Download Evolution of Development (EvoDevo) •Development is the process

Evolution of Development (EvoDevo)
•Development is the process of growing a complex,
multicellular, differentiated organism from a single cell
(fertilised egg).
•The evolution of genes involved in development is
considered to be central to the evolution of complex lifeforms.
• Each cell of our body contains exactly the same DNA
(with the exception of gametes, which only have half the
DNA, and certain cells in the immune system, where the
immune genes have been scrambled to create new
diversity). If the genes in each cell are the same, how, then,
do different parts of our body look become so plainly
different?
Homeotic Genes regulate specification of segment
identity
1. Homeotic genes turn on transcription of groups of
genes to make structures such as legs, wings,
antennae, etc.
2. Originally identified through mutations that cause
dramatic changes in body appearance.
3. Homeotic gene products are homeodomain-type DNA
binding proteins, regulating gene expression at the
level of RNA synthesis.
4. After segment pattern is established, homeotic genes
direct the developmental fates of particular groups of
cells.
•The answer lies in gene expression – i.e., the turning on
and off of genes. This is done in a very carefully regulated
way, which is why all humans, and indeed all vertebrates,
have limbs in the same relative positions.
•The genes that control gene expression in time and the
three-dimensional space of a growing multicellular organism
are called homeotic genes.
•In plants, the most important homeotic genes are called
the MADS-box genes
•In animals, the most important homeotic genes are called
the Hox genes.
•Hox genes occur in groups on chromosomes (a group of
Hox genes, one after another along a chromosome).
•All Hox genes have a perfect correlation between the
arrangement on the chromosome, and the position along
the anterior-posterior axis where they function.
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•So, the first gene (in the 3’-5’ direction along the
chromosome) determines the “head end” of the body, and
the last gene determines the “tail end”.
•We know this because if we create genetic mutants that
are missing one of the Hox genes, we observe certain body
features (e.g., legs) in the wrong place! That is, when certain
Hox genes are missing, the cells don’t know where they are
anymore.
Antennapedia complex. Defines
the identity of the anterior
segments of the fly from the head
through the second thoracic
segment.
Physical order of the homeotic genes
Physical order of the homeotic genes in these two clusters is
identical to the order in which these genes are expressed along the
anterior-posterior axis of the embryo during development.
Bithorax complex. Specifies the
identities of remaining thoracic
segments through the eighth
abdominal segment.
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The Homeobox: Mouse and Drosophila homeotic genes
have common features of organization
•Arrangement of gene order on
chromosomes is conserved.
•Amino acid sequences of genes are
conserved in homeo box domains.
•Gene complexes likely arose from a
common ancestor before
arthropod/vertebrate evolutionary
divergence.
•Minor changes in genes have an
immense effect on the overall body plan.
•Ordered nature of homeotic gene
clusters is highly conserved in evolution,
including man.
•Is there a segmentation plan in
mammalian development?
What can we learn about the evolution of these genes?
What animals have these genes? How do the genes look different? How do the
animals look different (morphology and complexity)?
•5 ancestral Hox genes (seen in Cnidarian)
•New genes were created by gene duplication
•More complex animals have more Hox genes along
the cluster
•Vertebrates (the most morphologically complex in this
comparison) have extra copies of the cluster (4
clusters)
• As a general rule we can say that more complex
animals have more Hox genes. We can relate the
evolution of these genes to the evolution of complex
development.
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