Lecture 15 - hox genes & evolutionary development biology Flashcards
What is a phylogenetic tree made from?
- originally assembled based upon morphological simmilarities
- now we also use molecular sequence data
What makes us morphologically different?
changes in expression of a common set of genes (60% the same genes as a banana)
What are transcriptional regulatory sequences?
enhancers
How do we determine if proteins are similar?
- input the amino acid sequence of the protein in question
- the last program searches huge databases for other proteins which have similar sequences
What does a similarity in protein sequence suggest?
- proteins evolved from the same common ancestor and that the proteins have similar molecular functions
How many vertebrate FGFs are there?
22 vertebrate FGFs that fall into 4 clusters based upon protein sequence alignment
- Ciona (a chordate) has single representatives in each of the 4 groups. This suggests that the common ancestor of the sea squirt and vertebrates had 4 FGFs
How do many FGFs arise?
Gene duplication - changes in ploidy & local duplication
- new copies of genes that arise in the genome are called paralogues
- ploidy = the number of sets of chromosomes in a cell
- there are 22 paralogues in the human genome
How does duplication of a chromosome or region of a chromosome occur?
after a duplication, it is likely that the duplicate genes are at first redundant
An extra copy can change by:
1. pattern of expression during development
2. structure of the protein
3. Both small changes caused by point mutations and but changes caused by domain swapping
This is GENE NEOFUNCTIONALISM
What is the most common driving force in the morphological evolution of animals?
changes in expression
Why are changes in expression patterns of genes thought to play a major role in evolution?
- because regulatory elements can change easily
How important is the exact position of the enhancer?
- usually unimportant, and the DNA sequence for transcription factor binding sites is simple
- therefore, it is relatively easy to add or delete sites by rearrangements, insertions, deletions or base pairs substitutions
What impact would changing the expression of a gene have on a protein?
changes the effect protein structure would have to be more precise as not to introduce a stop, change the reading frame, interfere with the protein’s folding or disrupt RNA splicing
What are HOX genes?
transcription factors that found in clusters in the genome
What can be suggested that HOX genes all have quite a similar sequence?
they originated from one gene
What is the result of changing Hox gene expression (in the embryo)?
results in changes in segmental identity
What are the Hox genes that specify vertebral segmental identity?
- cervical (neck)/Hoxc5
- Thoracic (chest)/Hoxc6
What is evidence that changes in the expression of genes has played a major role in morphological evolution?
Expression of the gene C6 starts more posteriorly in chick - this corellates with a longer neck (more cervical vertebrate) and less chest (fewer thoracic vertebrate) than mice
How may shifts in HOX gene expression explain the loss of limbs during snake evolution?
- up to 300 somites - mice have 60
- no forelimbs and severely reduced hind limbs
- vertebrae anterior to the hind have ribs and a mixed cervical/thoracic morphology
What could be the result of the expansion of HoxC6 & HoxC8?
could confer many of the morphological changes (shift towards thoracic) seen during the evolution of snakes
Why do crustaceans have legs on their abdomen but insect don’t?
it turns out that this is because of changes in the protein sequence
How may ubx evolution explain why insects don’t have legs on their abdomen?
- in fly larvae, the dlx transcription factor specifies leg precursor cells
- Ubx is expressed in the abdomen where it represses Dlx expression
- crustacean ubx doesn’t act as a repressor of Dlx
- Ubx is expressed in the abodmen of crustacean larvae, but it doesn’t turn off dlx expression
- it is thought that the ubx gene became able to repress dlx expression in an ancestor of Drosophila
What is easier - changing regulatory sequences or protein structure?
changing regulatory sequences
