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Why Sex Is Good
adapted from Clyde Freeman Herreid
Department of Biological Sciences
University at Buffalo, State University of New York
Birds do it. Bees do it. Even educated fleas do it. Let’s do it. Let’s fall in love.
—Cole Porter
Part I—”It”
Why do so many organisms go through sexual reproduction? It seems like every organism we
think about does it: clams, jellyfish, trees, and elephants. And while we’re thinking about it: why
only two sexes? It doesn’t have to be that way. Some fungi have dozens of sexes, enough to
keep a romance novelist and a scriptwriter of soap operas ecstatic for years.
Sex really isn’t necessary for reproduction. Bacteria and many one-celled organisms like
amoebae reproduce quite nicely by simply dividing in half (binary fission). They produce
identical copies of themselves, quite an efficient way of sending one’s genes on to the next
generation. They do it alone. For them, it doesn’t take two to tango.
Other organisms can do it too. Some lizard species have only one sex—females. They
reproduce parthenogenetically—that is, females produce eggs that spontaneously start
development without sperm being involved at all. They are completely asexual.
Some species have it both ways: they reproduce both sexually and asexually. You’ve learned
about the reproductive strategies of many protists and fungi, many of which can reproduce
either asexually or sexually. Queen bees when they produce females (workers) release sperm
out of a storage sac and fertilize the egg in the normal way, but when they want to produce
males (drones) they hold the sperm back and the eggs develop by parthenogenesis. Water flea
(Daphnia) populations seem to switch from asexual to sexual depending on environmental
conditions. And some species of fish actually switch from being one sex to the other depending
on which gender is in short supply.
So, this brings us to a fundamental question that biologists have not completely solved: If
organisms can survive well without sex—in fact, may do better without it—why has
sexual reproduction evolved?
Questions
1.
Propose three hypotheses to explain why sexual reproduction has evolved. (At least 20
have been suggested!) The following articles will help your hypothesis development:
http://science.sciencemag.org/content/324/5932/1254.full
To increase a populations survivability the many environmental changes that occur
throughout the decades.
To increase the possibility for genetic variation.
To allow for an organism to select what traits to pass down to future offspring.
2.
How might you test one of these hypotheses?
To test one of the hypotheses, I would compare the genetic variation between an
organism that only participates in asexual reproduction to one who reproduces sexually.
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Through genotyping ancestral organisms and looking at the genetic modifications that
arise with each organism.
Part II—Is It Always Good?
In a world without sex there would be no males and females. No flowers, no insects specialized
in pollinating them, no extravagant color and form like the peacock’s tail; and much animal
behavior would not exist. —Rolf Hoekstra
All of that is true, but so what? Who needs this stuff that Hoekstra is talking about for survival?
The great German biologist August Weismann proposed an answer to the question of “Why
sex?” He asserted that sex increases genetic variation. When two different individuals mate by
joining their gametes together, they produce a brand new genetic mixture and this promotes
evolutionary adaptation.
This idea held sway for a hundred years until a couple of authors, George Williams and
Maynard Smith, said, “Hold on. There are a couple of problems with this scenario.” Sex is not
always good.
1. Mixing of the genes tends to break up favorable combinations. Why break up a good thing?
2. Asexual reproduction is twice as efficient as sexual reproduction at sending one’s genes into
the next generation. Every time a sexual mother produces a child, that child only has one-half of
the mother’s genes; the other half is from dad. An asexual mother reproducing
parthenogenetically would give her child the complete set. In fact, it is better to have every
individual in a population capable of reproduction (i.e., be a female) than to have individuals
who are not (i.e., be a male). Such populations should rapidly out-reproduce a sexual
population. This has been called the “two-fold cost of sex.”
On both of the above counts, it seems clearly disadvantageous for individuals to reproduce
sexually! Yet sex has evolved and seems here to stay. Many scientists have tried to puzzle their
way out of this dilemma by testing some of the assumptions inherent in the argument.
Question
Design an experiment to test the hypothesis that asexual reproduction leads to a higher
evolutionary fitness (i.e., leads to more progeny) than sexual reproduction.
An experiment to test for this would be observation of two similar species, one with sexual
reproduction and one with asexual reproduction. Through taking the species differences into
account, you could track the reproduction of how many are produced as well as the how long
the reproduction cycle takes for completion. Typically, shorter reproduction cycles produce
greater amounts of progeny in a certain set time period.
Part III—Sex and Stress
There is a snail that lives in New Zealand lakes that has both asexual and sexual individuals.
Curtis Lively (currently at Indiana University) and his colleagues decided that the snails could be
used to test the hypothesis that a changing or stressful environment would favor sexual
reproduction—the logic being that if the environment changes, then variation (from sexual
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reproduction) is a good thing; some of your offspring might have the right genetic constitution to
survive.
Question
Why would sexually reproducing snails have greater variation than asexual snails?
Sexual reproducing snails have greater variation because when two snails come together, they
each bring their unique genome together. Through genetic recombination, they will have a more
diverse offspring with greater variation compared to the parents. This is different that asexual
snails which simply produce clones of themselves as offspring.
Here’s the situation the biologists found. The snails live in freshwater habitats and there are over
a dozen worm parasites that attack them. The scientists reasoned that there might be a
difference in the fitness of the asexual and sexual individuals in ponds where there were
different degrees of parasitism.
This is what they found: in ponds where there was a high degree of parasitism there was a
much higher percentage (2.5 times more) of sexually reproducing individuals.
Questions
1. Before carrying out the experiment, why did the scientists expect there would be a difference
in fitness between sexual and asexual snails in ponds with different degrees of parasitism?
As degree of parasitism increases, certain traits are less affected by the parasites. Snails that
aren’t as affected by the parasites will they be able to sexually reproduce more those snails that
are, thus allowing the traits which protect against parasites to carry on to new populations. The
offspring are better able to survive against the attacks from worms, and the cycle continues. The
genetic variation will increase until the snails are no longer affected by the parasite’s presence,
under the assumption that the parasite doesn’t evolve.
2. Are the data they obtained consistent with Weismann’s hypothesis (from Part II)? Explain
your thinking.
It appears to be consistent with the Weismann’s hypothesis. Weismann hypothesized that
sexual reproduction promotes genetic variation, giving the population the upper hand when
confronted with evolutionary changes to their environment (e.g. parasitism). This why the snail
populations with sexual reproduction capabilities have greater genetic variation to evolve to be
immune to the effects of the parasites.
Part IV—An Experiment
A team of scientists at the Imperial College London tackled the problem and published their
results in Nature magazine (March 25, 2005). They decided to use yeasts, which are singlecelled fungi, because they can reproduce both sexually and asexually, are easy to keep in the
lab, and reproduce very rapidly.
Yeasts normally reproduce asexually, but when they are stressed (starved, at high
temperatures, etc.) they will reproduce sexually. The scientists did not want this switching to
occur. So they genetically manipulated one asexual strain. They deleted the two genes (Spo11
and Spo13) required for normal meiosis, so that sexual reproduction was impossible. Now they
had two pure strains—one asexual and one sexual.
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The Imperial College team decided to compare the reproductive rate of the asexual vs. the
sexual yeasts in two different environments: harsh and benign. That is, “fitness” would be
measured by comparing the growth rate of the two strains. The benign environment had plenty
of nutrients although glucose was limited so that growth was not uncontrolled. The harsh
environment had the same glucose concentration but was at a higher temperature and had
more demanding osmotic conditions.
On the graph below plot the results you would expect if Weismann’s hypothesis were correct.
Plot a line that represents the changes in fitness over time for each strain: 1) sexual yeasts in
benign conditions, 2) asexual yeasts in benign conditions, 3) asexual yeasts in harsh conditions,
and 4) sexual yeasts in harsh conditions. Use different colors/pattern to represent each strain
and make a clear legend.
Condition
sexual yeasts in benign conditions
asexual yeasts in benign conditions
asexual yeasts in harsh conditions
sexual yeasts in harsh conditions
Color
Part V—The Results
Here are the results of the real experiment.
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Question
What conclusions can you make based upon the data? In other words, do they support the
hypothesis that sexual reproduction influences fitness in yeast? Explain your reasoning.
It appears that from the data from the graph that the yeast population that experienced the
highest relative fitness were in fact the yeast who were involved in the process of sexual
reproduction. This indicates that under harsh environments, the ability to produce sexually is
favored over the ability to reproduce asexually, according to the data.
Part VI – A New Study
Follow the link below to read the news release (9/10/2013) about a strange reproductive
strategy in a different species of fungus, the yeast (Cryptococcus neoformans):
http://www.livescience.com/39541-deadly-fungus-mates-with-clones.html
Questions
I.
What group would you classify this yeast with (Asexual, sexual)? Why?
II.
What is meant by unisexual reproduction? Why would this seem to be pointless?
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III.
What happens, which would be considered bad in humans, in C. neoformans when it
reproduces unisexually?
Now view the peer reviewed original article where the scientists report these results and read
the abstract:
http://www.plosbiology.org/article/info:doi/10.1371/journal.pbio.1001653
If you have any questions about the abstract discuss it with your group and then clarify
questions with your instructor.
In this figure (6B in the paper), XL280
and WT::NAT represent the original
strain (i.e., wild-type) of the fungus with
the normal number of chromosomes.
All the MN strains are ones that have
undergone aneuploidy and thus have
an extra chromosome. All strains were
then grown on petri dishes (like we did
for the soil bacteria). The YPD petri
dish is the nutrient rich condition, in
which all cells should grow. The FLC
media includes the antifungal drug
fluconazole, which inhibits fungal growth.
IV.
In Fig. 6B the % of each strain surviving after a given time period is shown. To read
this figure you want to compare the proportion of each colored strain to the black,
which represents the wild-type strain, on the YPD plate versus the Fluconazole plate.
What can you conclude about the relative survival of the aneuploid strains to the
wild-type strain.
Now examine Fig. 6C.
In
these graphs the y-axis is the OD600, which is the optical density at wavelength 600nm and
represents the population size of the different strains of fungus. So we are looking at how
population size of each strain changes over time under two conditions: ideal on the YPD media
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and “harsh” on the YPD + FLC media. Remember FLC is an antifungal drug, which is nasty if
you’re a yeast.
Questions
V.
How does the relative growth of the aneuploid strains (MN) of fungus compare to the
wild-type (XL280) strains in ideal conditions and harsh conditions?
VI.
Why might unisexuality (another form of sexual reproduction) be favored
evolutionarily in these pathogenic yeasts?
VII.
What implications do you think this has for human health? Remember that C.
neoformans is a parasitic yeast that can cause deadly brain infections in humans and
is typically treated with antifungal drugs.
VIII.
Now take a look at Fig. 7 and read the figure caption. Are these results consistent
with your response to question VII?
Figure 7. Aneuploid strains are pathogenic in the murine inhalation model.
Ten female mice per group were infected with 106 cells of aneuploid strains MN35, MN55,
MN77, and MN89. The mice were anesthetized by intraperitoneal injection of phenobarbital, and
they were infected through intranasal instillation. The animals were monitored daily for clinical
signs of cryptococcal infection and sacrificed at predetermined clinical points that predict
imminent mortality.
Question
From the results of this study, and those in Part III & IV, what can you conclude about the
evolution of sexual reproduction (including unisexuality) in general? If there is a “two-fold cost”
to doing it, why might it persist?
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