28/3/2022: Evolution and Development: Lecture 6: Regulatory evolution
Conserved genetic systems in diverse species:
Evolutionary biologists have often studied the distribution of alleles in populations, and the
genetic differences between species, in order to explain phenotypic diversity. However, it was
discovered that animals with very different phenotypes have surprisingly conserved genetic
systems that underlie their development. For example: the “eyeless” gene in Drosophila is
necessary (and sufficient) to create eyes.
For example: eyeless → this gene is necessary to create eyes → induction of eyes on each leg
by forcing expression of the eyeless protein → resulting in fully developed eyes on different
parts of the body (eg. leg or antenna), but cannot use these eyes → due to development of the
hox gene.
The really amazing thing is that these eyes can be induced by the expression of the
homologous gene from mice.
The genetic toolkit: Important transcription factors that consist of a large number of
fundamental components that can be used to build a wide variety of different end products
Genes like eyes are part of the evolutionary developmental genetic toolkit.
Genes that form the genetic toolkit are:
- Conserved across species
- Have a role in embryonic development
- Important: specify the identity of different body parts
- Are usually transcription factors that regulate large numbers of downstream genes in a
coordinated fashion
- Changes in the genes in the developmental toolkit underlie many large-scale changes in
the body organization of different species
Gene families in the genetic toolkit:
Many genes in the evo-devo genetic toolkit occur in gene families
Gene families arise through duplication errors in meiosis → this causes more than one copy of a
gene to exist, often adjacent to each other on the same chromosome. Natural selection can
then operate on each copy independently, such that they may end up with slightly different
functions, or be expressed in slightly different locations or at different times
, Example: hox gene family:
Hox genes are an important family of genes in the
genetic toolkit.
Here are four clusters of Hox genes on four
different human chromosomes. They are
color-coded to show homology between the
different chromosomes.
A few gains are gained or lost on each
chromosome, but the similarity between the
clusters is clear, showing repeated duplications of
these genes in our evolutionary history, and the
consequent expansion of the gene family
Over time, certain types get lost → 2 of the blue gene groups are removed (in this case after
duplication) → fundamentally disappear, and cannot return.
Hox genes are transcription factors that are expressed in a tissue-specific manner and regulate
the expression of large numbers of games that generate appropriate structures → hox genes
have in common a 60-amino acid long sequence = homeobox → functions in DNA binding.
Hox linear are co-linear → the order in which they are expressed corresponds with the order in
which they are located on the chromosome → head first, tail last.
Mutations result in different body parts being “mixed up”
The more hox duplications there are, the higher the chance that natural selection will tweak and
select for the most optimum type → more hox duplications = more complexity in body
organization.
Hox genes are also responsible for the internal organization (e.g. internal organs) → When
going down the phylogenetic tree, it is possible to predict the internal organization of the
common ancestor.
Hox genes are involved in the movement of different brain regions (either becoming bigger or
smaller) → by knocking out certain hox genes in mice → the brain obtained different structure →
consequences of the brain structure from the hox genes can be visualized (however, general
brain structure remains the same)
Conserved genetic systems in diverse species:
Evolutionary biologists have often studied the distribution of alleles in populations, and the
genetic differences between species, in order to explain phenotypic diversity. However, it was
discovered that animals with very different phenotypes have surprisingly conserved genetic
systems that underlie their development. For example: the “eyeless” gene in Drosophila is
necessary (and sufficient) to create eyes.
For example: eyeless → this gene is necessary to create eyes → induction of eyes on each leg
by forcing expression of the eyeless protein → resulting in fully developed eyes on different
parts of the body (eg. leg or antenna), but cannot use these eyes → due to development of the
hox gene.
The really amazing thing is that these eyes can be induced by the expression of the
homologous gene from mice.
The genetic toolkit: Important transcription factors that consist of a large number of
fundamental components that can be used to build a wide variety of different end products
Genes like eyes are part of the evolutionary developmental genetic toolkit.
Genes that form the genetic toolkit are:
- Conserved across species
- Have a role in embryonic development
- Important: specify the identity of different body parts
- Are usually transcription factors that regulate large numbers of downstream genes in a
coordinated fashion
- Changes in the genes in the developmental toolkit underlie many large-scale changes in
the body organization of different species
Gene families in the genetic toolkit:
Many genes in the evo-devo genetic toolkit occur in gene families
Gene families arise through duplication errors in meiosis → this causes more than one copy of a
gene to exist, often adjacent to each other on the same chromosome. Natural selection can
then operate on each copy independently, such that they may end up with slightly different
functions, or be expressed in slightly different locations or at different times
, Example: hox gene family:
Hox genes are an important family of genes in the
genetic toolkit.
Here are four clusters of Hox genes on four
different human chromosomes. They are
color-coded to show homology between the
different chromosomes.
A few gains are gained or lost on each
chromosome, but the similarity between the
clusters is clear, showing repeated duplications of
these genes in our evolutionary history, and the
consequent expansion of the gene family
Over time, certain types get lost → 2 of the blue gene groups are removed (in this case after
duplication) → fundamentally disappear, and cannot return.
Hox genes are transcription factors that are expressed in a tissue-specific manner and regulate
the expression of large numbers of games that generate appropriate structures → hox genes
have in common a 60-amino acid long sequence = homeobox → functions in DNA binding.
Hox linear are co-linear → the order in which they are expressed corresponds with the order in
which they are located on the chromosome → head first, tail last.
Mutations result in different body parts being “mixed up”
The more hox duplications there are, the higher the chance that natural selection will tweak and
select for the most optimum type → more hox duplications = more complexity in body
organization.
Hox genes are also responsible for the internal organization (e.g. internal organs) → When
going down the phylogenetic tree, it is possible to predict the internal organization of the
common ancestor.
Hox genes are involved in the movement of different brain regions (either becoming bigger or
smaller) → by knocking out certain hox genes in mice → the brain obtained different structure →
consequences of the brain structure from the hox genes can be visualized (however, general
brain structure remains the same)
