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Homeobox Genes and
Evolution
Lecture 3
Hox Gene Function
3’
Gene A
Gene B
Gene C
Gene D
5’
Which phenotype would you predict from loss of
3’
Gene D function?
Gene A
Gene B
Gene C
Gene D
5’
Which phenotype would you predict from loss of
Gene D function?
A
B
C
D
Hox Gene Function
3’
Gene A
Gene B
Gene C
Gene D
5’
Which phenotype would you predict from loss of
3’
Gene D function?
Gene A
Gene B
Gene C
Gene D
5’
Which phenotype would you predict from loss of
3’
Gene B function?
Gene A
Gene B
Gene C
Gene D
5’
Which phenotype would you predict from loss of
Gene B function?
A
B
C
D
Hox Gene Function
3’
Gene A
Gene B
Gene C
Gene D
5’
Which phenotype would you predict from loss of
3’
Gene B function?
Gene A
Gene B
Gene C
Gene D
5’
What order would you expect the
Hox genes to be in on the
chromosome?
•
•
•
•
3’-A-B-C-D-E-5’
3’-C-A-E-B-D-5’
3’-B-D-E-A-C-5’
3’-D-B-E-A-C-5’
Gene D
Gene B
Gene E
Gene A
Gene C
Mutations in Hox genes can lead to
what type of phenotype?
a. The anterior portion of the embryo does not develop
b. Several adjacent segments will be missing in an
otherwise intact embryo
c. The affected segment will develop like its posterior
neighbour
d. Duplication of a segment
Which statements describe the
phenotype of the Hox mutant?
mutant
a.
b.
c.
d.
Anterior segments have been transformed into
posterior ones
Abdominal segments develop as thoracic segments
Posterior segments have been transformed into
anterior ones
An example of a homeotic transformation
Which gene is predicted to control the
development of the most anterior structures?
5’
A
B
C
D
3’
The Antennapedia Mutation
Wild-type
Antennapedia mutation
Why do Antennapedia (Antp) mutants
have legs where their antennae should
be?
• Absence of Antp gene function in the head
transforms that segment’s appendage into one
normally found in the thorax
• Antp is needed for normal antennae development
and is missing in these mutants
• The mutants misexpress Antp in the head,
transforming that segment’s appendage into one
normally found in the thorax
• They have no head
How to get legless
The vertebrae of snakes show
homeosis
Pythons have >300 vertebrae
Very few cervical (no ribs)
vertebrae: lost to form thoracic
(rib bearing) vertebrae
Whole body resembles thorax
No forelimbs
Greatly reduced hindlimbs
Evolution of modern snakes
Hox gene expression boundaries
correlate with morphological
boundaries
Changes in body plan correlate with
changes in Hox expression
Chicken
HEAD
limb
HoxC6
HoxC8
TAIL
limb
Python
HEAD
TAIL
limb
What do you predict the pattern of HoxC6
and HoxC8 look like in python embryo?
What do you predict the pattern of HoxC6
and HoxC8 look like in python embryo?
a) Same as the chicken
b) HoxC6/C8 are not expressed in python
c) HoxC6/C8 expression is expanded
anteriorly and posteriorly
d) HoxC6/C8 expression is expanded
anteriorly
Changes in body plan correlate with
changes in Hox expression
Chicken
HEAD
limb
HoxC6
HoxC8
TAIL
limb
Python
HEAD
HoxC6
HoxC8
Expansion of Hox expression
domains creates thoracic, ribbearing vertebrae along
almost entire body length
TAIL
limb
Also results in loss of
forelimb, through expansion
of expression into anterior
somites
Role of Hox genes in evolution
1. Most, if not all, bilaterally symmetric animals, possess
one or more Hox clusters that are arranged co-linear
with their head to tail expression domains
2. The Hox cluster functions during development to
determine head to tail organisation by controlling
region specific gene expression
3. Changes in Hox gene expression can be correlated
with changes in head to tail organisation
4. New body designs DO NOT require new genes,
rather the modification of the function of existing ones
Role of Hox genes in evolution
1. Most, if not all, bilaterally symmetric animals, possess
one or more Hox clusters that are arranged co-linear
with their head to tail expression domains
2. The Hox cluster functions during development to
determine head to tail organisation by controlling
region specific gene expression
3. Changes in Hox gene expression can be correlated
with changes in head to tail organisation
4. New body designs DO NOT require new genes,
rather the modification of the function of existing ones
Role of Hox genes in evolution
1. Most, if not all, bilaterally symmetric animals, possess
one or more Hox clusters that are arranged co-linear
with their head to tail expression domains
2. The Hox cluster functions during development to
determine head to tail organisation by controlling
region specific gene expression
3. Changes in Hox gene expression can be correlated
with changes in head to tail organisation
4. New body designs DO NOT require new genes,
rather the modification of the function of existing ones
Role of Hox genes in evolution
1. Most, if not all, bilaterally symmetric animals, possess
one or more Hox clusters that are arranged co-linear
with their head to tail expression domains
2. The Hox cluster functions during development to
determine head to tail organisation by controlling
region specific gene expression
3. Changes in Hox gene expression can be correlated
with changes in head to tail organisation
4. New body designs DO NOT require new genes,
rather the modification of the function of existing ones