A new study in worms may help explain why sexual reproduction with a partner beats going it alone, evolutionarily speaking of course.
Through a three-year experiment, University professor Patrick Phillips has answered the question of why two sexes exist and why they are so important to reproducing and adaptation.
The answer? Sexual reproduction as opposed to asexual reproduction leads to decreased susceptibility to genetic mutations.
Although the answer might seem obvious, scientists behind the study admit, Phillips believes this to be one of the biggest evolutionary studies done to date.
University student Michelle Parmenter, a lab technician in Phillips’ lab, joined graduate teaching fellow Levi Morran in answering an idea of evolution that the lab had been working on for several years.
Phillips says that questions on sexual reproduction have been around for more than 200 years, and that his work as the director of the Center for Ecology and Evolutionary Biology has focused on the features of the mating system for some time. Parmenter and Morran conducted 100 mini-evolution experiments involving nematode worms and putting them through a series of mutations to determine their adaptability and survival rate in harsh environments, the study was published on a early release date in the journal Nature on Oct. 21.
The worm is the Caenorhabditis elegans — or C. elegans — a small (about 1 mm long) soil nematode. One benefit Phillips found to using this worm in their study is their four-day generation life, allowing Morran and Parmenter to test over 60 generations of the worm in a contained evolutionary process.
Morran separated the worms into three categories, two of which underwent
mutation to prevent certain sexual reproduction processes. The XOL is a population of hermaphrodite worms that produce both sperm and eggs and were mutated to reproduce with themselves — selfing; the Wild Type, a natural collection of the C. elegans worms that were untouched and comprised of 25 percent males; and the FOG population that was mutated to only reproduce with a partner.
Morran said that to genetically mutate the XOL population he had to expose the population to a harmful bacterial pathogen that eats the worms from the inside out, killing all male specimens, leaving the hermaphrodites alone to reproduce.
The outcrossing population, which Parmener says is just a fancy word for sexual reproduction with a partner, resulted in both higher levels of adaptability to harmful environments and were less susceptible to accumulating harmful mutations.
“By having sex with a male partner you are able to escape the harmful effects of
mutations,” Morran said.
Phillips says that previous theories have suggested that selfing populations are able to purge many of these mutations, but this study found that the ability to sufficiently purge was overwhelmed by slight increases in mutation rates.
With careful study, Phillips and his team found that selfing populations lead to fixation in adapting where any new adaptive mutations will tend to become trapped within selfing generations, and they could not respond to evolutionary mutations like those of the
outcrossing populations.
“We didn’t know exactly what the outcome would be,” Phillips said. “We expected organisms that could, to have sex alone, and thought that outcrossing worms were at a huge disadvantage in that selfing populations vastly out produced them.”
Though the results were somewhat predicted, all three did not expect the outcrossing populations to produce such drastically good results.
“On a graph it would look like a linear slope,” Parmenter said. “The selfing worms had no adaptations, the wild type had a good amount, but the outcrossing worms had a substantially better adaptation to harmful pathogens.”
To Phillips, the results verify what scientists have thought for over thirty years, and often
receive a “well, duh” reaction from friends and faculty.
“When I told my mother the results she simply said, ‘Well, of course,’” Phillips said. “This answers a big question that we all have pre-conceived thoughts on.”
While the experiments have a serious scientific impact it doesn’t stop Morran and Parmenter from receiving raised eyebrows when they explain what they have been up for the past three years.
“There seems to be a general intelligence on this subject,” Morran said. “But our sexual habits have definitely have been under fire.”
“It is a funny thing to explain to people,” Parmenter said.
Morran and Parmenter are currently studying the adaptation of the outcrossing populations in the C. elegans, and trying to understand what happens in the worm’s genome that allowed them to adapt so efficiently.


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