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Credit: UNIV. OREGON MEDIA RELATIONS

Biologists have wondered for more than a century why separate sexes exist when many plant and animal species reproduce through self-fertilization (selfing). Two theories had emerged to explain why cross-fertilization, or outcrossing, might be beneficial to species despite the cost of a separate gender. One theory is that it reduces the probability that harmful mutations will become fixed in all future generations as a result of inbreeding. The other is that it boosts the chance that two beneficial mutations arising in two different individuals will appear in future generations and allow them to adapt to new environments. Now Patrick Phillips, an evolutionary biologist at the University of Oregon in Eugene, and his colleagues have tested the effects of new mutations and environmental change (see page 350) by breeding 50 generations of the nematode Caenorhabditis elegans, an organism capable of both selfing and outcrossing. He tells Nature more.

Why are you interested in this question?

Sexual reproduction is a major feature of life on Earth and it's so common we take it for granted. But, in fact, it requires explanation.

What makes C. elegans the ideal model system for this kind of study?

Producing each new generation of C. elegans took only about 4 days, so this part of each experiment, following 50 generations, lasted less than a year. This would be a difficult study to do in plants because it would take a huge amount of time and space. Yeasts wouldn't work because most don't self-fertilize. We know so much about the genetic basis of sex determination in C. elegans that we can use specific mutations to yield strains that either only self-fertilize or only cross-fertilize. Other animal species can't be tested because no such mutations exist that alter the mating system in the right way to conduct the study.

How did you test the 'adaptation' theory?

We exposed C. elegans to Serratia marcescens, a bacterial pathogen that kills the worms. After 40 generations of worms, neither the groups that only selfed nor the groups that outcrossed at low levels could adapt to the pathogen. But the high-outcrossing populations did.

Do species alternate from selfing to outcrossing?

They don't seem to be able to. When you take worms from different selfing lineages and cross them together, their offspring are sicker. If this happens in other species, it could explain why, once selfing occurs, it rarely moves into outcrossing, even though outcrossing may be more beneficial for the species.