Homeobox Genes and Evolution

1
Homeobox Genes and Evolution

• Lecture 3

2
Hox Gene Function
3
Which phenotype would you predict from loss of
Gene D function?
3

Gene A

Gene B

Gene C

Gene D
5
4
Which phenotype would you predict from loss of
Gene D function?
A
B
C
D
5
Hox Gene Function
6
Which phenotype would you predict from loss of
Gene D function?
3

Gene A

Gene B

Gene C
Gene D
5
7
Which phenotype would you predict from loss of
Gene B function?
3

Gene A

Gene B

Gene C

Gene D
5
8
Which phenotype would you predict from loss of
Gene B function?
A
B
C
D
9
Hox Gene Function
10
Which phenotype would you predict from loss of
Gene B function?
3

Gene A
Gene B

Gene C

Gene D
5
11
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
12
Mutations in Hox genes can lead to what type of
phenotype?

• The anterior portion of the embryo does not
  develop
• Several adjacent segments will be missing in an
  otherwise intact embryo
• The affected segment will develop like its
  posterior neighbour
• Duplication of a segment

13
Which statements describe the phenotype of the
Hox mutant?
mutant

• 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

14
Which gene is predicted to control the
development of the most anterior structures?
A
5
D
B
C
3
15
The Antennapedia Mutation
Antennapedia mutation
Wild-type
16
Why do Antennapedia (Antp) mutants have legs
where their antennae should be?

• Absence of Antp gene function in the head
  transforms that segments 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 segments appendage into one
  normally found in the thorax
• They have no head

17
How to get legless

• The vertebrae of snakes show homeosis
• Pythons have gt300 vertebrae
• Very few cervical (no ribs) vertebrae lost to
  form thoracic (rib bearing) vertebrae
• Whole body resembles thorax
• No forelimbs
• Greatly reduced hindlimbs

18
Evolution of modern snakes
19
Hox gene expression boundaries correlate with
morphological boundaries
20
Changes in body plan correlate with changes in
Hox expression
Chicken
thoracic
cervical
lumbar
TAIL
HEAD
HoxC6
HoxC8
limb
limb
What do you predict the pattern of HoxC6 and
HoxC8 look like in python embryo?
21
What do you predict the pattern of HoxC6 and
HoxC8 look like in python embryo?

• Same as the chicken
• HoxC6/C8 are not expressed in python
• HoxC6/C8 expression is expanded anteriorly and
  posteriorly
• HoxC6/C8 expression is expanded anteriorly

22
Changes in body plan correlate with changes in
Hox expression
Chicken

• Expansion of Hox expression domains creates
  thoracic, rib-bearing vertebrae along almost
  entire body length
• Also results in loss of forelimb, through
  expansion of expression into anterior somites

thoracic
cervical
lumbar
TAIL
HEAD
HoxC6
HoxC8
limb
limb
Python
thoracic
TAIL
HEAD
HoxC6
HoxC8
limb
23
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

24
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

25
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

26
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