Transcript Evolution is chance caught on the wing.

“Evolution is chance caught on the wing.”
• Henry Walter Bates and Batesian Mimicry
– Bates collected 14,712 different animal species during his 11 years on
the Amazon.
• Returned to England just prior to publication of Darwin’s The Origin of Species.
• In one of his first letters to Darwin, Bates stated, “
– Bates studied insects, especially butterflies, where protection from
predators was provided by __________________________________
_________________________________________________________
• Bates observed that birds found certain butterflies to be edible and others
noxious. Birds learned to distinguish between the two types based only on a few
experiences.
• ___________________________________________________________________
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– Bates wrote, “. . . On these expanded membranes nature writes, as
on a tablet, the story of the modification of species, so truly do all
changes of the organization register themselves thereon.”
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– “As the laws of nature must be
the same for all beings, the
conclusions furnished by this
group of insects must be
applicable to the whole organic
world. . . “
– The butterfly wing served as a
canvas for the evolution of
thousands of color patterns.
– __________________________
__________________________
____________________________
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• The Coloration of Butterfly Wings. . . Pure chance or could it be. . .
In current times the observations begs the
question “what are the genetic and
developmental mechanisms for making
these coloration patterns?”
___________________________________
___________________________________
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The Ground Level Plan For
Butterfly Wing Pattern
Stichophthalma camadeva
A different degree of
representation of the ground plan
Taenaris macrops
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• What Did Butterflies Invent?
1) Wing Scales
2) Coloration
3) Geometrical Patterning System
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1) Scales
– Greek, lepis, meaning scale or flake; ptera, winged creation:
Lepidoptera
– One scale is produced by a single cell.
• Scales have been found to evolve as modifications of the sensory
bristles on insects.
• They flattened and widened.
• ________________________________
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– Tool Kit Genes
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2) Coloration
- Each scale is a particular color, which
can be seen at high magnification.
- Individual scales may be entirely
different hue than their neighbors.
- _______________________________
_______________________________
_______________________________
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3) Geometrical Patterns on the Wings
• There are developmental
pathways that organize the
pattern.
• Eyespots: concentric rings of
scales of different colors.
• Proposed role: to where are
“you,” the predator , attracted?
Or to what part of the butterfly
are you attracted?
• How did all these pattern
elements get made or evolve?
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• Making Eyespots: Teaching Old Genes New
Tricks
– It All Begins In the Caterpillar
• Each wing forms from a flat disc of
cells that grows a lot during larval
development.
– Then you have the chrysalis. . . And
just before the butterfly emerges the
final color pattern is filled in.
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– The position of the eyespots is
decided in the caterpillar.
• Concentric ring patterns of the
eyespots are induced by an
“organizer,” which is at the center
of the developing eyespot.
• 1980: Fred Nijhout of Duke Univ.
killed a tiny patch of cells and no
eyespot formed.
• When this group of cells was
isolated from the developing
butterfly wing in the first day of
the chrysalis stage and
transplanted to a site elsewhere
in the wing, a new eyespot now
appeared.
• Only cells at the future center of
the eyespot had this property. . .
Called the “focus.”
– So what are the genes responsible for
this organizer???
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• Finding the Eyespot Organizer’s Gene: The Questions
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• How Could They Ever Find A Starting
Place?
– They knew a lot about fruit fly
wings.
• Insect wings evolved only once
so what they knew about the
tool kit genes of insect wings
should apply to their
counterparts of the butterfly
wing.
– They compared Buckeye butterfly
tool kit genes and fruit fly tool kit
genes that were homologous. That
is, they were both involved in the
building and patterning of wings.
– But just because their genes were
there. . .
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• They needed to prove that the genes were actually involved. . Active. .
. Being transcribed. . for the purpose of wing patterning and at the
time when patterns were actually being set up.
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• Their Experiments
– Location of Expressed Genes
• ______________________________________________________
______________________________________________________
• ______________________________________________________
______________________________________________________
• Some Results
– All of the butterfly genes were expressed in parts of the butterfly
wing disc that corresponded to the same geographical region where
they were expressed in the fruit fly. So that is a “good” sign.
• Conclusion: the butterfly wing had common geography to the
developing fruit fly wing.
– Top, bottom, front, rear of each wing and the wing edges
were all delineated by the same genes in both species.
– Conservation of an ancient wing design.
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– But there were some patterns of gene expression in the butterfly
wing that had no counterparts in the fruit fly. Aha!!! A difference!
• There were spots in the caterpillar discs precisely where the
eyespots would appear 1 week later in development.
• And these spots were made by just one of several genes.
• It was named Distal-less or Dll.
– Significance? Well, it wasn’t simply finding the gene’s
function but that this particular gene had other functions in
other organisms . . . building fruit fly limbs and arthropod
limbs.
» _______________________________________________
_______________________________________________
» Dll turned out to be deployed in the distal parts of
butterfly limbs, limbs of crustaceans, spiders and
centipedes.
» _______________________________________________
– __________________________________________________
__________________________________________________
__________________________________________________
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» When mutated, Dll caused the distal parts of fly limbs to
be lost.
» Dll turned out to be deployed in the distal parts of
butterfly limbs, limbs of crustaceans, spiders and
centipedes; vertebrates and invertebrates.
» It played a role throughout arthropod limb formation.
» In completely unrelated species, chicken’s legs, fish fins,
tube feet of sea urchins, all have a Dll gene and it is
associated with things that stick out of animal’s bodies.
– This was a tool-kit gene involved in building very different
structures that only share, at most, the common feature of
projecting away from the main body.
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• How did Distal-less or Dll learn the new trick of making spots in the
wing?
– Dll retained its old job but this tool kit of genes and the spots of Dll
expression were a new trick.
– So the role of Dll is at another time and place in wing spot
development and it controls a different pattern and all this has to do
with predator / prey relationships. . . Fitness. . . Natural
selection. . . Being around or not being around.
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• How did Distal Less “Learn” This New
Trick of Making Spots In the Wing?
– ___________________________
– The gene acquired a new switch
that coordinates the longitude
and latitude on these spots on
cells.
– ____________________________
____________________________
____________________________
____________________________
____________________________
____________________________
____________________________
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• Is That All There Is?
– No, there were other proteins,
tool kit proteins, involved in
making these concentric rings.
– So somehow each ring of scales
receives different instructions.
• That’s pretty darn complicated!!
– Could signals from the “focus”
induce the surrounding cells to
be different colors????
• And different instructions at the
different distances?? Ah, come on.
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• Two more took kit proteins
– Spalt and Engrailed
– Both are expressed, spalt
in a spot and engrailed in
a ring, in the African
species, Bicyclus
anynana.
– Spalt tool kit protein
pattern precisely marked
the future black ring.
– Engrailed pattern did the
same for the gold ring.
– The genes for spalt and
engrailed are both “old.”
So the new role in
butterflies is due to a
new genetic switch
controlling each gene
thus provided a new job.
Bicyclus anynana: the eyespot has a
white center, surrounded by the black
ring which in turn is surrounded by a
gold ring.
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• The Distal-less gene switches.
– _________________________________________________________
____________________________________________.
embryo
E
L
W
________ // ________// _______________
gene
leg
wing
– In butterflies, however, a new switch, S, has evolved that controls the
expression in eyespots.
embryo
E
L
W
S
________//________//______//__________
gene
leg
wing
eyespots
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• How Do Butterflies Change Their Spots?
– The Distal-less eyespot switch, S, can
be modified.
– Environmental changes in rainfall or
temperature may affect hormone
presence or absence and the
hormones will affect the genetic
switches.
– Within the genetic switch there are
specific places where tool kit
proteins bind. These specific places
or signature sequences can also
change with time.
Dry Season
Wet Season
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• How Do Butterflies Change Their Spots?
– Within the genetic switch there are
specific places where tool kit
proteins bind. These specific places
or signature sequences can also
change with time.
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• Mimicry and Color Pattern Evolution
– We’ve basically said that the difference in appearance between
species or between individuals of the same species, is due to
different spatial patterns of pigment synthesis and scale structural
colors.
– How have these differences, slight or large, affected the evolution of
mimicry and its role in natural selection?
• ______________________________________________________
______________________________________________________
______________________________________________________
______________________________________________________
• The goal is to ascertain the connection between fitness, genes
and the forms of the colorful patterns.
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