Lecture 6

Question 1

What are the four main stages of the drosophila life cycle?

Answer 1

The adult form, the embryonic form, the larval form and the pupa form
9 days are taken to complete the lifecycle

Question 2

Where does oogenesis occur in drosophila?

Answer 2

In the ovary of the female drosophila which has 16-20 ovarioles made of anterior germarium and posterior vitellarium

Question 3

What makes up the germarium in the ovarioles of the drosophila?

Answer 3

Germ line stem cells

Question 4

What makes up the vitellarium in the ovarioles of drosophila?

Answer 4

A chain of developing egg chambers

Question 5

What makes up a mature egg chamber?

Answer 5

A mature oocyte, 15 nurse cells and a large number of follicle cells

Question 6

How does oogenesis occur in the drosophila?

Answer 6

Stem cells in the germarium divide to give rise to one stem call and one cystoblast
The cystoblast then undergoes 4 rounds of mitotic division with incomplete cytokinesis giving rise to a syncytial cyst of 16 cells
One of the 16 cystocytes which is connected to 4 neighbouring cells undergoes meiosis to form the oocyte while the remaining cystoblasts go on to form nurse cells

Question 7

What connects all the 16 cells of the syncytial cyst?

Answer 7

Ring canals which forms cytoplasmic bridges from arrested mitotic cleavage furrows
There is also microtubule-based fusome which runs between the all of the ring canals
This forces the cells to divide in a specific known pattern

Question 8

How can the fusome indicate which cystoblast is most likely to give rise to the oocyte?

Answer 8

The cystoblast which contains the most of the fusome is typically the one which goes on to become the oocyte

Question 9

What is the function of nurse cells in drosophila?

Answer 9

The produce mRNAs and proteins into the oocyte which are moved by motor proteins along the fusome and throguh ring canals to establish the maternal factors in the oocyte

Question 10

What is the fusome made of?

Answer 10

Continuous elements of the endoplasmic recticulum and cytoskeletal proteins including polarized microtubules which have their minus ends restricted to the future oocyte

Question 11

Where does the fusome come from?

Answer 11

It is a product of the asymmetric cell division which forms the cystoblast

Question 12

What is the function of the fusome?

Answer 12

It controls the microtubule driven localization of the oocyte fate as it marks the anterior of the oocyte

Question 12

What is the function of the fusome?

Answer 12

It controls the microtubule driven localization of the oocyte fate as it marks the anterior of the oocyte

Question 13

How are nurse cells aided in their function of mass producing mRNAs and proteins for the oocyte?

Answer 13

They become polytene increasing the rate of transcript accumulation

Question 14

How do nurse cells become polytene?

Answer 14

When multiple rounds of replication produce many sister chromatids that remain synapsed together

Question 15

What cells other than nurse cells are associated with the drosophila egg?

Answer 15

1000 small follicle cells which are mesodermal and pass nutrients from the haemolymph into the developing egg and lay down the vitelline membrane and the egg shell or chorion

Question 16

What happens to the mRNA when it is placed inside the oocyte?

Answer 16

It is relocalised based on the oocyte cytoskeleton, this process helps to set up the body axis before sperm has even entered the egg

Question 17

How does fertilization of the oocyte in drosophila occur?

Answer 17

The sperm are stored in the seminal receptacle of the female and the eggs are fertilized in the uterus, where the sperm enters the egg through the micropyle, a small anterior protrusion

Question 18

Why is there no block to polyspermy in drosophila?

Answer 18

The huge sperm structures blocks anything else from entering

Question 19

What occurs in the first 10 nuclear divisions of a drosophila egg?

Answer 19

They are very rapid with each division taking 10 minutes there is no G1 or G2 phase
There is no cytoplasmic division resulting in the formation of a syncytium
From this point division will get progressively slow

Question 20

What are energids and how are they formed?

Answer 20

At about the 9th division the nuclei in the syncytium of the drosophila will move out from the center to the surface forming the syncytial blastoderm
The nuclei and their surrounding island of microtubules and microfilaments are called energids

Question 21

How many nuclei formed in the syncytial phase of drosophila development are not used in the embryo proper?

Answer 21

About 200 nuclei remain in the centre of the yolk and do not contribute to the embryo proper

Question 22

How and when are the pole cells formed in drosophila development?

Answer 22

In the beginning of cycle 9 about 5 nuclei reach the posterior pole of the egg so they can from the pole cells which will be the future germ cells
They are also the first cells to become covered by a cell membrane
The pole plasm which becomes incorporated into posterior cells that form around the nuclei controls germ cell determination through polar granules

Question 23

What is the mid blastula transition in drosophila?

Answer 23

After 13 divisions mitotic synchrony is lost, transcription increases and the normal G1 and G2 phases of the cell cycle are establshed and all non-germ cells are delineated

Question 24

What is the difference between syncytial blastoderm and the cellular blastoderm?

Answer 24

After all peripheral cells have been delineated and covered in a cell membrane the syncytial blastoderm is now referred to as the cellular blastoderm

Question 25

What is contained inside a polar granule?

Answer 25

Proteisn such as oskar, vasa, staufen and tudor, mRNAs for oskar, mitochondrial coded ribosomal RNA and polysomes

Question 26

How does gastrulation proceed in the drosophila?

Answer 26

Gastrulation begins soon after the mid blastula transition when 1000 ventral cells express twist and snail causing the cellular blastoderm invaginate ventrally and elongate to form the germ band There is an invagination in the anterior and posterior end which meet and fuse together to form the gut

Question 27

What are the features of the first instar larva of drosophila?

Answer 27

Hatches about 22 hours after fertilization | Has a spatially patterned cuticle with anterior mouthparts and segmentally repeated ventral dentricle bands

Question 28

What is pupation with regards to drosophila?

Answer 28

A phase about 120 hours after fertilization where the larva stop eating and the external cuticle hardens to become the puparium

Question 29

What occurs during the metamorphosis phase of drosophila?

Answer 29

Over the four days after pupation begins larval tissues degenerate within the puparium and form new adult tissues Most of the increasingly polyploid larval tissues die and are replaced by nests of diplid cells which remained undifferentiaited during larval growth

Question 30

What are the undifferentiated groups of cells in the drosophila which serve as precursosrs to the adult system?

Answer 30

``` Imaginal disc (9 pairs plus a central genital disc) form cuticular structures of the adult (wings, leg, head etc) Abdominal histoblasts which are segmentally repeated structures which give rise to the adult abdomen Imaginal nests clustered within each larval organ to give rise to adult organs like salivary glands ```

Question 31

What does the imaginal disc do during metamorphosis?

Answer 31

The undifferentiated cells proliferate further and differentiate under the influence of moulting hormone 20-hydroxyecdysone and change shape evertign the disc to form the adult structure

Question 32

What is transdetermination?

Answer 32

When an imaginal disc cells in drosophila differentiate into inappropriate structures