Sexual reproduction protects species from harmful parasites, extinction, IU researchers say

As common as birds, bees and clichés, sex is everywhere in nature.

But it’s hardly necessary. In fact, scientists say asexual reproduction is a more efficient way of expanding a population, as there’s no need to find a partner.

So why, then, is sexual reproduction so prevalent?

Biologists are now closer to that answer, thanks to evidence found by researchers at the IU Department of Biology.

Sexual reproduction as we know it may have started as a way to avoid infection and death from parasites — infections that species enjoying asexual reproduction are particularly susceptible to.

“It’s been a fairly major problem for evolutionary biology,” said Levi Morran, lead author of the report that presented the research. “There should be a lot more examples of self-fertilization and asexual reproduction out there than there is. This brings us a step closer to understanding why that is.”

With offspring of asexual reproduction, the gene pool is considerably more limited, Morran said.

This limitation means if a parasite can infect one organism, it can infect an
entire species. Sexual reproduction may be a result of combatting this, allowing parents to produce offspring more resistant to parasites.

This would save a species from extinction.
 
The findings affirm an evolutionary theory known as the Red Queen Hypotheses, which suggests that sexual reproduction through cross-fertilization helps species stay a step ahead of the parasites that would infect them.

The name of the theory comes from the villainous monarch in Lewis Carroll’s “Through the Looking Glass,” the sequel to “Alice in Wonderland.”

“Now, here, you see, it takes all the running you can do to keep in the same place,” the Red Queen said to Alice during a foot race in the novel.

In that same regard, hosts and parasites are constantly evolving as fast as they can without really getting anywhere, Morran said.

“Basically, the theory is that hosts and pathogens are constantly locked in this evolutionary arms race,” he said. “Pathogens are constantly evolving so they can infect hosts. And hosts are constantly evolving so they can evade pathogens.”

Based on this theory, Morran said he was involved in an experiment in Oregon as a graduate student that formed the basis of the
IU study. There, he studied how worms that reproduce sexually under changing conditions evolve at a greater rate than those who use self-fertilization.

But what were those conditions in nature?

At IU, he partnered with three undergraduate students in the department to figure the problem out.

In this study, the researchers used a microscopic roundworm called Caenorhabditis elegans as a host and a pathogenic bacteria called Serratia marcescens to create a host-parasite co-evolutionary system in a controlled
environment.

They conducted more than 70 evolution experiments to test the Red Queen Hypothesis, altering the mating system of the worms.

Now, different populations of the worms mated sexually, while others mated through self-fertilization.

Some experiments used a mixture of both within the same population. The researchers then exposed the roundworms to the bacteria.

The parasites were either allowed to co-evolve with the worms or were prevented from evolving. Through this, the researchers then determined which system gave populations an evolutionary advantage.

When the bacteria did not co-evolve with the worms, self-fertilization evolved as the preferred form of reproduction. When the bacteria were allowed to co-evolve with the worms, sex became the dominant form.

Morran said the findings could be particularly significant in the fields of medicine, agriculture and conservation.

“Sex is important because it creates a situation where genetic variation can happen,” Morran said. “And genetic variation is very important.”