Why Two Sexes Are Better Than One

Unisex. Even organisms that seem to have just one gender, like this Chlamydomonas alga, have two cryptic "mating types."

Step into a singles bar and it's pretty clear that having humanity divided up into two sexes can be frustrating--it cuts the potential mating pool in half. Biologists have long puzzled over why this should be. After all, with only one sex, everybody can be a potential mate, so why bother with two? Now, a computer simulation of early life may have found the answer.

The fact that sexual reproduction involves males and females seems so obvious that we often do not realize that it is possible with fewer than two genders. Scientists believe that before the two-gender state that most animals, plants, and fungi now find themselves in, there was only one sex, and everybody could mate with everybody else. In fact, says theoretical biologist Tamás Czárán of Eötvös Loránd University in Budapest, Hungary, many theoretical studies and computer simulations show that a one-sex situation is not likely to evolve into something else. So biologists had always wondered how nature got into the apparently wasteful habit of supplying species with an extra gender.

To solve this problem, Czárán and evolutionary geneticist Rolf Hoekstra of Wageningen University, the Netherlands, reexamined previous computer simulations. They found that all had assumed that species live in the equivalent of a busy town square, where all individuals bump into each other all the time. But in real life, organisms are much more likely to bump into a neighbor than someone from the other side of the tracks. The researchers created a model that took this into consideration and populated it with the hypothetical ancestors of all animals, plants, and fungi: single-celled creatures that reproduce by cloning most of the time, but regularly engage in sex, making either sperm, eggs, or wildcard sex cells that can mate with sperm, egg, or other wildcards. They ran the model 2420 times on 10 PCs simultaneously, taking a total of about 10,000 hours of computation. "It was very demanding," says Czárán.

The outcome was worth it: The model showed that sperm and egg sex cells normally outbred wildcard sex cells, the pair reports online 21 September in BMC Evolutionary Biology. The researchers say the reason is that the latter would more often mate with their relatives, because during the cloning phase, the neighborhood would be flooded by their wildcard offspring and suffer lower fitness due to inbreeding.

Evolutionary biologist Joel Parker of the University of Lausanne, Switzerland, is impressed with the new paper, which he calls "brilliant." He hopes Czárán and Hoekstra will continue their work to find out why more than two genders is also an evolutionary no-no.

Related sites
The article in BMC Evolutionary Biology
Rolf Hoekstra's site
Joel Parker's site