3. A-P patterning in Drosophila Flashcards
What are the axes that need to be patterned in Drosophila?
Axes to be patterned:
- A-P
- D-V
- L-R
When does Drosophila A-P patterning begin?
In oogenesis in the mother - maternally deposited factors - RNA + proteins
Whidh factors determining A-P pattern are deposited in oogenesis?
A-P patterning maternally deposited factors:
- A: bcd RNA
- P: Oskar RNA -> nanos RNA
Which genes are in turn influenced in response to the maternally deposited A-P determing factors?
bcd and nanos RNA are maternally deposited in A and P of oocyte
caudal and hb RNA are maternally deposited all over in oocyte - no localisation
Once bcd and nanos are translated into protein:
- bcd inhibts caudal RNA -> protein => caudal protein localised in P -> activated by nanos
- nanos inhibits hb RNA -> protein => hb localised in A -> activated by bcd
Explain Drosophila ovary
2 ovaries - consist of 16-20 ovarioles - tubes with linear arrangement of egg chambers with developing oocytes - move as they develop from ovariole tip into the oviduct
Explain how an egg chamber is formed in oogenesis
Ovary <- ovariole <- egg chambers at different stages of development
Egg chamber consists of:
- nurse cells
- follicle cells
- oocyte
- MTOC + microtubules
Egg chamber formed at A end of germaniurm where germline stem cells (GSC) divide assymetrically -> self-renew + produce a cystoblast
Cystoblast divides in incomplete cell divisions leaving ring canals and fusomes between the divided cells - vary in connection # between 1-4 => 16 cells from which 2 cells with 4 connections - one of the 2 will become the oocyte - others nurse cells
Follicle cells: germarium also has somatic follicle stem cells (FSC) - when GSCs divide so do FSCs ->
Which are germline and which somatic: nurse cells, oocyte, follicle cells?
Nurse cells: germline - germline stem cells (GSC) divide assymetrically in germarium -> cystoblast - nurse cells come from cystoblast divisions
Oocyte: germline
Follicle cells: somatic - follicle stem cells (FSC) in germarium divide to give rise to follicle cells in egg chamber
Why does the oocyte form from the cystoblast divided cell with 4 connections rather than with 1?
4 connections allow accummulation of more proteins -> promotes MTOC development - microtubule start stretching through ring canal into - microtubules import more products into oocyte - allows the oocyte to become different from 15 other sister germ precursos which become nurse cells
How does the oocyte move in the egg chamber to get positioned?
Oocyte moves by:
- Microtubule organising center (MTOC) aligns microtubules -> +/- ends -> oocyte gets positioned at distal end between follicle cells
- division direction of nurse cells
- at young egg chamber stage - nurse cells produce Delta signal - influences adjacent follicle cells different - turns unpaired expression in follicle
- adhesion ECad molecule produced in oocyte + in follicle cells near the oocyte -> stick together => oocyte pulled towards distal end
Explain the role of Delta signal in oocyte positioning in ovariole?
In young egg chamber - oocyte pulled distally by older egg chamber by:
older egg chamber secretes Delta signal - activates Upr signal via JAK-STAT pathway -> signal increased ECad on oocyte surface towards older egg chamber follicle cells => adhesion - oocyte is pulled distally as disc changes into a ball of cells
Explain how oocyte microtubule reorganisation is induced
Oocyte reorganises its microtubules in response to:
1. Delta signal
Older egg chamber activates Delta signal - Upr signal via JAK-STAT pathway signals to oocyte - oocyte increased ECad expression on its surface facing the follicle cells near the older egg chamber
- Gurken (EGF) signal
More Upr signal activates microtubules to organise (-) end to oocyte edge, (+) end to nurse cells - transport gurken RNA into oocyte - gurken gets translated and diffuses into nearby terminal posterior follicle cells - Unknown back signal
In response to gurken follicle cells return with an some kind of ‘back’ signal which signals for microtubule reorganisation in oocyte: induces myosin -> recruits PAR-1 -> inhibits MTOC at distal end + microtubules degraded - New microtubules and MTOC form
What is microtubule reorganisation needed for?
Microtubules reorganised to later position maternal factors and the nucleus
What are the oocyte microtubules reorganised into after microtubule reogranisation?
Unorganised mesh of microtubules:
many different MTOC more A end with (-) and (+) microtubule ends - messy but polarised A(-)<->(+)P
How does bcd and oskar get localised in oocyte?
bcd and oskar expressed all over - microtubules arranged with polar ends A(-)<->(+)P
Motor proteins:
- dynein goes (+) -> (-) carrying bcd RNA into A
- kinesin goes (-) -> (+) carrying oskar PROTEIN into P => nanos localises with oskar in P
bcd RNA<————> nanos RNA
What is the difference between RNA and protein localisation in oocytes / embryos?
- RNA is localised, no moving - no gradient
TRANSLATION - protein diffuses and creates a gradient
Explain how bcd RNA influences AP patterning in embryo
bcd deposited in A of oocyte - after fertilisation 60min gets translated into protein - constantly produced because bcd decays quickly - diffuses and creates bcd gradient in the syncytial cytoplasm of the embryo
bcd - a TF - turns on genes at different conc along the gradient -> induces zygotic transcription of
