The arms race between a parasite and its host | Ellen De Caestecker | TEDxFlanders


Good afternoon, everyone. I’m going to indeed introduce you to the magnificent
world of the waterflea and its parasites. They are very small organisms that you can
find them everywhere if you look hard enough. I will tell you more about their adaptation
— how one adapts to the other — and the other way around. And then I will try to explain to you how
we can come to the story of Alice in Wonderland and that queen dynamics. But if I’m talking about adaptations I have
to talk about the person that you have met probably last year but we should not forget
about — that is Darwin. I insist in referring to him again because
he is really the person who taught us that adaptation and natural selection is the basis
of evolution. And he came to that concept after his big
voyage with the ship the Beagle around the world. So there he met different populations and
he came into the insight that adaptation and natural selection are important processes. In his book On the Origin of Species, he taught
us that a lot species actually have the potential to populate the whole world if they want to. But, on the other hand, he also noticed that
populations stay naturally constant. So that led him to conclude that every organism
is in the struggle of life and it’s only the best adapted at a certain circumstance that
will win and pass on his own genes. That’s the survival of the fittest. So everyone is on this world born to maximize
its fitness and to produce its genes to the next generation. Darwin came to that idea after his traveling
around the world and one example to clearly show you how evolution and natural selection
work is the example of the giant tortoises at the Galapagos Islands. So the Galapagos Islands are relatively new
— so geologically-speaking they are new. So what has happened probably and what is
most likely is that one female pregnant tortoise came on the Galapagos Islands and she has
a lot of offspring that spread around all the different islands of the Galapagos. Now each island of the Galapagos differs in
the circumstances — you have dry islands and you have more wetter islands. So the dry islands, they typically have high
foliages with high-growing plants. So this leads to the diversification of a
few tortoises. So we have typically the saddle type with
the long neck, and the domed type with a short neck and the intermediate type. So now you all may think, Oh no, she is going
to explain the story of Jean-Baptiste Lamarck. No, I’m not. I’m going to show you Darwin clearly differs
from Lamarck. What Lamarck says is that we all inherit acquired
and we give it to our offspring, but that’s individual-based. There’s a big difference with Darwin. Darwin’s natural selection theory is based
on populations, so on how groups and all of you. So what actually happened with this tortoise
is that at a certain island they come, they adapt, and the best adapted individuals will
pass on their genes to the next generations. So in a dry island with high foliage, those
with high necks will have a higher reproductive capacity and they will have more offspring. So the next time, the next generation that
you look at, the percentage of long-necked tortoises has changed. It has increased. So this sort of frequency change that we see
in environments over time in natural selection. Now I have to tell you a small story about
Lonesome George. Lonesome George is one one of the tortoises
that is the last descendant of the Pinta Island on the Galapagos. So what happened? The people came on the Galapagos. They reduced the total population and then
in the 60s and 70s the last individual that was found was Lonesome George — who is also
called the rarest living creature. And this is because he was totally alone and
there was not so much hope for Lonesome George actually because when they found him, they
kept him in captivity and they placed him with other tortoises of the islands nearby,
from Isabela Island. And he was totally not interested in these
female tortoises so it was a pity so they always thought, Okay, so Lonesome George will
stay alone. But apparently recently there is more hope
for Lonesome George because he showed some mating activities with his female partners. And also they found a new individual who is
actually a hybrid, so a cross between the species of Lonesome George and another one
of Isabela. So there is hope for conservation biology
that they may have a new Lonesome George in the end after a while. But having said that I actually until now
I mainly focused on — if we think about evolution, we think about macro-evolution, we think about
the diversification of species, but I’m going to prove to you that evolution is going much
more faster than we think. And it’s also around us. So this is the normal macro-evolution — we
go to a shorter time scales — and then another example is the Darwin finches, which may be
familiar to you as well. What you see here is that the Darwin finches
— you have different species; they all look pretty similar but if you look very well you
see that they differ in the beaks — so in the type of the beak. That’s actually a way of reducing competition,
so all competition between people who are here — an important lesson from the Darwin
finches that are here is try to diversify as much as possible. You have to find your niche that the other
one didn’t find. This is why we have to come up with different
ideas. And this is what the Darwin finches also did. So they each became specialized in the type
of food that they ate so we had an evolution towards different species. And Peter and Rosemary Grant, they could show
that after a few years that they studied the Darwin finches, at the Galapagos Islands that
the evolution can go fast. What you see on the slide here — on the left,
you see that in 1976 — some of you may remember, although you all look pretty young — but
we have a very dry year. So what happened is that there was not much
food; there was only big seeds, so we actually got a shift in the beak size from the small
size to the big size. And importantly, they could also show that
this has a genetic basis. So here we see evolution, we see genetic changes
over time in a few traits. That is adaptation. They also calculated that if those dry periods
would happen a few times, we will become a new species every 20 years so that’s already
pretty fast. But not so fast as evolutionary parasites,
and more particularly, co-evolution because co-evolution refers to the evolution that
two species exert on each other because they adapt to each other. And then they diversify. But parasites are pathogens — so we all know
the viruses and bacteria — who are becoming very violent and resistant. They typically adapt very, very fast. Another example of striking behavior in co-evolution
— just to show you how cool evolution can be — is with this worm, this Dicrocoelium
dendriticum. This worm actually induces changes in behavior
of ants. So you see the ant over there — what this
worm typically does is going to different species. So it has a typical host. It goes from the ant to the sheep and after
the sheep to the snail and then again to the ant. All the time. So what this worm can do is from the moment
it comes into the ant, it will migrate to the brain of the ant. However you need to call it. The haemocoel of the ant — actually it sticks
to that and what happens is that the ant is crawling on the grass, on top of the grass,
and it freezes. Until a sheep comes because then it gets eaten
faster. So that’s a very small behavior change of
the parasite because it also does it every day and on very strict time points and these
time points are related to the sheep predation activity. So this shows that these adaptations can be
very peculiar and that they are very strong. But then to our very fast adapting parasites
and the most fast ones are the viruses and the bacteria. So what you see on the slides here is that
if there is no selection we get multiple strains, so we all differ, we all are different types,
but from the moment that we exert selection and that’s what has happened for instance
with this Staphylococcus aureu which may be more familiar to you as the “ziekenhuisbacterie.” So what has happened is that if you have selection
— if you use current antibiotics that have Methicillin, you will only have a few lines
that can survive the antibiotics and they will become very resistant. So persons in hospitals have a high chance
to become more exposed to those most-resistant mutated bacteria lines — if you use a lot
of antibiotics. And then I have almost come to my research
but what I want to stress here is that given that even after this very fast adaptations
between hosts and parasites we can actually have an co-evolutionary arms race. With this I mean that there is no absolute
goals, so they actually fight off each other all the time and with only that thing in mind
that they won’t be relatively better than the other one. Referring to the Cold War between the USSR
and the States and maybe also in government — it tends to show some co-evolutionary arms
races as well if you ask me. So a solution for that may be the people in
the government should listen. In this arms race with this host and parasites
we can have Red Queen dynamics. What are those Red Queen dynamics? First of all, the main principle is actually
that you have to use as much different possibilities to fight off your parasites as possible. So using antibiotic cocktails seem to be very
efficient because it lowers the resistance adaptation of the parasites. And another solution that nature has found
is sexual recombination because apparently there are lots of costs to sexual reproduction
for one person more than the other. But the good thing is that you have genetically
diversified offspring. And having all these different types makes
it more difficult for the parasite to adapt. You can see that on the left — if you have
an asexual population — so assume worms that are reproducing all the time asexually, then
the parasites will win easily. But if you have an organism that can reproduce
sexually, it is much more difficult for the parasite. And this adaptations actually leads to this
Red Queen dynamic. You don’t have to understand this graph, but
you do have to understand is that you see that they fluctuate all the time. So we get fluctuations in allele frequencies
over time if you want to say it more difficult. So we get all this fluctuation and the result
is that no one wins. No one will get to the directional fixed point. Or you can also say the net effect on the
fitness of the genetic contribution changes only relatively. Those who stand still loses the game clearly. And this is why it’s called the Red Queen
because it’s referring to the story of Alice in Wonderland — where the Red Queen is saying
to Alice, “Now, here, you see, it takes all the running you can do, to keep in the
same place. If you want to get somewhere else, you must
run at least twice as fast as that!” So we have to be faster than these bacterial
lines. Okay, we knew all that so we were busy with
that during my PhD I was developing all those ideas and then I said, “Aw, that’s a pity,
we don’t have real evidence for that process. We see that bacterial lines mutate and that
there is some adaptation but it’s very difficult to follow those lines over time.” So then we came up with our system, which
is water fleas — Daphnia — and the nice thing with water fleas is actually they are
very small. They are not fleas; they do not fly around. They do no jump around. They jump in the water, so they are actually
small shrimps. And what they can do is combining asexual
reproduction or sexual reproduction. So if everything goes good, if there’s enough
food, they will reproduce asexually, so they are clones. We have different clones. But if things go bad, they start to produce
males and also haploid eggs and then you have these resting eggs which goes to the bottom
of the lake. So very interestingly Daphnia only need males
in bad situations. So what they then do — the result is that
if you have these resting eggs, they actually have new layers every year, so if you then
take a sediment core out of your lake and you slice it at different depths, you can
take the resting eggs out and we can also hatch them. So we can actually revive the Daphnia. This means we can go back in time. So we can, we have actually the unique possibility
to reconstruct the genetic structure and evolutionary changes from the past. So what did I do? I went to a nice pond in Heverlee near Brussels
and Leuven and I took some sediment cores in which there was a population of water fleas
— on the right you see a sick water flea, a sick Daphnia — it’s full of those bacterial
spores and they typically kill the eggs of the Daphnia, so you see this sick one on the
right doesn’t have eggs and healthy one has. This is very strong competition because killing
off your eggs means that you don’t have genetic contribution to the next generation. So we have a hard competition between our
water flea and our parasite. So we took sediment cores, we sliced them,
we took resting eggs, we revived the Daphnia and we also took the parasite spores out of
the different types. So in this way we could go back into time
and actually reconstruct co-evolution in the past because we had an archive of our host
and we had an archive of our parasite, so we could look at what happened in that time. And this is how we reconstructed co-evolution. We then took the Daphnia of the different
depths and we exposed them — in black — to the parasites from the layer below — so the
parasites from the past; the same layer, the contemporary parasites we call them; and then
the parasites from the layer above — these are the parasites of the future. And if everything goes well we should have
a short intermission of a movie now, but I don’t know if it’s coming. [movie in German] Sorry it’s in German, my apologies for that,
but I think you understood. So what it showed was actually that you have
the resting eggs; they go to the bottom and you can revive them again and so we can reconstruct
the co-evolution from the past. So in this different slices we took the host
and the parasite and then we looked at what happens. So this is what I said before. But interestingly, our results showed that
there is indeed an arms race going on. So our Red Queen arms race is because we see
our the graph you clearly see that on average the current or the contemporary association
so if you take the parasite and the host from the same depth that the parasites do best. They infect better the Daphnia than parasites
from the depth below. So best parasites are parasites of the future. And then if you look over the whole range,
we saw that there is no directional increase in infectivity over time. So this shows that no one wins. You gets this continuous adaptation between
the host and the parasite, resulting in no final or absolute goal. So we had empirical evidence for host parasite
Red Queen dynamics and I hope I could convince you that even by using such a small animal
as the water flea that we can see the important evolutionary processes going on. And they are important not only for the Daphnia
but what we then did is putting our results in models and mechanically extrapolate our
models to other models, such as the human immune system. And there we can also see that there is a
lot of genetic variation on resistance in humans — this is being proved more and more
now. And apparently we also found a cure-all to
biodiversity and this comes to the biodiversity year because the fact that sexual reproduction
is important — we have to create genetic variation; we have to create diversification
and in this way we will be able to conserve our biodiversity. Maybe. Thank you for your attention. [applause]

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2 thoughts on “The arms race between a parasite and its host | Ellen De Caestecker | TEDxFlanders

  1. i just found out about her research because profesoor Raoul Moulder mention it in the animal behavior class that i am taking at coursera, it is a really interesting investigation. What a privilege to have her as your teacher. and I need to work in my english too :p

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