Rotifers are tiny aquatic creatures with options: They can have sex, or they can simply clone themselves. And this ability to go both ways has made them interesting subjects in the study of the evolution of sex.

Now scientists find when these animals are moved between varied environments, they became more likely to seek out a mate, rather than creating the next generation on their own, sans sex.

Why do scientists care? Because sex is messy and costly and is not the easiest way for organisms to propagate, so scientists have long wondered why it evolved at all from the original method — asexual reproduction, aka cloning.

This study indicates that certain environmental conditions allow for the evolution of a higher rate of sex, in spite of the costs it entails. This finding could help explain why sex evolved, according to the researchers. [Why We Have Sex: 237 Reasons

The drawbacks to sex

Sex between parents shuffles their genes before passing them on — not the simplest way to create offspring.

"It would be so much easier to clone yourself. You don't have to find a mate, you don't have to spend time mating," Lutz Becks, an evolutionary biologist at the University of Toronto and a study researcher told LiveScience.com.

The list goes on: Sex also has the potential to break up advantageous genetic combinations and, assuming each individual produces the same number of offspring, it produces half as many individuals by the third generation as compared with asexual reproduction.

Theories explaining the evolution of sex focus on its role in shuffling genes as a means of purging harmful mutations or by allowing host species to evolve new defenses to fight off parasites. There is also theoretical evidence that sex evolves more easily in a spatially varied environment, which the researchers tested.

A complicated life cycle

Becks and University of Toronto colleague Aneil Agrawal gave the rotifers either high- or low-quality algae to eat. They then moved some of the rotifers between the high- and low-quality food environments. This "migration" created varied environments for them.

Rotifers have complicated sex lives. If conditions are right, they can switch from asexual reproduction to sexual reproduction, but the change requires two generations — one generation for asexual females to switch from laying cloned eggs to laying eggs that produce sexual females. During the second generation, those sexual females then produce males, and the males and females can mate, producing eggs. Rotifers produce about one offspring per day.

To study how much sex was going on among the rotifers, the scientist looked at the proportion of clones that switched to sexual reproduction and the percentage of sexually produced eggs.

After 13 weeks, they found that sex had declined rapidly among the rotifers living in the homogenous environments of either all high- or all low-quality algae. However, among the rotifers that had to adjust to different environments as the scientists moved them around, the decline in the rate of sex was negligible.

To get a better idea of what was going on, the scientists restarted the experiment, mixing all of the animals together. Because sex among the animals in the homogenous environments had declined markedly, this brought the overall rate of sex down below where it had been 13 weeks prior. However, after the experiment was restarted, the rate of sex among the animals in the varied environments climbed toward its previous rate, leading the scientists to believe that the rate of sex was reaching an equilibrium determined by the environment.

This finding could potentially help explain the mysterious evolution of sex, said Becks. 

"There are so many theories out there, and we don’t really know what the answer is, why sex evolved," he said. 

The research appears in tomorrow’s (Oct. 14) issue of the journal Nature.