By Dean A. Haycock
Special To BioWorld Today
We might forfeit some initial pleasure, but wouldn't it be better to just split in two when we wanted to reproduce?
Despite our culture's obsession with sex, there are still more questions than answers surrounding it. Marketing and advertising experts may have it figured out to their satisfaction, but scientists still have a few things they would like cleared up. For instance, what advantage does sex provide over asexual reproduction?
A report in the July 31, 1997, issue of Nature, titled "The advantage of sex in evolving yeast populations," suggests one advantage of sex may relate to the ability of sexual reproducers to rid themselves of deleterious mutations.
The authors are Clifford Zeyl, who was a Ph.D. student under co-author Graham Bell, professor of biology at McGill University and Redpath Museum, both in Montreal. Zeyl completed the paper while holding a postdoctoral fellowship at Michigan State University, in East Lansing. He is now an assistant professor at Wake Forest University, in Winston-Salem, N.C.
The work is of interest from the basis of pure biology, of course, but also because it provides experimental data in an area of evolutionary biology where such data is rare.
"What is outrageous is that the people who are doing these experiments are the theorists who have a stake in this business, myself included," said Alexy Kondrashov, associate professor at Cornell University. "Then there are hordes of experimental evolutionary biologists who are doing God knows what."
The theoreticians who dominate the field have proposed several theories to explain Nature's apparent predilection for sex. Such theories must account for the observations that sexual reproduction requires each parent to pass on only half of his or her genes to an offspring. This might seem to dilute or waste a useful combination of genes built up over many generations. Obligatory sex also cuts the potential rate of reproduction in half because males don't give birth while the products of parthenogenesis, or straightforward splitting-in-two, each can continue to replicate themselves.
One of the best-known theories assumes that the advantage of sex can be found somewhere in the resulting differences between parents and offspring. Different genes provide a greater chance of adapting, if necessary, to the challenges presented by a changing environment, according to this explanation. Helpful mutations that pop up independently could be concentrated into a line of descendants.
New Theory Of Sex Investigated
Another explanation holds that sex offers the advantage of concentrating deleterious, or harmful, mutations in a stable environment. Once these mutations are concentrated in an unfortunate offspring, natural selection eliminates the offspring along with his or her undesirable genes and rid the population of them. These are not mutually exclusive explanations for sex, but they remain theory. More effort has gone into testing such ideas in models and simulations than in living organisms.
Zeyl and Bell studied Saccharomyces cerevisiae, a yeast organism capable of both sexual and asexual reproduction. They provided the yeast with a stable environment by including glucose in the their culture media. Yeast prefer this sugar as an energy source. A challenging environment was provided by a culture media in which glucose was replaced with another sugar, galactose. Yeast grow more slowly in this sugar and must adapt their metabolism to use it efficiently.
The researchers found that sex increases the fitness of yeast in the stable, unchanging environment but does not improve their lot in an environment that presented a challenge. Also, the sexual populations grew more quickly on both types of media than the asexual populations did. The work thus supports the notion that the advantage of sexuality is its ability to remove deleterious mutations from a population in a comfortable environment.
More Study Needed
"It is a good paper but the evidence for deleterious mutations is still indirect," Kondrashov told BioWorld Today. "I am a little bit obsessed with direct evidence of deleterious mutation rates."
Kondrashov points out that we still don't know genomic rates of deleterious mutations in any organism and that obtaining them will be incredibly difficult.
"We are nowhere close to the having the sequencing capacity (required to obtain these rates)," Kondrashov noted.
Zeyl acknowledges that his is indirect evidence. It is, however, actual experimental data in a field that can use it. Zeyl already has begun experiments that might provide direct evidence to support his findings.
"I've taken sexual and asexual populations and hit them with a mutagen, a chemical that produces mutations, and then looked at the effect of sex on fitness," Zeyl said.
His preliminary results support the conclusion of the Nature paper.
"I did not mean to suggest in the paper that the alternative explanation for sex, the faster adaptation in a new environment, is necessarily rejected. I didn't see it in this case but the difference between the two environments was a fairly simple one that may not necessarily reflect what typically happens in nature. So I am also going to try experiments in which the environment is different in more ways, to give a better opportunity for sex to accelerate adaptation," Zeyl said. He cited changing variables such as pH and temperature.
The work is of interest from the aspect of pure science. And from a practical standpoint, it confirms the long held theoretical belief that a population of asexually reproducing cells will decline in fitness over the long term.
"This is one explanation for why so many familiar organisms, including almost all vertebrates, have chosen in an evolutionary sense to reproduce in what seems to be a very inefficient way," Zeyl said. *