Lecture 6: intro to Drosophila

Question 1

Why are Drosophila a useful animal model?

Answer 1

• accessible embryology and adult developmental stages
• very low cost, fast
• excellent genetics
• no ethical concerns
• can control life cycle with temp i.e., 10 days @ 25 degrees and 21 days @ 18 degrees
• whole genome sequenced
• have many conserved genes with vertebrates
• a range of techniques can be used to visualize, misexpress or reduce gene expression e.g., enhancer trap, Gal4/UAS, RNAi etc.

Question 2

Describe the Drosophila life cycle.

Answer 2

Fertilised egg -> cleavage -> syncytial blastoderm -> gastrulation -> embryo -> hatching -> larva:1^st->3^rd instar -> pupa -> metamorphosis -> adult fly ->

Question 3

Describe some features of Drosophila mating.

Answer 3

• One female and one male fly can produce over 200 flies in less than two weeks
• Flies display courtship behaviours to attract a mate e.g., tapping, wind vibration and licking. The most reproducible behaviours are genetically encoded

Question 4

Explain the role of nurse cells in oogenesis.

Answer 4

• In ovarioles: two stem cells – one becomes future egg and one becomes a nurse cell which undergo endoreduplication of their contents (double their DNA exponentially). Proteins and RNA are made in the nurse cells.
• Products of the nurse cell transferred into the oocyte via ring canals in a process called cytoplasmic dumping to provide nutrients to the embryo at the expense of the nurse cell i.e., they sacrifice themselves to feed the oocyte

Question 5

Describe the process of subcellular localisation of maternal factors in the egg.

Answer 5

• Oskar mRNA deposited from the maternal nurse cells is localized in the pOsterior
• Bicoid (morphogen) mRNA deposited from the maternal nurse cells is localized in the anterior
• First signs of patterning
• Surrounding follicle cells secrete the egg shell
• Filaments present through which the egg breathes
• Hydrophobic layer under the shell prevents embryo drying out

Question 6

Describe the process of nuclear division.

Answer 6

• The sperm and egg nucleus fuse and divide
• After 8 cycles there are a number of nuclei spread throughout the cell
• Continues for 14 divisions as nuclei migrate to the outside of the egg – small number remain in the yolk (syncytial blastoderm)
• A small number move to the posterior end of the embryo and form the pole cells which end up on the outer surface of the blastoderm. The pole cells eventually give rise to the germ cells which form the gametes. This separation is a common strategy in animal development.
• All nuclei in a common cytosol
• Bicoid and Oskar diffuse freely in the space until after the 14^th division where cellularization occurs – membranes grow from the surface to enclose the nuclei
• Then one cell turns into thousands of cells in a few minutes (cellular blastoderm). The cellular blastoderm (i.e., minus pole cells) is a single epithelial layer and gives rise to all other future tissues.

Question 7

Explain the difference between the Bicoid gradient in Drosophila and a morphogen gradient in vertebrates.

Answer 7

Drosophila Bicoid:

A gradient of Bicoid TF is formed in the fertilised egg - syncytial blastoderm stages when the embryo is not multicellular. The Bicoid protein moves down a concentration gradient from its source a one end of the embryo to the other. Bicoid then functions as a morphogen as different concentrations regulate different target genes at different positions of the embryos A-P gradient.

Vertebrate Morphogens:

A morphogen is a secreted signalling ligand that is distributed in an EC concentration gradient. Different concentrations of ligand stimulate different levels of signalling pathway activity within target cells and these regulate different genes due to different TF activation. Both Bicoid-like and secreted morphogens induce specific cell fates in a concentration dependent manner.

Question 8

Provide experimental proof that Bicoid is necessary and sufficient for the development of anterior structures in the Drosophila embryo.

Answer 8

• extract anterior cytoplasm from a WT embryo and inject into a KO embryo at the posterior end. Embryo develops head structures => sufficient.
• Can be shown also to be a morphogen by implanting in the middle of a KO mutant. Head grows in the middle and thoracic segments on either side of the head in a mirror image.