homeobox genes and evolution

Homeobox Genes and Evolution

Lecture 3

Hox Gene Function

5’

3’

Gene A

Gene B

Gene C

Gene D

Which phenotype would you predict from loss of Gene D function?

5’

3’

Gene A

Gene B

Gene C

Gene D

Which phenotype would you predict from loss of Gene D function?

A B

C D

Hox Gene Function

5’

3’

Gene A

Gene B

Gene C

Gene D

Which phenotype would you predict from loss of Gene D function?

5’

3’

Gene A

Gene B

Gene C

Gene D

Which phenotype would you predict from loss of Gene B function?

5’

3’

Gene A

Gene B

Gene C

Gene D

Which phenotype would you predict from loss of Gene B function?

A B

C D

Hox Gene Function

5’

3’

Gene A

Gene B

Gene C

Gene D

Which phenotype would you predict from loss of Gene B function?

5’

3’

Gene A

Gene B

Gene C

Gene D

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 AGene E

Gene B

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?

a. Anterior segments have been transformed into posterior ones

b. Abdominal segments develop as thoracic segments

c. Posterior segments have been transformed into anterior ones

d. An example of a homeotic transformation

mutant

Which gene is predicted to control the development of the most anterior structures?

5’ 3’A DB C

The Antennapedia Mutation

Antennapedia mutationWild-type

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

cerv

ical

thor

acic

lumba

r

HEAD TAIL

limb limbHoxC8

HoxC6

Chicken

What do you predict the pattern of HoxC6 and HoxC8 look like in python embryo?

thor

acic

HEAD TAIL

limb

Python

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

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

cerv

ical

thor

acic

lumba

r

HEAD TAIL

limb limb

thor

acic

HEAD TAIL

limb

HoxC8HoxC6

HoxC8HoxC6

Chicken

Python

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