When Procreation Is a Matter of Real Estate

Sexual female rotifers, top, carrying resting eggs (the darker eggs) along with asexual females carrying lighter-shaded amictic eggs, and a single asexual female with an attached asexual egg.

The choice to have sex has everything to do with location, at least for tiny freshwater creatures called rotifers.

Rotifers can reproduce sexually or asexually, and the decision to go one way or another depends on the animals’ habitat, according to a new study in the journal Nature.

The researchers bred rotifers in three different environments: one in which the quality of available food was high, one in which it was low and one in which it was mixed. The rate of sexual reproduction remained the same where the food quality was consistently high or low, but it increased significantly in the mixed region over generations, the researchers found.

In the mixed environment, asexual females were more likely to produce sexually reproducing female offspring. In the two homogenous regions, females tended to produce asexual females — carbon copies of themselves.

The researchers believe that a more diverse set of genes is a useful survival tool in a heterogeneous environment.

“That would be the explanation as to why sex is beneficial and why the rate of sex goes up,” said Lutz Becks, an evolutionary ecologist at the University of Toronto and the study’s first author. “You are mixing your genes.”

After 12 weeks, or about 80 generations of rotifers, the researchers found that about 80 percent of the population in the heterogeneous group was sexual, compared with only about 40 percent of the homogenous groups.

“Nature is, of course, different from our simple laboratory environment, but this allows us to follow the rate of sex in real time,” Dr. Becks said.

Sindya N. Bhanoo, New York Times


Full article and photo: http://www.nytimes.com/2010/10/19/science/19obrotifer.html