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Behavior is written in the genes.

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All of us have innate behaviors;

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all animals are born with behaviors.

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Behaviors which through evolution 
have become more complex.

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In many aspects,

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behavior in males and females
of the same species is different.

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These differences in behavior

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are due to differences in the wiring
of the nervous system.

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At the same time, these differences
in wiring are established genetically.

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Aggression is a behavior
that in general is specific to males.

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We see it in species
that are very different,

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evolutionarily apart from each other.

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The way each one
of these species fights is different

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but the mechanisms are similar

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and the reasons are almost
always the same.

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Males fight for females,
for food, or for territory.

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I study aggression in flies.

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You are about to see an attack.

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One animal is going to stand up

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and is going to hurl its body-weight
at his opponent.

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The other one, meanwhile,
is going to try to escape.

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Why do I study aggression in flies?

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Because the behavior,
that I'm interested in understanding,

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is the same, and the system
is much simpler.

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It's like studying a model.

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Sometimes it happens
that the other animal,

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instead of escaping,
stays and starts to fight,

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and then something
we call "boxing" occurs.

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Something crucial about attacking
is that only the males do it.

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The one who attacks first
is much more likely to win.

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And males only attack other males.

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Females never attack.

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So, why study behavior in flies?

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Because their behaviors
are the same as ours:

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Flies sleep, eat, court, fight,
learn, and remember.

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The nervous system is much simpler;

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and the genes that play a role
in this behavior are very similar.

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So, studying aggression in flies
allows us to understand

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how this behavior develops
in other species.

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A key element when deciding
whether to court or attack

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is determining the sex
of the other animal.

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All males are programmed to decide
between courtship or aggression,

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but the sensory signals they use

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to see if what is in front of them
is a male or a female are different;

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for example, in our species
visual signals are very important.

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This is what a fly has in front of itself.

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So, how does it decide?

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Evidently, it somehow makes a decision

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because what it is going to do
is very different in each case.

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When there is a male
entering its territory,

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it is going to start attacking

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and to chase him everywhere
until the other one decides to escape.

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When there is a female,
he extends his wing for her.

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As you can see, he literally courts her.

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He chases her everywhere,

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he sings to her until
he has finally won her over.

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So the question is, how does it know?

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How does it decide?

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What are the key signals that it uses
to decide if it has to court or attack?

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If we can identify these signals,
can we change them?

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Can we invert this decision?

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In insects, like in many species,

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chemical signals called pheromones
are very important.

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These signals are produced
by one animal and detected by another.

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So, then something we decided to do

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was to switch the pheromones,
change the scent.

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To make it happen,

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we manipulated the expression
of one gene, called "transformer".

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"Transformer" is turned on in females
and turned off in males.

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So, something that we can do
is to mute the "transformer" in females

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and force its acquisition in males.

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In doing so, we masculinize females
and feminize males.

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But not all over, we aren't going
to manipulate the entire animal.

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We are going to choose certain parts,

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in particular the cells
which produce pheromones.

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What we can achieve are females
that have masculine pheromones

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or males that have feminine pheromones;

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then we are going to see
what is going to happen

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when a normal male, unmanipulated,

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meets a female
that has masculine pheromones.

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This is what happens, it attacks her.

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When she enters his territory,
he does something

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that he would never do
in front of a normal female,

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which is start attacking her.

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When we carried out
the reciprocal experiment,

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what we found was that males
with feminine pheromones were courted.

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What does this mean?

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That males are
genetically programmed to court

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when they find feminine pheromones

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and to attack when they find
masculine pheromones.

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But, this behavior,

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this innate behavior
that is wired in the brain --

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court if there are feminine pheromones,
attack if there are masculine pheromones --

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can it be changed?

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Could it be that an animal
as simple as a fly

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could learn from its experience
and modify a behavior which is innate,

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something which is wired
in its nervous system?

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To respond to this question,
we did a similar experiment.

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Again, we manipulated the "transformer",

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but this time, in the females' brains.

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These females still have
feminine pheromones

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but they act like males.

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We muted their "transformer" in the brain
to turn them into aggressive females.

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They not only dislike courtship completely
but they also attack.

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We then asked ourselves,

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what is a male going to do
when it meets a female

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that has female pheromones
but is aggressive?

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The first thing it's going to do
is court her as much as possible.

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It chases and chases her,
extends its wing, sings to her,

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but the moment comes
when he decides to change his behavior.

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The moment comes
after so much ineffective courting

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and in addition to being attacked by her;

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he says, "enough is enough"
and ends up attacking her.

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This means that even an animal
as simple as a fly

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can learn from its experience
and modify its behavior.

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And not only that.

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When there are fights between two males,
there is always a winner and a loser.

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After a male loses many times

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what happens is that
he suppresses this behavior.

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He stops fighting.

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This means that even in an animal
as simple as a fly

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not only behavior is in the genes,

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but the ability to learn from experience
and modify it is there, too.

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Thank you very much.

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(Applause)