Hox genes and evodevo Flashcards
whats The phylogenetic tree of animals
-Originally these trees were assembled based upon morphological similarities (for comparisons)
-Now we also use molecular sequence data
why are most of the genes are the same in all animals investigated.
-much of what makes us different (morphologically) is thought to be caused by changes in expression of a common set of developmental genes.
-timing and level of expression and which tissues are expressed are all important
-% =how similar to humans:
=cats 90%
=chimpanzees 96%
=banana 60%
=cows 80%
=mouse 85%
How do we determine if proteins are similar
-Blast protein alignment
-Input the amino acid sequence of the protein in question (query)
-The Blast program searches huge databases for other proteins which have similar sequences.
-using protein zen: Similarity found between protein sequence suggests that the proteins evolved from the same common ancestor and that the proteins have similar molecular functions.
molecular phylogeny explained
-There are 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 (in red). This suggests that the common ancestor of the sea squirt and vertebrates had 4 FGFs.
-How do so many FGFs arise? Gene duplication: changes in ploidy and local duplications.
-New copies of genes that arise in the genome are called paralogues.
-ploidy = the number of sets of chromosomes in a cell
whats Duplication of a chromosome or region of a chromosome
-After a duplication, it is likely that the duplicate genes are at first redundant.
-The extra copy can change in the following ways:
1) pattern of expression during development
2) structure of the protein. Both small changes caused by point mutations and big changes caused by domain swapping.
-This is gene neofunctionalisation
-Changes in expression are thought to be the most common driving force in the morphological evolution of animals.
Why are changes in expression patterns of genes thought to play a major role in evolution
-Because regulatory elements can change easily
-The exact position of an enhancer is usually unimportant, and the DNA sequence for transcription factor binding sites is simple. Thus, it should be relatively easy to add or delete sites by rearrangements, insertions, deletions or base pair substitution.
-Changes that effect protein structure would have to be more precise so as not to introduce a stop, change the reading frame, interfere with the protein’s folding or disrupt RNA splicing.
how do Hox genes specify segment identity
-Hox genes are transcription factors that are found in clusters in the genome
-They all have sequence similarity suggesting that they originated from one gene
-They are expressed in specific segments in the embryo
-Changing hox gene expression(in the embryo) results in changes in segmental identity.
vertebral identity
-Cervical(neck)/ Hoxc5
-Thoracic(Chest)/ Hoxc6
what does shift in how gene expression explain
-loss of limbs during snake evolution
-Up to 300 somites – mice have 60
-No forelimb and severely reduced hindlimbs.
-Vertebrae anterior to the hindlimbs have ribs and a mixed cervical/thoracic morphology
-Expansion of HoxC6 and HoxC8 expression could confer many of the morphological changes (shift towards thoracic) seen during the evolution of snakes
where do crustaceans have legs
-abdomens, insects dont have this
-It turns out that this may be because of changes in the protein sequence
what does 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.
why does Crustacean Ubx not act as a repressor of Dlx.
-Ubx is expressed in the abdomen of crustacean larvae, but it doesn’t turn of dlx expression.
-It is thought that the Ubx gene became able to repress Dlx expression (changes made in the protein sequence) in an ancestor of Drosophila.
