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

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)

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.

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.

(Growth Factor)

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

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

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

Scr Antp T2 T1 T3 A8 Ubx AbdB

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

WT Antp incorrectly expressed in head antenna T2 legs