2. Importance of Drosophila melanogaster
• D. melanogaster is one of the most useful organisms in which to study development. Among its
advantages:
• They are easy to breed and keep alive
• We know its entire genetic sequence
• The DNA replicated many times without separating
• They have a short reproductive cycle
3. Early Drosophila Development
• Fruit flies can go through syncytial specification
• This means that the nuclei are not surrounded by a
membrane until after the 13th cell division
• At one point there is a large cell containing around
6000 nuclei
• They share a common cytoplasm
• The dorsal/ventral and anterior/posterior axes are
specified by interactions within this syncytium
• The axes are fixed prior to fertilization
4. Fertilization
• When the sperm gets to the egg, the egg is already activated
• This means that the egg start mitotic divisions.
• When the sperm does enter the egg, it uses a micropyle. This is a tunnel in
the chorion (eggshell). This lets in one sperm at a time in.
• The sperm enters an egg that has already begun to specify the body axes
• The sperm and egg do not fuse. They interact.
• The maternal and paternal chromosomes remain separate until the end of
miosis I
5. Cleavage
• In most insects, there is superficial cleavage (the
cleavage is along the rim of the egg and the yolk
is in the middle.
• In drosophila, the nuclei divide without dell
division—the result is the syncytium.
• After about nine cycles of division, about five
nuclei reach the posterior side of the embryo and
are enclosed with membrane. These pole cells
will be future gametes.
6. Cleavage (continued)
• The syncytium is not uniform. There
are concentrations of various proteins
in it.
• In the 10th division, many nuclei
move to the periphery and become
energids—cells with a membrane and
surrounded by microtubules and actin
filaments.
• At around the 13th division the cell
membrane folds inward to section off
each of the nuclei (cellularization)
along the periphery. This creates the
cellular blastoderm
7. Mid-Blastula Transition
• Up to this point, this is directed by proteins and
mRNA placed in the egg during oogenesis (in other
words, it is all maternal)
• In mid-blastula transition, control of division is
from the zygote itself
8. Gastrulation
• The drosophila will start to form a body plan
• Head and tail end
• Segments in the thorax and abdomen
• Cells on the ventral side invaginate to form a ventral furrow. This will be the
future mesoderm. The furrow will pinch off and become a ventral tube
• At the anterior and posterior ends of the furrow, cells invaginate to make
two pockets which will become future endoderm
• The outer layer starts to bend and form the cephalic (head) furrow which is
ectoderm
9. Segmentation and the
Anterior-Posterior Body Plan
• The molecules that contribute to
embryogenesis came from oogenesis. The
oocyte comes from the female germ cell
(the oogonium).
• The oogonium consists of 16
interconnected cells. 15 of them are nurse
cells.
• Nurse cells make mRNA and proteins
that will get transported to the one cell
that will grow to be the oocyte
10. Continued
• The mRNA and proteins are controlled by various
genes:
• GAP genes- divide the embryo into rough
segments and control the expression of pair-rule
genes
• Pair-Rule genes- divide the embryo into
segments
• Segment Polarity genes- establish the anterior-
posterior orientation for each segment
• Hox genes (homeotic selector)- cause each
segment to develop into specific body parts
• Other genes:
• Acron- helps to form the terminal portion of the
hear
• Telson- responsible for the tail
11.
12. Proteins Acting as
Morphogens
• Bicoid-
• High levels produce head structures
• Lower levels produce mouth structures
• A moderate level produces the thorax
• Its absence produces the abdomen.
• Caudal-
• It activates genes for posterior
development
13. Dorsal Ventral Axis
• Another morphogen helps to determine this axis
• Dorsal Protein is the product of the dorsal gene
