SEX: WHY BOTHER? EVOLUTIONARY MYSTERIES
PROBED AT UH
Ricardo Azevedo’s Research on Pros,
Cons of Sexual Reproduction Explained in Nature Magazine
HOUSTON, March 1, 2006 – What advantage did sex offer when
it first appeared and why does sex persist in modern organisms,
stopping them from becoming asexual again? One University of Houston
professor thinks he may have uncovered some new clues in answering
these questions.
By studying one of the great mysteries of biology – the evolution
of sexual reproduction – Ricardo Azevedo, an assistant professor
in the department of biology and biochemistry at UH, has found in
a study using a computational model that a leading theory may be
more plausible than previously thought, as a consequence of sexual
reproduction itself. His findings are described in a paper titled
“Sexual Reproduction Selects for Robustness and Negative Epistasis
in Artificial Gene Networks,” appearing in the current issue
of Nature, the weekly scientific journal for biological and physical
sciences research.
Collaborating with Christina Burch from the University of North
Carolina at Chapel Hill, Azevedo and his team created a very simple
model of how genes interact with each other to produce an organism
and simulated the evolution of this simple genetic system under
different conditions. What they found was quite surprising –
sexual reproduction itself can lead to the evolution of a special
feature of the genetic architecture known as negative epistasis
that, in turn, confers an evolutionary advantage to sexually reproducing
organisms. In other words, sexual reproduction may be self-reinforcing.
They also found that sexually reproducing populations evolved an
increased robustness to mutations when compared to asexual ones.
These findings suggest a good news/bad news scenario when it comes
to the evolutionary implications of sex. Sexual populations adapt
better to their environments and become more resistant to harmful
mutations, but these advantages are more likely to benefit our natural
enemies.
According to Azevedo, the issue is that there are many costs associated
with sexual reproduction. First, sexually transmitted diseases are
widespread in sexually reproducing populations, making sex risky.
Second, there’s the so-called “twofold cost of sex,”
such that if females carry most of the burden in mammalian sex,
this appears to be true in evolutionary terms, as well. A mutant
human female able to reproduce asexually and give birth to more
females like her would give rise to a population with twice the
reproductive rate per capita of the normal human population and
would become dominant within a few centuries.
While a switch to asexual reproduction is extremely unlikely to
happen in humans due to a genetic quirk of mammals called genomic
imprinting, asexuality can and has re-evolved many times in animals
such as reptiles, fish and insects. However, despite its many costs,
sexual reproduction is widespread and asexual populations tend to
be relatively short lived in an evolutionary time scale.
“Asexuality seems to be an evolutionary dead end,” Azevedo
said. “So sex must have its benefits.”
Many benefits of sex have been proposed over the last century, but
scientists have had a hard time figuring out which ones are decisive.
One being examined here, known as the mutational deterministic hypothesis
(MDH), postulates that sexual reproduction confers an advantage
by helping natural selection remove harmful mutations from the population.
“According to MDH, in order for sexual populations to overcome
the twofold cost of sex, two things must be true,” he said.
“The production rate of harmful mutations must be relatively
high, such that each individual acquires on average one or more
harmful germline mutations not inherited from its parents. The second
is that these harmful mutations must interact in a special way,
called negative epistasis, such that adding more and more harmful
mutations makes you progressively worse off.”
For example, if a single harmful mutation lowers fitness by 5 percent
on average, then successive mutations are expected to lead to a
progressive decline in 5 percent steps if the mutations don’t
interact with each other. Negative epistasis, however, comes into
play, for example, if the second mutation decreased fitness by 10
percent, the third by 15 percent and so forth.
While biologists have been trying to figure out just how prevalent
negative epistasis is in nature to test MDH, relatively little attention
has been paid to the question of what conditions could lead to the
existence of negative epistasis in the first place. If those conditions
were known, it would help scientists decide whether it’s even
worth looking for it or not. Azevedo’s study suggests that
it is. In many of their simulated worlds, sexual reproduction generated
negative epistasis, thus creating the conditions required for its
own maintenance. If this is true about the real world, this would
constitute a spectacular example of evolution forging its own path.
Although the thought that sex may have evolved as a kind of “genetic
waste disposal” mechanism would seem depressing, it gets worse.
The evolutionary benefits of sex are likely reaped most effectively
by organisms with fast generation times and large population sizes,
such as disease-causing microorganisms. That sex also may confer
an increased ability to fight back parasites, as proposed by another
theory for the evolution of sex, probably serves as little consolation.
But it’s exactly why scientists, like most other human beings,
find sex so intriguing.
For a copy of the article, visit http://www.nature.com/nature/journal/v440/n7080/abs/nature04488.html.
Visit Ricardo Azevedo’s Web site at http://www.bchs.uh.edu/faculty.php?155622-961-5=razeved2.
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