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reed magazine logoMarch 2010

Sex, Worms, and Darwin



Birds do it. Bees do it. At first blush, sexual reproduction seems like such a ubiquitous fact of life on earth that its prevalence can be taken for granted. To evolutionary biologists, however, this plenteous procreation is a bit of a puzzle, because of an awkward paradox known as “the twofold cost of males.”

Patrick Phillips

Patrick Phillips in his University of Oregon labratory.

Consider two species. One consists of “outcrossers,” that is, males and females who reproduce by methods familiar to many readers. The second consists of “selfers,” or hermaphrodites who replicate all by themselves without the fuss and bother of finding a mate. All other things being equal, selfers should quickly overwhelm outcrossers, because in selfing species, all individuals produce offspring, whereas among outcrossers, half the population lounges around watching TV and scarfing down the food supply. (In mathematical terms, selfers should outnumber outcrossers by 2n-1 to 1 after n generations, assuming equal proportion of males and females.)

So why isn’t selfing more common?

“It’s a profound evolutionary problem,” says Patrick Phillips ’86, who directs the Center for Ecology and Evolutionary Biology at the University of Oregon. “In theory, selfing species should take over the world.”

To explore the puzzle, Patrick turned to a remarkable worm, Caenorhabditis elegans, that can reproduce either by outcrossing or—when the mood is right and neither end has a headache—by selfing.

Using genetic manipulation, Patrick’s research team at UO put a stop to this decadent gender-bending, creating a colony of outcrossers and a colony of selfers, and then subjected the worms to various indignities to test their fitness. In one case, the worms were exposed to a chemical that dramatically increased their likelihood of genetic mutations, and then forced to work harder to find food. In another, the worms were infected with a virulent bacterial pathogen with a high mortality rate.

Caenorhabditis elegans

Those remarkable worms, Caenorhabditis elegans

After 50 generations, the two groups showed pronounced differences: the outcrossers had adapted to their new environment, evolved resistance to the bacteria, and gave every indication of being a happy, healthy, writhing mass of worms. The selfers, on the other hand, were substantially weaker, were less able to compete for food, and generally lacked backbone. In the most extreme case, their overall fitness had declined by 80 percent. “They were headed for extinction,” says Patrick.

The fundamental problem with selfing, it turns out, is that there is no way to combine advantageous mutations. One lineage may develop resistance to a pathogen, and another may develop resistance to cold, but the two lineages cannot create offspring that share these attributes. By contrast, outcrossing is an excellent mechanism both for purging harmful mutations and for sharing beneficial ones; over time, new generations tend to inherit all the strengths of older ones.

These results, published last year in the journal Nature, prompted a flurry of amusing headlines (Oregonian: “Why Bother with Sex?” Daily Emerald: “Why Mating Matters.” eSarcasm: “Scientists: Please Have Sex with Others.”), and some lively email (see below). More important, they also provided the first real scientific demonstration that males, despite certain inherent design weaknesses, perform a vital role in the propagation of species.

“As a male, it’s reassuring to find out you have a useful biological function,” chuckles Patrick, who credits professors such as thesis adviser Bob Kaplan and Steve Stearns in biology and Allen Neuringer in psychology for awakening his interest in the natural world. “Going to Reed was like watching this giant world of ideas come into focus,” he says. Patrick won a Guggenheim fellowship in 2006 and is continuing his research into the genetic basis of complex traits such as sexuality, aging, and longevity.

Further Reading

“Mutation load and rapid adaptation favour outcrossing over self-fertilization,” Levi T. Morran, Michelle D. Parmenter, & Patrick C. Phillips, Nature, 21 Oct 2009.

“Lonely worm seeks mate, C elegans,”

reed magazine logoMarch 2010