Development and Axis Specification in drosophila

1. Drosophila cleavage is superficial. The nuclei divide 13 times before forming cells. Before cell
formation, the nuclei reside in a syncytial blastoderm. Each nucleus is surrounded by an actinfilled
cytoplasm.
2. When the cells form, the Drosophila embryo undergoes a midblastula transition, wherein the
cleavages become asynchronous and new mRNA is made. The amount of chromatin determines
the timing of this transition.
3. Gastrulation begins with the invagination of the most ventral region, the presumptive
mesoderm. This causes the formation of a ventral furrow. The germ band expands such that the
future posterior segments curl just behind the presumptive head.
4. Maternal effect genes are responsible for the initiation of anterior-posterior polarity. Bicoid
mRNA is sequestered by its 3´ UTR in the future anterior by the cytoskeleton; nanos mRNA is
sequestered by its 3´ UTR in the future posterior pole. Hunchback and caudal messages are seen
throughout the embryo.
5. At fertilization, bicoid and nanos messages are translated. A gradient of Bicoid protein
activates more hunchback transcription in the anterior. Moreover, Bicoid inhibits the translation
of caudal mRNA. A gradient of Nanos in the posterior inhibits the translation of hunchback
mRNA. Caudal protein is made in the posterior.
6. Bicoid and Hunchback proteins activate the genes responsible for the anterior portion of the
fly; Caudal activates genes responsible for posterior development.
7. The unsegmented anterior and posterior are regulated by the activation of the Torso protein at the
anterior and posterior poles of the egg.
8. The gap genes respond to concentrations of the maternal effect gene proteins. Their protein products
interact with each other such that each gap gene protein defines specific regions of the embryo.
9. The gap gene proteins activate and repress the pair-rule genes. The pair-rule genes have modular
promoters such that they become activated in the seven "stripes." Their boundaries are defined by the gap
genes. These genes form seven bands of transcription along the anterior-posterior axis, each one
comprising two parasegments.
10. The pair-rule gene products activate engrailed and wingless expression in adjacent cells. The engrailedexpressing
cells form the anterior boundary of each parasegment. These cells form a signaling center that
organizes the cuticle formation and segmental structure of the embryo.
11. The homeotic selector genes are found in two complexes on chromosome 3 of Drosophila. Together
these are called Hom-C, the homeotic gene complex. The genes are arranged in the same order as their
transcriptional expression. These genes specify each segment, and mutations in these genes are capable of
transforming one segment into another.
12. The expression of each homeotic selector gene is regulated by the gap and pair-rule genes. Their
expression is refined and maintained by interactions whereby the protein products interact with genes,
preventing the transcription of neighboring Hom-C genes.
13. In Ultrabithorax mutations, the third thoracic segment becomes specifed as the second thoracic
segment. This converts the halteres into wings. When Antennapedia is expressed in the head as well as in
the thorax, it represses antenna formation, allowing legs to form where the antenna should be.
14. The targets of the Hom-C proteins are the realisator genes. These include Distal-less and Wingless
genes (in the thoracic segments).
15. Dorsal-ventral polarity is regulated by the entry of the Dorsal protein into the nucleus. Dorsal-ventral
polarity is initiated by the nucleus being positioned in the dorsal-anterior of the oocyte and transcribing the
gurken message. This message is transported to the region above the nucleus and adjacent to the follicle
cells.
16. The gurken mRNA is translated into the Gurken protein, which is secreted from the oocyte and binds to
its receptor, Torpedo, on the follicle cells. This dorsalizes the follicle cells, preventing them from
synthesizing Pipe.
17. The Pipe protein in the ventral follicle cells modifies an as yet unknown factor that modifies the Nudel
protein. This allows the Nudel protein to activate a cascade of proteolysis in the space between the ventral
follicle cells and the ventral cells of the embryo.
18. As a result of the cascade, the Spätzle protein is activated and binds to the Toll protein on the ventral
embryonic cells.
19. The activated Toll protein activates Pelle and Tube to phosphorylate the Cactus protein, which has been
bound to the Dorsal protein. Phosphorylated Cactus protein is degraded, allowing Dorsal protein to enter
the nucleus.
20. Once in the nucleus, Dorsal protein activates the genes responsible for the ventral cell fates and
represses those genes whose proteins would specify dorsal cell fates. Since a gradient of Dorsal protein
enters the various nuclei, those at the most ventral surface become mesoderm, those more lateral become
neurogenic ectoderm.
21. Organs form at the intersection of dorsal-ventral and anterior-posterior regions of gene expression.
Ref:Developmental biology Gilbert