Animal Development Drosophila axis formation Part 1: A-P patterning. [email protected] Problem: starting point where all cells have the same developmental potential (because they have the same DNA).
However, the end point is the production of nerve cells, muscle cells, epithelial cells etc. Therefore differentiation happens. Mechanism?
Breakage of symmetry
Diagrams representing a group of mesenchymal cells that emit a signal (black dots)
Salazar-Ciudad I Development 2010;137:531-539
THEY LIVE…. a signal (black dots)
Developmental biology: a signal (black dots)
Drosophila segmentation and repeated units
* egg: generate the system
* larva: eat and grow
* pupa: structures in
larvae grow out to form
adult fly: metamorphosis
(Drosophila is a
Eye mutant C
Wing mutant A
Leg mutant B
The unusual feature of the Drosophila early embryo is that the first 13 mitoses are nuclear divisions without concomitant cytoplasmic division, making the embryo a syncitium-a multinucleated cell. After division 9, the plasma membrane of the oocyte evaginates at the posterior pole to surround each nucleus thus creating the pole cells, which will form the fly’s germ line.
Segments in embryos are maintained throughout development a signal (black dots)
The Hunt for Mutants a signal (black dots)
30,000 independently-derived mutants in genes required for survival.
8,000 mutants define genes required for embryonic survival (these became the focus the study).
750 mutants have specific effects on A/P or D/V patterning.
150 genes with specific effects on A/P or D/V patterning identified by the 750 mutants (average of ~ 5 alleles per gene).
Denticle bands on a 1 a signal (black dots)st instar larva.
A Detour Into Embryonic Anatomy – Denticle Bands
Denticle bands are hair-like projections on the ventral cuticle of an embryo.
Denticle bands provide an easily visualized marker of embryonic/larval pattern.
Maternal effect genes a signal (black dots)
Maternal effect genes Anteroposterior and Dorsoventral Axes
Essential for many fundamental processes:
Hamilton et al 2012, Biophysics for the life sciences Chapter 11
EGF signalling between the oocyte nucleus and follicle cells localisation by Gurken signalling.
Isolating the ovary localisation by Gurken signalling.
Weil, Parton, Davis, Jove (2012)
Stage 9 localisation by Gurken signalling.
Weil, Parton, Davis, Jove (2012)
mRNA (in situ)
Protein (Ab staining)
Bicoid is an Anterior Morphogen localisation by Gurken signalling.
Note that bicoid (and other maternal effect gene products) diffuse in the shared cytoplasm of the syncytial blastoderm.
This is a unique feature of insect embryogenesis.
BCD acts as a concentration-dependent manner localisation by Gurken signalling.
Thresholds can turn gradients into sharp boundaries localisation by Gurken signalling.
Bicoid binds the 3’ UTR of caudal mRNA and suppresses translation.
Caudal protein enters the nuclei at the posterior end of the syncytial blastoderm and helps specify posterior fates
Posterior group genes:
The role of nanos is to disable hb maternal mRNA at the posterior end of the egg.
Terminal group genes Anteroposterior and Dorsoventral Axes
Torso: TRK signalling via MAPK Anteroposterior and Dorsoventral Axes
SUMMARY Anteroposterior and Dorsoventral Axes
Nurse cells surrounding the oocyte in the ovarian follicle provide it with large amounts of mRNAs and proteins, some of which become localised in particular sites. The oocyte produces a local signal, which induces follicle cells at one end to become posterior follicle cells. The posterior follicle cells cause a re-organisation of the oocyte cytoskeleton that localises bicoid and hunchback mRNA to the anterior end and other mRNAs such as oskar and nanos to the posterior end of the oocyte. Following fertilisation, development starts and these mRNAs are translated. Subsequently, gradients of the BCD and HB proteins define the anterior nuclei-the embryo is still a syncytial blastoderm, while inhibition of translation of their mRNAs by Nanos define the posterior cells. Nuclei in between receive a variable amount of BCD and HB resulting in differential activation or repression of target genes and finally in different developmental cell fates.