Lecture 4

Question 1

What are the features of C.Elegans?

Answer 1

Mainly hermaphrodites capable of self-fertilization although there are small numbers of males (no females exist)
The egg is small and ovoid with rapid embryo development is rapid the larva hatching after 15 hours at 20 degrees Celsius
Maturation to adulthood via four larval stages takes about 50 hours

Question 2

How is the anterior-posterior body axis formed in c.elgans?

Answer 2

This is the first thing established in the c. elgans and it occurs at fertilization
The sperm enters the egg and its pronuclei and centrosome locates to the posterior pole
The centrosome then interacts with the actin microfilaments at the posterior end driving contraction in an anterior direction driving cytoplamsic flow to the posterior end
This allows asymmetric distibution of maternal factors such as mRNAs and proteins including par 2 and 3
par 2 accumulates at the posterior end while par 3 accumulates at the anterior end
An unequal cell division then occurs forming a large AB cell containing on par 3 and a smaller P cell containing only par 2

Question 3

What occurs at the second cleavage of c. elegans?

Answer 3

There is rotational cleavage as there is rotation of the centrosomes in the P1 cell due to the asymmetric distribution of the Par proteins in the P1 and AB cells
The AB cell gives rise to the ABa cell and the ABp cell while the P1 cell divides meridonally to give rise to the EMS cell and the P2 cell

Question 4

What happens at the anterior end of the fertilized egg of c.elegans prior to the first cleavage?

Answer 4

A cap of microfilaments forms and the P granules become localized at the posterior of the egg

Question 5

What are P granules in the C.Elegans?

Answer 5

Granuleswhich contain mRNAs which are essential for germ cell development

Question 6

Why can the main axes of c. elegans be identified at the second cleavage?

Answer 6

Because P2 is posterior and ABp is squeezed into a dorsal-posterior position with EMS in the ventral position

Question 7

When is handedness etablished in c.elegans?

Answer 7

At the third division

Question 8

How is the fate of ABp determined relative to ABa?

Answer 8

The P2 cell is in contact with the ABp cell and signals it via notch-delta/GLP-1 APX-1 pathway
The ABa cell however does not have this contact and therefore does not receive the signal

Question 9

What is the difference between the notch-delta signalling pathway and the GLP-1-APX-1 pathway?

Answer 9

The notch-delta pathway is found in drosophila while the GLP-1-APX-1 is found in C.elegans

Question 10

How does the GLP-1/APX-1 signalling pathway function in C.Elgans?

Answer 10

GLP is present in ABp and APX-1 is present in P2
they are both transmembrane proteins and deliver a signal requiring cellular contact delivering a spatially precise signal
The APX-1 ligand binds to the GLP receptor causing a cleavage of the APX-1 intracellular domain and its translocation to the nucleus where it acts as a transcription factor to activate the transcription of genes specifying ABp

Question 11

Why is GLP-1 protein only present in the AB cell at the two cell stage?

Answer 11

Its translation is repressed in the P cell lineage

Question 12

What happens to the EMS cell when it undergoes cell division and how is this controlled?

Answer 12

The EMS cell undergoes a cell division to form the MS and E cell which go on to form very different tissues in the organism
This differing fate of the daughter cells is controlled by a Wnt/MOM pathway

Question 13

What is the difference between the wnt pathway and the MOM pathway?

Answer 13

Drosophila use Wnt while C.Elegans uses the homologes MOM-2 and MOM-5

Question 14

How does the MOM pathway proceed in c.elegans?

Answer 14

There is signalling between the P2 cell and the EMS cell with MOM-2 from the P2 cell binding to the MOM-5 receptor on the EMS cell leading to a complex of WRM-1 and a protein kinase LIT-1 accumulating in the E cell nucleus
LIT-1 then phosphorylates the transcriptional regulator POP-1 causing it to exported from the E cell nucleus and allowing SYS-1 to enter the E cell nucleus (but not the MS cell nucleus) where it acts as a co-activator for POP-1 and switch on endodermal-specifying genes
In the MS cell there is no LIT-1 complex so the POP-1 regulator stays in the nucleus preventing the endodermal genes from being transcribed

Question 15

What does the P2 cell divide to form?

Answer 15

The C cell and P3
P3 will then divide to give D and P4
the C cell forms hypodermis, neurons and body wall muscle
The D cell forms muscle exclusively
P4 retains the P granules and gives rise to the germ line

Question 16

What is the post-embryonic development that occurs in C.Elegans?

Answer 16

There are four larval stages seperated by moults the newly hatched larva is similar to the adult but smaller and immature containing only 558 nuclei to the 959 of the mature adult hermaphrodite and 1031 of the mature adult male
There is also a dower stage which the c.elegans can enter into if conditions are bad where development can be halted for up to 4 months

Question 17

What controls the timing of the moulting of the larval stages in c. elegans?

Answer 17

Controlled by the concentrations of LIN 14 with decreasing concentrations marking each stage in the larval life cycle
LIN 14 is controlled by miRNA LIN-4

Question 18

How does miRNA inhibit protein synthesis?

Answer 18

A pre-miRNA is produced as a hairpin loop, Dicer then cleaves this into a single-stranded miRNA, this then binds to RSC to target ti to bind to certain mRNAs via complementary base pairs and to target them for degradation

Question 19

How is the vulva formed in c.elegans?

Answer 19

There are 22 cells in the adult vulva in c.elegans there are four cells which control its specification, the gonadal anchor cell which is the inducing cell and three responding cells (P5p, P6p, P7p).
The responding cells come from a set of six equivalent cells (P3p-P8p) theoretically any of these cells could form the vulva depending on which cells the anchor cell signals
The anchor cell gives a primary inductive signal to one of the cells confering a primary vulval fate on it and a secondary vulval stat on the two adjacent cells brought about by lateral inhibition of the primary cell
The cells that do not recieve any signal will adopt a tertiary fate and go on to form parts of the hypodermis

Question 20

What is the signal given by the anchor cell which will induce the formation of a vulva in C.Elegans?

Answer 20

The secreted product of the lin-3 gene whichi s the equivalent of EGF, the receptor is the product of the let-23-gene a receptor tyrosine kinase equivalent to EGFR

Question 21

What is the lateral inhibitory signal provided by the primary vulval cell in c. elegans?

Answer 21

A signal composed of 3 proteins from the delta family which interact with the product of lin-12 which is a receptor of the Notch family