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Axis Specification in
Drosophila
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
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
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
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.
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
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
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
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
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
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
Dorsal Ventral Axis
• Another morphogen helps to determine this axis
• Dorsal Protein is the product of the dorsal gene

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BIO420 Chapter 09

  • 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