Dear all,
It’s great you’re all here to witness the defence of my thesis. A quick overview of what is
gonna happen the next hour and a half. I start with giving a short talk of about 10
minutes on the work I did these last few years. Then the committee will come in and the
actual defense will start. After 45 the pedel will show up to tell that the defense is over.
The committee leaves and we have break of 15 minutes to chat a bit. Finally the
committee will be back for the ... Of the ... And the judicium. But let’s start with
explaining my research.
My thesis is entitled sexual selection in fungi, and as you might have understood from
the title it has something to do with fungi and sex. And that is correct, what you maybe
not got, is that it actually is all about evolution.
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As you most likely know, Darwin was one of the first to extensively describe how
evolution can occur, and how species can adapt to their environment. He is most famous
for the theory of natural selection as described in his important book on the origin of
species. What many of you probably don’t know is that he wrote another important
work (by the way he wrote many books, notes and papers, he was quite a productive
chap) but one of those works written about a dozen years was the descent of men, in
which he clearly describes how sexual selection can drive evolution.
Sexual selection has been studied extensively in animals, and to a lesser extend in
plants, but had not been considered to act in fungi too. In my studies I investigated how
sexual selection affects evolution in fungi. Before I’ll get to the fungi, I’ll shortly explain
how natural selection sexual and selection works.
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Natural selection describes how a trait can evolve. For most traits variation is present
and this variation is often heritable, up to a certain level. I this example there are 5 birds
that vary in the length of their tails.
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And natural selection leads to evolution of intermediate tail-length. Survival is however
not the only thing that is of importance for reproduction and thus for evolution.
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In order to reproduce an individual first needs to find a partner to reproduce with! Here
there are 5 male and 5 female birds. There is an important difference between the male
and female individuals, there are many differences, but one is very important for sexual
selection. The females are limited in the number of offspring they can produce. One
female can produce maybe 10 eggs per year, but not many more. On the other hand,
one male can mate with all five females, and thus produce 10 offspring per female,
which results in 50 offspring. The more matings, the more offspring. However, this is
true for all males, and thus competition for mating will occur. If a male has a
characteristic that gives it an advantage in competition for matings, this trait will be
selected. The females can mate with either male, and thus be choosy.
Suppose females prefer a long tail. I’ll not go into why they do this, because that is quite
a complicated story, but let’s just assume they do. The males with the longest tails will
mate and thus reproduce.
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The other males will not produce any offspring. Sexual selection thus leads to increased
tail length in males, because it increases their chance of mating. The females will have
the optimal intermediate taillength, which is good for survival, as females don’t benefit
from long tails at all.
That is what makes sexual selection so interesting. It can lead to evolution of traits that
do not seem to be beneficial at all.
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Here are some examples of sexually selected traits in animals. At the top, extravagant
traits in males evolved to increase attractiveness to females. The examples at the
bottom increase competitiveness between males by signalling its strength as in the
lizard or by actual fighting.
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So how does this relate to fungi?
I performed all the experimental research described in the thesis with the mushroom
fungus S. commune. It’s very nice to work with as we can easily manipulate it and grow
it in the lab. It is maybe difficult to imagine how sexual selection occurs, as there are no
obvious traits as in the previous examples, but also this species needs to find a mate and
competition for mates can arise, thus giving the potential for sexual selection. Including
sexual selection as a factor for evolution might explain some characteristics that we can
observe in fungi, that we previously could not explain.
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To better understand this I’ll quickly explain the lifecycle of a mushroom fungus. The
lifecycle starts with a spore that is produced by the mushroom. The spore travels
through the air until it finds a substrate, wood for Schizophyllum, and germinates. It then
forms a mycelium, which is a network of connected cells. Each cell contains a haploid
nucleus, this is an envelope that contains a single copy of the genome. Such a mycelium
is called a monokaryon. The monokaryon continues to grow, until it finds another
individual. This is when mating occurs. The two mycelia fuse and start exchanging nuclei.
Nuclei from one individual are transported into the others mycelium and vice versa. The
final result of the mating is that the entire mycelium is composed of cells that contain
two nuclei. This is something which is unique to fungi. When an egg becomes fertilized
by a sperm, the two nuclei fuse together to form one nucleus that contains two copies
of the genome, it is diploid, but not in fungi, here they remain separately. A mycelium
with two nuclei is called a dikaryon. The dikaryon will grow a bit more and can form
mushrooms that can produce haploid spores again.
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There are a few specific questions I investigated.
First of all we need to see if sexual selection is at all possible in fungi. For this we need
to investigate the moment of fertilization and see if some individuals are more
successful in mating than others. I looked at a specific type of mating...
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In which a monokaryon is fertilized not by another monokaryon but by a dikaryon. This
type of mating is expected to occur in nature regularly. In this case the two nuclei in the
dikaryon are competing with each other for fertilization. We were interested to see if it
is always the same nucleus that wins, and it so, who is in charge. Is it the grey mycelium
on the right that prefers one of the nuclei, or if the two nuclei fight it out between
themselves.
Our results showed that indeed almost alwaus the same nucleus wins competitions, and
that most of the times the interaction is one that resembles male-male competition as
depicted in the lower figure, but that sometimes the female is in charge.
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So is sexual selection possible. Yes, some males are more successful in mating.
But showing that under some circumstances there is variation in mating success does
not prove that sexual selection plays a role in nature.
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For this I collected some trees in the woods and investigated the genetics of individuals
on the tree. From this I could deduced how mating had occurred. The results showed
that many matings occured and that these might very well have led to sexual selection.
Because I wasn’t there when mating actually occurred, I cannot claim that sexual
selection takes place, but there are many indications it might occur.
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Does sexual selection occur in nature. Very likely.
But even if there is competition during mating, this does not necessarily mean that
competition leads to evolution of traits. To actually test if sexual selection drive
evolution I performed an evolutionary experiment.
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In this experiment I had my fungus evolve in the laboratory under strong competition for
mating. I repeatedly had nuclei compete with each other to fertilize a monokaryon. Only
the few nuclei that managed to perform this fertilization make it to the next transfer. A
trait that increases your competitive ability will thus be selected. Indeed I saw that after
20 transfers some of the evolved strains were much better in performing fertilizations.
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So also in fungi, sexual selection can lead to traits that increase fitness due to a better
competitive ability.
With these questions answered we know that at least in schizophyllum sexual selectoin
is likely to play a role. To see whether also in other fungi sexual selection plays a role I
investigated the literature to look for examples of traits that might have arisen due to
sexual selection. Many examples were found. Many traits that we found are difficult to
explain if no sexual selection is present. Overall it seems that sexual selection acts in
fungi. With this knowledge we can further understand how and why fungi evolved
different mechanisms that act during mating, but also gain knowledge on how general
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