March 1, 2006
UNIVERSITY OF HOUSTON
Office of External Communication
Media Contact: Lisa Merkl
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SEX: WHY BOTHER? EVOLUTIONARY
MYSTERIES PROBED AT UH - Ricardo Azevedo’s
Research on Pros, Cons of Sexual Reproduction Explained
in Nature Magazine
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. 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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