2. Embryonic development:
Fertilization….
Day 1: development ,embryo hatches out of the egg shell to
become a larva
Day 2,03 and so : larva- three stages/instars, separated by
molting
end of third stage, Pupa forms.
Inside a pupa a radical remodeling of the body - a process
called metamorphosis
After Day 9:Adult fly/ Imago
3.
4. Egg- series of nuclear division (every 8 minute) without cell devision creates a
Syncytium
1st
9 divisions – cloud of nuclei is formed- from middle to surface of egg
moment- form monolayer called Syncytial blastoderm
4 nuclear divisions
Few nuclei to extreme posterior end – few cycles (9)- pole cells- give rise to
germ cells.
When the nuclei reach the periphery of the egg during the tenth cleavage cycle,
each nucleus becomes surrounded by microtubules and microfilaments. The
nuclei and their associated cytoplasmic
islands are called energids. (up to 12th
cycle)
5. Plasma membrane grow inward converting syncytial blastoderm into
cellular blastoderm (13th
cycle)
Cell division slows down, asynchronous, and transcription rate is increased.
Blastoderm fate map:
6.
7. Transcription from the nuclei (which begins around the eleventh cycle) is
greatly enhanced at this stage. This slowdown of nuclear division and the
concomitant increase in RNA transcription is often referred to as the mid
blastula transition.
Gastrulation: The first movements of Drosophila gastrulation segregate the
presumptive mesoderm, endoderm, and ectoderm. The prospective mesoderm
about 1000 cells constituting the ventral midline of the embryo folds inward
to produce the ventral furrow.
This furrow eventually pinches off from the surface to become a ventral tube
within the embryo. It then flattens to form a layer of mesodermal tissue
beneath the ventral ectoderm.
2:50 h
3:40 h
4:20 h
8. The prospective endoderm invaginates as two pockets at the anterior and posterior ends of the
ventral furrow. The pole cells are internalized along with the endoderm. At this time, the
embryo bends to form the cephalic furrow.
The ectodermal cells on the surface and the mesoderm undergo convergence and extension,
migrating toward the ventral midline to form the germ band, a collection of cells along the
ventral midline that includes all the cells that will form the trunk of the embryo.
The germ band extends posteriorly and, perhaps because of the egg case, wraps around the top
(dorsal) surface of the embryo
9.
10. While the germ band is in its extended position, several key
morphogenetic processes occur: organogenesis, segmentation, and the segregation
of the imaginal discs. Imaginal discs are those cells set aside to produce the adult
structures.
In addition, the nervous system forms from two regions of ventral ectoderm.
Neuroblasts differentiate from this neurogenic ectoderm within each segment (and
also from the Non segmented region of the head ectoderm). Therefore, in insects
like Drosophila, the nervous system is located ventrally.
11. Body plan of Drosophila:
Head
Three thoracic segments (T1 T2 T3)
Eight abdominal segments (A1 to A8)
The first thoracic segment, has only legs; the second thoracic
segment has legs and wings; and the third thoracic segment has
legs and halteres (balancers).
12. The anterior-posterior and dorsal-ventral axes of Drosophila form at
right angles to one another, and they are both determined by the
position of the oocyte within the follicle cells of the ovary.
