Environmental Factor, April 2007, National Institute of Environmental Health Sciences
Biologist Explores Reasons for Sexual Reproduction
By Eddy Ball
Biologist Matthew Meselson, Ph.D., addressed the question "Why Does Sexual Reproduction Exist?" during a March 9 talk in Rodbell Auditorium. Meselson is the Thomas Dudley Cabot Professor of the Natural Sciences at Harvard University, an adjunct scientist at the Marine Biological Laboratory in Woods Hole, Cape Cod, Mass., and winner of the prestigious Lasker Award.
"This question of why is it that it takes two individuals to produce one," Meselson explained as he began his talk, "...is a fairly old question." Although Darwin offered no explanations and, in fact, wrote that the reasons are "still hidden in darkness," his theory of evolution sparked interest in the question among scientists. Today, the question of why such an involved process is necessary continues to be intriguing.
There are many costs associated with sexual reproduction, Meselson noted. These include the cost of finding a mate, or failing to find a mate, sexually transmitted diseases and parasites, and the inefficiency of a process that demands twice as many individuals to produce offspring.
Meselson noted that there are more than 20 different hypotheses as to why sexual reproduction predominates among organisms with a high taxonomic rank. One of the earliest was the proposal by the 19th century German evolutionary theorist August Weismann that sex exists because it generates the variation upon which natural selection can act. More recent theories have pointed to the proliferation of beneficial mutations - or the avoidance of harmful ones - as advantageous to sexually reproducing organisms as a way of "mixing things up."
In contrast to the mixing theories, Meselson's own current hypothesis focuses on an organism's ability to keep the proliferation of parasitic retrotransposons from overwhelming it. Retrotransposons are mobile genetic elements that insert themselves into chromosomes and are present in virtually all eukaryotes. This kind of mutation, according to Meselson, "is particularly obnoxious... [because it] can replicate itself."
Although such elements reproduce autonomously, their increase is constrained by processes dependent on sexual reproduction. In most cases, abandonment of sexual reproduction results in the number of such elements increasing indefinitely, eventually driving the asexual lineage to extinction.
Interestingly enough, Meselson found support for his ideas in an exception to this pattern of extinction - the lack of retrotransposons in bdelloid rotifers. Bdelloids are an ancient class of invertebrate animals that are characteristically found in the water films of mosses and lichens, in rain gutters and in other ephemerally aquatic habitats. Even though scientists have studied these animals for hundreds of years, they have never observed any indication of sexual reproduction in this ancient life form.
According to Meselson, the key to their survival seems to lie in the remarkable ability of bdelloids to survive desiccation, or dying up, at any stage of their life cycle by entering a metabolically quiescent state and rapidly recovering as water becomes available. It is likely the adaptation of bdelloids to desiccating habitats and the associated DNA damage and repair that have depleted their genomes of retrotransposons and made it possible for them to "cross over" to avoid extinction.
At the end of his talk, Meselson conceded that "since this is the field of theoretical population genetics, it's very difficult to really prove something [definitively]... The situation is so complex." However, the bdelloids, "these marvelous creatures," may offer the best example yet of "nature's experiment" demonstrating why sexual reproduction exists.
The Laboratory of Molecular Genetics (LMG) trainees sponsored Meselson's talk. Postdoctoral Fellow Libertad Garcia-Villada was the host of the lecture.
The Many Aspects of Matthew Meselson's Life in Science
In the course of his long career as a scientist, Meselson has made several important contributions to molecular biology. These contributions include the development of equilibrium sedimentation of macromolecules in density gradients, the experimental proof of the semi-conservative nature of DNA replication, the demonstration of a break-reunion mechanism of genetic recombination in phage lambda, the discovery of the methylation-based mechanism that protects DNA against restriction enzymes, and the phenomenon of DNA mismatch repair. He is also well known for his expertise in chemical and biological warfare, which led him to ascertain the source of the putative poison "yellow rain" in Vietnam and the hidden story behind a lethal outbreak of pulmonary anthrax in Sverdlovsk.
While still a graduate student under Nobel Prize winner Linus Pauling at the California Institute of Technology, he became interested in DNA replication and repair. In 1957, he and colleague Frank Stahl performed the landmark experiment confirming that DNA replicates as predicted by the double helix structure Watson and Crick had recently proposed. This work is the subject of the book by Frederick Lawrence Holmes, Meselson, Stahl, and the Replication of DNA: A History of "The Most Beautiful Experiment in Biology," published in 2001 by Yale University Press.
Meselson also has been politically influential as a vocal opponent of biological warfare. He worked at the Arms Control and Disarmament Agency and became a friend of Henry Kissinger in the 1960's. He served as an advisor to the Biological Weapons Convention in 1972. Despite his influence during Nixon's presidency, Meselson also ended up on one of the former president's two enemies' lists. In the years since, he has been active in the Harvard-Sussex Program on Chemical and Biological Weapons.