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CHAPTER 25
LECTURE
SLIDES
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Evolution of Development
Chapter 25
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• To explain differences in species, we
need to look at changes in genes that
have their effect by altering
development and thus phenotype
• Phenotypic diversity has resulted not
from large changes in genes but from
changing patterns of expression
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• 2 closely related sea urchin species have very
different developmental patterns
• 2 forms have very similar genes
• Dramatic changes in gene expression during
development
• Adult forms nearly the same in both
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• About 2 dozen gene families regulate
animal and plant development
• Hox (homeobox) genes establish the
body plan by specifying when and
where genes are expressed
– Hox genes code for proteins that bind to
regulatory region of other genes
• Plants: shoot growth and leaf development
• Animals: establish body plans
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• MADS box genes code for a DNAbinding motif
– Establish the body plan of plants,
especially flowers
– MADS box highly conserved but variation
in other coding region sequences
• Transcription factors and genes
coordinate development
– Changes can have dramatic effects on the
development of an organism
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Heterochrony
• Alterations in timing of developmental events
due to a genetic change
– Could affect a gene that controls transition of plant
from juvenile to adult
– Mutation results in small plant that flowers quickly
• Most mutations that affect developmental
regulatory genes are lethal
• If mutation leads to increased fitness, then
new phenotype will persist
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Homeosis
• Alterations in the spatial pattern of gene
expression
• 4-winged Drosophila requires mutation in 3
genes in Bithorax complex
• Drosophila Antennapedia has a leg where an
antenna should be
• Mutations can arise spontaneously or by
mutagenesis in the laboratory but bizarre
phenotypes would have little survival value in
the wild
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• Coding sequence of a gene can contain
multiple regions with different functions
• Regulatory region change may alter
time or place of gene expression
– Hetrochrony or homeosis
– Downstream targets the same but cells
that express target genes or timing of
expression could change
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Gene Mutations
• Brassica oleracea subspecies have very
diverse phenotypes
– Wild cabbage, kale, tree kale, red
cabbage, green cabbage, brussels sprouts,
broccoli, and cauliflower
• CAL and Apetala1 gene mutation
changes regular flowers into masses of
arrested flower buds
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• CAL was cloned from Brassica
– Stop codon, TAG, was found in the middle of the
sequences for broccoli and cauliflower
– Stop codon appeared after ancestors of broccoli
and cauliflower diverged but before broccoli and 13
cauliflower diverged from each other
• Cichlid fish jaws demonstrate
morphological diversity
– Cichlid fish in Lake Malawi in East Africa
– In less than a few million years, hundreds
of species evolved from a common
ancestor
– Different species acquired different niches
based on feeding habits
• Rammers: long snouts
• Biters: intermediate snout
• Bottom feeders: short snouts
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• 2 genes, of yet unknown function, are
likely responsible for the shape and size
of the jaw
• Regulating the length versus the height
of the jaw may well be an early and
important developmental event
• Range of jaw forms appears to have
persisted because the cichlids establish
unique niches for feeding within the lake
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Same gene, new function
• Evolution of chordates can partially be
explained by the co-option of an existing
gene for a new function
– Ascidians have a notochord but no
vertebrate
• Brachyury gene encodes a transcription factor
expressed in developing notochord
• Homologues found in invertebrates
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– Region of Brachyury gene encodes protein
domain called T box, transcription factor
• Transcription factor turns on or off a gene or
genes
• In mice and dogs, Brachyury mutation causes a
short tail
• Humans have wild-type Brachyury so other genes
must be needed for a tail
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• Limb formation
– Most tetrapods have 4 limbs – 2 forelimbs
and 2 hindlimbs
– Bird wings and human forearms are
homologous structures
– Humans and birds both express Tbx5 gene
in developing limb buds
– What seems to have changed as birds
and humans evolved are the genes that
are transcribed because of the Tbx5
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protein
• In the ancestral tetrapod, perhaps Tbx5 protein bound to only
one gene and triggered transcription
• In humans and birds, genes are expressed in response to Tbx5
protein, but they are different genes
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Different Genes,
Convergent Function
• Homoplastic (analogous) structures
– Same or similar functions
– Arose independently
• Phylogenies reveal convergent events
• Origin of convergence difficult to understand
– Different developmental pathways may have been
modified
– In other cases not clear whether it is the same or
different genes responsible
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• Insect wing patterns
– Can protect them from predation or allow
them to thermoregulate
– Origins of patterns best explained as
recruitment of existing regulatory programs
for new functions
– Sensory bristles and scale development
initiated by achaete-scute transcription
factor
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• Scales derived from bristles
– Structure inverted
– Daughter cell neuronal connection dies
– Pigment production triggered
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• Flower shapes have also altered in a
convergent way
– Radially symmetrical flower: two identical
parts when cut across center
• Daisies, roses, tulips
– Bilaterally symmetrical flowers have mirrorimage halves
• Snapdragons, mints, peas
• Shape may be important in their evolutionary
success
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• Cycloidia (CYC) gene responsible for bilateral
symmetry of snapdragon
– Snapdragons with mutations in CYC have radially
symmetrical flowers
• Radial symmetry is the ancestral condition
• Gain of bilateral symmetry arose
independently among some species because
of the CYC gene
– Convergent evolution through mutations of the
same gene
• In other cases, CYC is not clearly responsible
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for the bilateral symmetry
Gene duplication and
divergence
• Gene duplications of paleoAP3 led to
flowering-plant morphology
– MADS box gene duplicated
– Gave rise to PI and paleoAP3 genes
– In ancestral plant, genes affected stamen
development
• This function has been retained
– paleoAP3 duplicated to produce AP3 and
an AP3 duplicate
– AP3 gained a role in petal development
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• Two gene
duplications
resulted in the
AP3 gene in
the eudicots
that has
acquired a role
in petal
development
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• Gene divergence of AP3 altered function to
control petal development
• Mutant ap3 plants do not produce either
petals or stamens
• PI and AP3 proteins can bind to each other
– Regulate transcription of genes needed for
stamen and petal formation
• No petals will form if C-terminus motif of AP3
absent
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Petals Present
Stamen Present
YES
YES
AP3 Gene Construct Added to
AP3 Mutant Arabidopsis
Complete
AP3 Gene
MADS
AP3 C terminus
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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Petals Present
Stamen Present
YES
YES
NO
NO
AP3 Gene Construct Added to
AP3 Mutant Arabidopsis
Complete
AP3 Gene
MADS
No AP3
C terminus
MADS
AP3 C terminus
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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Petals Present
Stamen Present
YES
YES
NO
NO
NO
SOME
AP3 Gene Construct Added to
AP3 Mutant Arabidopsis
Complete
AP3 Gene
MADS
No AP3
C terminus
MADS
PI C terminus
replaces AP3
C terminus
MADS
AP3 C terminus
PI C terminus
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Functional Analysis
• Range of experiments designed to test the
actual function of a gene in different species
• Sequence comparison essential
– Need to distinguish paralogues from orthologues
• Single base mutation can change active gene
into an inactive pseudogene
• Therefore, function can be inferred from
sequence data but actual function has to be
demonstrated experimentally
• Functional genomics – experimenting to
demonstrate actual function of the gene
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Diversity of eyes case study
• Explaining complicated structures was one of
Darwin’s greatest challenges
• Incremental improvements in function could
build a complex structure through natural
selection
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• Eyes of organisms are extremely
different in many ways
• Eyes are an example of convergent
evolution
– Homoplastic (analogous) structures rather
than homologous
• Morphological evidence indicates eyes
evolved at least twenty times
– Most recent common ancestor of all these
forms had no ability to detect light
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• Genes discovered that code for
transcription factor important in lens
development
– Pax6 in mice, eyeless in flies
• Sequence of genes highly similar –
homologues
• Walter Gehring inserted mouse Pax6
into genome of a fruit fly
– Created transgenic fly
– Pax6 gene turned on by regulatory factors in the
fly’s leg
– Eye formed on leg of fly
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Mouse Pax6 makes an eye on the leg of a fly
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• Transgenic fly results completely
unexpected
– Insects and vertebrates diverged more
than 500 MYA
– Large differences in eye structure
– Expected eye development to be controlled
by completely different genes
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• Pax6 and eyeless cave fish
• Pax6 gene expression reduced
• Eyes start to develop, then degenerate
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• Ribbon worms, Lineus sanguineus, also
rely on Pax6 for eyespot development
• Pax6 homologue has been cloned and
has been shown to express at the sites
where eyespots develop
• In contrast, planarian worms do not rely
on Pax6 for eyespot development
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Eyespot
Regenerated
head
Eyespot
Regenerating
head
Probe for
Pax6
Probe for
Pax6
transcripts
Pax6 expression correlates with ribbon
worm eyespot regeneration
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SCIENTIFIC THINKING
Hypothesis: Pax6 is necessary for planaria eyespot regeneration
which occurs when planaria are cut in half longitudinally.
Prediction: Eyespot regeneration will not occur without Pax6 protein.
Test:
Cut
Eyespot
Eyespots
Result: Eyespots regenerate.
Conclusion: Pax6 is not required for eyespot regeneration in
planaria.
Further Experiments: Can you design an experiment to test if the
planaria Pax6 gene can initiate eyespot development in a ribbon
worm or eye development in a fly?
Pax6 is not required for planaria eyespot
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regeneration
Several explanations for Pax6
• Eyes in different types of animals evolved
truly independently, as originally believed
– Possible Pax6 had role in forehead development
and this role has been co-opted time and time
again for eye development
• Many think this unlikely since gene
sequences and functional roles so similar
– Suggests to many that Pax6 acquired its eye
development role only a single time in the
ancestor to all organisms using Pax6 for eye
development
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