1. Establishing positional information along anterior-posterior axis
bicoid (bcd)
Maternally transcribed bcd mRNA localized at anterior end of oocyte.
mRNA
A P
Protein
Translated following fertilization forms gradient of bcd protein by diffusion.
Bcd protein = TF

2. Bcd regulates txn of Gap genes
(zygotically expressed genes)
High levels of Bcd activates txn of hb
Hunchback (hb)
High levels of Bcd represses txn of Kr
Kruppel (Kr)
Slightly lower levels of Bcd are able to repress txn of kni
Knirps (kni)

3. Gap genes also encode TFs
They regulate each others’ txn:
Ex: Hb represses txn of Kr, helping to refine anterior boundary of Kr expression.
They also regulate txn of other genes:
Ex: Pair Rule genes:
Each is expressed in 7 stripes.

4. Pair rule genes also encode TFs
They regulate each others’ txn.
Also regulate txn of Segment Polarity genes:
Each is expressed in 14 stripes along A-P axis.
Some encode TFs.
Others encode proteins involved in cell-cell communication.

5. (Growth Factor)

6. Drosophila embryogenesis
Cellular blastoderm
pole cells (germ line)
Anterior
Posterior
10 h
Segmented embryo
22 h
1st instar larva

7. TFs
Divide the embryo into segments
Regulate txn of homeotic genes
TFs
TFs
Cell-cell communication

8. (master control genes)
Homeotic genes = Selector genes
Selector Gene Model
Selector Genes
(master control genes)
Effector Genes

9. Scr
Antp T2 T1 T3 A8
Ubx
AbdB

10. Homeotic Phenotypes WT Ubx T3 Effector Genes Ubx mutant
Development of an inappropriate body part in place of the correct body part. WT T1 T2 T3
Ubx
T3 Effector Genes
Ubx mutant
Ubx Antp off in T3 cells
Ubx- Antp on in T3 cells

11. WT Antp incorrectly expressed in head
antenna
T2 legs