpedo receptor present in follicle cells The follicle cells send a signal which causes a reorganization of microtubules in the egg this causes the nucleus to move to the dorsal anterior region and establishes a nearby posterior fate

Question 26

What is the signalling pathway the torpedo receptor activates causing a posterior fate in the drosphila fruit fly?

Answer 26

The signal from the follicular cells activates protein kinase A in the oocyte leading to dissociation of the microtubules allowing them to reform from the anterior cortex

Question 27

What is the role of the segmentation genes in the drosphila embryo?

Answer 27

They establish the number and polarity of the segments in the adult fly, functioning through the embryonic parasegments which behave as independent developmental units

Question 28

What are the three classes of segmentation genes in order of expression?

Answer 28

Gap genes which activate Pair rule genes which activate segment polarity genes

Question 29

What occurs in mutants of the gap genes?

Answer 29

Mutants lack large regions of the body

Question 30

What occurs in mutants of the pair rule genes?

Answer 30

Mutants lack portions of every other segment

Question 31

What occurs in mutants of segment polarity genes?

Answer 31

Mutants have defects in every other segment

Question 32

What is the function of the gap gene in the drosophila embryo?

Answer 32

Gap genes mark out the coarsest subdivisions of the embryo and are the first zygotic genes to be activated which occurs by their interaction with specific maternal factors They code for short-lived transcription factors which cause regions of expression, these regions may be stabilized by repressive actions of pairs of non-adjacent gap genes

Question 33

What are the broad groups in the gap gene family?

Answer 33

The head gap genes, the central gap genes and the terminal gap genes

Question 34

What is an example of a head gap gene?

Answer 34

buttonhead

Question 35

What are some examples of central gap genes?

Answer 35

hunchback, giant, krupel, knirps

Question 36

What are some examples of terminal gap genes?

Answer 36

tailless and huckebein

Question 37

What are the pair rule genes?

Answer 37

Segmentation genes activated by the gap genes and expressed at cellularisation, they code for tanscription factors and are grouped into primary and secondary Define the boundaries of the 14 parasegements

Question 38

What is the difference between primary and secondary pair rule genes?

Answer 38

Primary (even-skipped) pair rule genes have their expression depend on maternal and gap genes while secondary has their expression depend on primary Primary specifies odd parasegments with a gene repressor acting on ftz Secondary (fushi tarazu) specifies even parasegements and is a gene activator

Question 39

What occurs to drosophila with mutations in pair rule genes?

Answer 39

cause a series of deletions which affect alternate sections of the embryo so there are only half as many segments as normal

Question 40

How is each segment formed by the pair-rule genes in the drosophila embryo formed?

Answer 40

They are each formed independently of each other

Question 41

How is the second even skipped stripe (parasgement 3) formed in drosphila?

Answer 41

There is a promoter which can bind Biocid and hunchback to activate it and giant and kruppel which are repressors this enables giant to set the anterior parasegment boundary and kruppel to set the posterior segment boundary

Question 42

What are the segment polarity genes?

Answer 42

Engrailed, wingless, hedghog, dishevelled and others these genes have there expression affected by pair-rule genes and function to specify the boundaries of each segment during cellularisation (long term control of segmentation as opposed to gap and pair rule genes)

Question 43

What is the role of engrailed in the drosophila embryo?

Answer 43

Delineates the anterior of each parasegment, even skipped activates odd numbered engrailed stripes, while Fushi tarazu activates the even stripes So engrailed is in every parasegement it its anterior cells or the posterior compartment of each adult segment

Question 44

How is wingless expressed in the drosophila embryo?

Answer 44

Wingless is expressed in the absence of fushi tarazu and evenskipped which occurs in the posterior of the parasegment due to decay of evenskipped and fushi tarazu signalling protiens Slopp-paired protein however is contained in the posterior of the parasegment and this activates wingless expression

Question 45

What is the role of wingless in the drosophila embryo?

Answer 45

It defines the posterior of the parasegment or the anterior compartment of the adult segment

Question 46

How does wingless and engrailed signalling interact with each other?

Answer 46

There is a reciprocal signalling loop where engrailed activates hedgehog which signals across the parasegmental boundary to its receptor patched, this stops smoothened from inhibiting wingless expression Cubitus interuptus then mediates wingless transcription Wingless then diffuses posteriorly binding to frizzled to activate dishevelled, inhibiting Zeste-white 3 freeing armadillo activating engrailed transcription

Question 47

What is the purpose of the wingless and engrailed signalling loop?

Answer 47

This provides stable parasegmental boundary