This document discusses Hox genes, which are homeotic genes that encode transcription factors important for segment identity in animals. It describes two major homeotic gene complexes, ANT-C and BX-C, which determine segment identity in the head/thorax and posterior thorax/abdomen, respectively, in Drosophila. Mutations in these genes can lead to homeosis, where one body part develops with the identity of another part. The document also discusses how Hox genes regulate segment identity through binding DNA in a specific combination, the co-linear expression of Hox genes, and how changes in Hox gene expression have contributed to the evolution of arthropod body plans.

1. Hox genes
(Developmental Biology)
By
Mrs Sanchita Choubey
(M.Sc., PGDCR, Pursuing Ph. D)
Assistant Professor of Microbiology
Dr. D Y Patil Arts Commerce and Science College Pimpri, Pune

3. Homeotic gene complexes in Drosophila
 ANT-C (Antennapedia
complex) is largely
responsible for
segmental identity in
the head and anterior
thorax.
 BX-C (Bithorax
complex) is responsible
for segmental identity
in the posterior thorax
and abdomen.

4. HOMEOSIS
 Homeosis or homeotic transformation, is the development of one
body part with the phenotype of another.

5. The bithorax mutations
 This class of loss of
functions mutations
cause the entire
third thoracic
segment to be
transformed into a
second thoracic
segment giving rise
to flies with four
wings instead for
the normal two.

6. The Tab dominant mutations
 These gain of
function mutations
transform part of
the second thoracic
segment into the
sixth abdominal
segment.

7. The Antennapedia mutations
 These gain of function
mutations transform antenna
into leg.

8. The Homeodomain
 The homeotic genes encode transcription factors of a
class called homeodomain proteins. The homeodomain
is a 60aa protein domain, which binds DNA. Hox genes
bind DNA regulatory elements of their target genes in a
specific combination so that the expression pattern in
each of the different segments is unique.

9. The co-linearity principle: Homeotic gene expression in
Drosophila
 The anterior boundary of homeotic gene expression is
ordered from SCR (most anterior to ANTP, UBX and ABD-B
(most posterior). This order is matched by the linear
arrangement of the corresponding genes along chromosome
3.

10. A2
A1
T2
T3
T
1
Gebelein et al, Dev. Cell, 2002
Dll repression:
a paradigm for the study of Hox/Exd interaction
DME-lacZ / Ubx
Ubx
AbdA
Dll
Exd
+

11. The DNA sequence motif hypothesis
 Different combination of DNA modules would
give different combination of co-factors bound
on the promoter and thus a different array of
transcriptional interactions with each Hox
protein (Li et al, Development 126, 5581-5589,
1999).

12. Developmental strategies in
animals are ancient and
highly conserved. In essence,
a mammal, a worm and a fly-
three very different
organisms-are put together
with the same basic building
blocks and regulatory devices.

14. Wild type
Hox mutant
Lewis et al. 2000

16. Is there a “ground” state?
What could constitute a
“ground” state?
Where all HOX genes are
expressed in all segments

18. Changes in Hox gene expression can
help explain the evolution of arthropod
body plans

19. species 1 species 2
Evolution of crustacean maxillipeds

20. Triops (no mxp): Ubx expression in all thoracic segments

21. T1
T2
T3
T1
T2
T3
Mysid (1 mxp): Ubx expression from T2 to the posterior

22. Changes in Hox genes
 Duplication of genes or whole clusters (mammals) by unequal crossing
over
 Following duplication there is diversification of both coding and
regulatory sequences
 Changes (rare) in coding sequences (Ubx in Diptera vs. Ubx in
Lepidoptera)
 Changes in the expression of Hox targets