Flashcards in Lecture 7 Deck (47):
What is necessary to define each cell geographically in the blastoderm?
Two coordinates set up by two different signalling systems, one for the anterior-posterior axis and one for the dorsal ventral axis
How is the drosophila a segmented organism?
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
Why is it hard to determine precise numbers of segments for different body parts of the drosophila?
Portions of the head and abdomen fuse during development
What is the fundamental unit of embryonic gene expression in the drosophila?
What is the parasegment in drosophila?
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
What is the heirachy of the three classes of gene involved in generating the anterior-posterior axis in drosophila?
The egg-polarity genes which control the segmentation genes which control the segment polarity genes
How is polarity established in the egg?
The anterior, posterior and unsegmented terminal regions are specified by organizing centers established by the egg-polarity genes
How is the drosophila body segmented?
The organising centres establish a periodic pattern of segments involving the segmentation genes
How are the segments formed in drosophila able to identify what structure they are to become?
The influence of the homeotic genes
What are the egg polarity genes?
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
Where do the egg-polarity genes in drosophila come from?
They are transcribed from the maternal genome during oogenesis by nurse cells and transferred into the egg via microtubule transport
What are the major maternal effect genes, egg-polarity genes in drosophila?
Bicoid and hunchback (maternal) in the head and thoracic regions
Nanos and caudal in the abdominal region
How does the bicoid gradient form in the drosphila embryo?
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
what is the effect of bicoid protein in drosphila?
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
What is the effect of the maternal hunchback gene in drosophila?
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
What is the relationship between nanos and hunchback in drosphila?
Where nanos is high, hunchback is low
How can bicoid function as a repressor in drosophila?
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
What egg polarity genes are high in the posterior end of the drosophila embryo?
Nanos and caudal
What egg polarity genes are high in the anterior end of the drosophila embryo?
Huncback and bicoid
What occurs in the offspring a female drosophila who is a mutant in the bicoid gene?
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
What occurs in the offspring of a drosophila who is a mutant in the nanos gene?
There is no nanos present in the embryo, as this is the posterior morphogen the resulting offspring will have no abdomen
How does nanos get into the drosophila embryo?
It is produced by nurse cells and some of it is trapped in the posterior pole of the oocyte
How does nanos become trapped in the posterior pole of the oocyte?
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
Why isn't all of the nanos trapped at the posterior pole of the drosphila embryo?
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
What is the role of the gurken-torpedo signalling pathway in the drosophila embryo?
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
What is the signalling pathway the torpedo receptor activates causing a posterior fate in the drosphila fruit fly?
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
What is the role of the segmentation genes in the drosphila embryo?
They establish the number and polarity of the segments in the adult fly, functioning through the embryonic parasegments which behave as independent developmental units
What are the three classes of segmentation genes in order of expression?
Gap genes which activate Pair rule genes which activate segment polarity genes
What occurs in mutants of the gap genes?
Mutants lack large regions of the body
What occurs in mutants of the pair rule genes?
Mutants lack portions of every other segment
What occurs in mutants of segment polarity genes?
Mutants have defects in every other segment
What is the function of the gap gene in the drosophila embryo?
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
What are the broad groups in the gap gene family?
The head gap genes, the central gap genes and the terminal gap genes
What is an example of a head gap gene?
What are some examples of central gap genes?
hunchback, giant, krupel, knirps
What are some examples of terminal gap genes?
tailless and huckebein
What are the pair rule genes?
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
What is the difference between primary and secondary pair rule genes?
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
What occurs to drosophila with mutations in pair rule genes?
cause a series of deletions which affect alternate sections of the embryo so there are only half as many segments as normal
How is each segment formed by the pair-rule genes in the drosophila embryo formed?
They are each formed independently of each other
How is the second even skipped stripe (parasgement 3) formed in drosphila?
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
What are the segment polarity genes?
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)
What is the role of engrailed in the drosophila embryo?
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
How is wingless expressed in the drosophila embryo?
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
What is the role of wingless in the drosophila embryo?
It defines the posterior of the parasegment or the anterior compartment of the adult segment
How does wingless and engrailed signalling interact with each other?
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