Hox genes and EvoDevo Flashcards
The phylogenetic tree of animals
Concept originates from Carl Linnaeus (Carl von Linné)
Brach points are last common ancestor
Originally these trees assembled based on morphological similarities
Now use molecular sequence data
Most genes are same in all animals investigated
Much of what makes us morphologically different is thought to be caused by changes in expression common set of genes
How d we determine if proteins are similar?
Blast protein alignment
Input amino acid sequence of proposed protein
Blast program searches massive databases for other proteins with similar sequences
Similarity found between protein sequence suggests proteins evolved from same common ancestor and that proteins have similar molecular functions and roles e.g. transcription factor
Molecular phylogeny
There are 2 vertebrate FGFs that fall into 4 clusters based upon protein sequence alignment
Ciona (a chordate) has single representatives in each 4 groups (red on diagram)
This suggest common ancestor of sea squirt and vertebrates had at least 4 FGFs
Gene duplication commonly takes place in changes in ploidy (not full separation of chromosomes) and local duplication (small regions chromosomes that gets duplicated) is what causes so many FGFs to arise
New copies of genes that arise in same genome are called paralogues
Duplication of chromosome or region of a chromosome
After duplication, likely duplicate gene at first redundant
The extra copy can change in:
- Pattern of expression during development
- Structure of protein, both small changes caused by point mutations and big changes caused by domain swapping
Changes in expression are thought to be most common driving force in morphological evolution of animals
Why are changes in expression patterns of genes though to play major role in morphological evolution?
Because enhancers can change easily
Accidents during meiosis could bring a new enhancer close to the gene
If gene redundant copy then new enhancer doesn’t cause problems
Exact position of enhancer usually unimportant, and DNA sequence for transcription factor binding sites is simple
Thus should be relatively easy to add or delete sites by rearrangements, insertions, deletions or base pair substitutions
Changes that affect proteins structure would have to be more precise so as not to introduce a stop, change reading frame, interfere with protein’s folding or disrupt RNA splicing
Hox genes specify segment identiti
Hox genes used to see differences in morphological evolution
Hox genes transcription factors that are found in clusters in genome, they regulate lots of transcription factors
Have similarity suggesting they originate from one gene
Changing hox gene expression results in changes in segmental identity e.g. flies having bithorax phenotype
evidence that changes in expression of genes has played major role in morphological evolution
Look at diagram
expression of gene C6 starts more posteriorly in chick - this correlates with longer neck (more cervical vertebrae) and less chest (fewer thoracic vertebrae) than mice
Shifts in Hox gene expression may explain loss of limbs during snake evolution
No forelimb and severely reduced hindlimbs
All vertebrae anterior to hindlimbs have ribs and mixed cervical/thoracic morphology (except for the atlas)
Expansion of HoxC6 and HoxC8 expression could confer many morphological changes (shift towards thoracic) seen during evolution snakes
look at diagram
Further evidence for changes in expression acting during morphological evolution
By experimentally changing expression of single gene, can create ectopic organs
Organs are very complex structures, so genes are capable of such big tasks called master regulatory genes
These genes regulate whole gene networks
Often these organs are function, this adaptability that takes place during development sometimes referred to as evolutionary robustness
E.g. if you take eye imaginal disk and place it on leg imaginal disk then then fly will have the ey gene artificially expressed in leg precursor cell
Crustaceans have legs on their abdomens but insects don’t, turns out may be because of changes in protein sequence
ubx evolution may explain why insects don’t have legs on abdomen
If fly embryos, Dlx transcription factor specifies leg precursor cells and is active in thorax
Ubx expressed in abdomen where it repressed Dlx expression so don’t get legs on abdomen
Crustacean Ubx doesn’t act as repressor of Dlx
Ubx expressed in abdomen of crustacean embryos, but doesn’t turn off Dlx expression
Is thought that Ubx gene became able to repress Dlx expression in ancestor of Drosophila
Summary
The toolbox of proteins was established early in animal evolution and most animals have a similar set of genes
Genome sequencing and protein alignment can be used to increase our understanding of the evolutionary relatedness of species and protein families
Duplication of genes provides dispensable genes for evolution to act upon
The morphological evolution of animals has been primarily driven by changes in gene expression.
Hox genes confer segmental identity – having a modular structure (eg somites) facilitates evolution
Changing regulatory sequences is easier than changing protein structure
