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9.35 Homeotic gene expression in Drosophila
After the parasegmental boundaries are set, the pair-rule
and gap genes interact to regulate the homeotic
selector genes, which determine the identity of each
segment.
Antennapedia complex:
labial (lab), Deformed (dfd)- head
Antennapedia (Antp), sex comb reduced (scr)- thoracic
segments
proboscipedia (pb)- appears to act only in adults, but in
its absence, the labial palps of the mouth are
transformed into legs.
9.35 Homeotic gene expression in Drosophila
Bithorax complex:
Ultrabithorax (Ubx)required for the identity of
the 3rd thoracic segment.
abdominal A (abdA) and
abdominal B (abdB)responsible for the
segmental identities of the
abdominal segments.
Homeotic complex (Hom-C)
9.36 A four-winged fruit fly constructed by putting together three mutations in
cis regulators of the Ultrabithorax gene
Wild type: 3 thoracic segments. 2nd segment produces a pair of
wings and the 3rd segment produces a pair of wings and a pair of
balancers known as halteres.
When the Ultrabithorax
gene is deleted, the 3rd
thoracic segment is
transformed into another 2nd
thoracic segment.
9.37 (A) Head of a wild-type fruit fly. (B) Head of a fly containing the Antennapedia
mutation that converts antennae into legs
When Antennapedia gene is
expressed in the head, legs rather
than antennae grow out of the
head sockets.
Initiating the patterns of homeotic gene expression
The initial domains of homeotic gene expression are
influenced by the gap genes and pair rule genes.
- Expression of abdA and abdB genes is repressed by the gap
gene proteins Hunchback and Kruppel.
- The Antennapedia gene is activated by particular levels of
Hunchback.
Gap genes and pair-rule genes are transient.
Polycomb proteins- repression of homeotic genes
Trithorax proteins- maintain the chromatin active conformation
9.38 Antibody staining of the Ultrabithorax protein in (A) the wing disc and (B) the
haltere disc of third instar Drosophila larvae
The homeotic genes work by activating or repressing a group
of “realisator genes”, which are the targets of homeotic
gene proteins and which function to form the specified
tissue or organ primordia.
Ultrabithorax protein is able to repress the expression of wingless in
those cells that will become the halteres of the fly.