Sex evolved to prevent parasite infections, say scientists

Sexual reproduction that involves two partners is far less efficient than self-fertilization — at least, from the perspective of evolution. So why did creatures like humans ever start having sex with each other? According to a new study, we did it to fight parasites. We spoke with the researchers to discover what this says about sex.

Biologists at the University of Indiana found some of the most convincing evidence yet that the evolutionary driver of sexual reproduction is a need to avoid death by parasites. The basic logic is that, if an organism reproduces asexually, then the genetic variation of its species as a whole will slowly grind to a halt, and it becomes increasingly likely that a parasite that can kill one member of the species can wreak havoc on the entire population. (For proof of that, just look at bananas.)

Sexual reproduction, then, serves as a way to keep introducing genetic variety, a process that has to constantly be repeated in order to continue staving off attacks the latest and deadliest parasites. This is known as the "Red Queen Hypothesis", taking its name from a line in Lewis Carroll's Through the Looking Glass in which, "It takes all the running you can do, to keep in the same place."

To understand what this all means, we spoke with the lead author of the new study, Indiana post-doctoral researcher Levi Morran. He explained to us what drew him to this subject, why self-fertilization only seems like it's the more efficient method of reproduction, how their experiment worked, and why he finally might have found some evolutionary justification for the existence of the male gender!

io9: What is your background? What made you interested in this topic?

LM: I graduated from Indiana University with my BS in biology in 2004 and from the University of Oregon with my PhD in evolutionary genetics in 2009. Now I'm back at Indiana University working as an NIH postdoctoral fellow in Curt Lively's lab, which is also where I worked as an undergraduate in 2003 and 2004.

As for being interested in the topic of sex, who isn't? I think sex is interesting to most people from both a recreational and biological perspective. Most people seem to be curious about their origin and everyone is the product of sexual reproduction, so its relevant to almost everyone on some level.

I became interested in researching the evolution of sex while in grad school working with Dr. Patrick Phillips at the University of Oregon. We were studying the nematode Caenorhabditis elegans, which can self-fertilize but also keeps some males around to have sex. We read a paper that said males in this species were just evolutionary relics. I just didn't buy that explanation for the existence of males in the species and decided I would try to figure out why they kept males around. Being a male myself, I think deep down I wanted some evolutionary justification for the existence of males.

Why does self-fertilization appear to be so much more efficient than sexual reproduction?

Imagine a population in which mothers can produce daughters that can self-fertilize and produce offspring all by themselves, or they can produce males that need to have sex with another individual to be able to reproduce. A mother under natural selection is more successful when she produces all daughters in this case, because producing all daughters gives a mother more grandchildren than another mother that wastes time and energy producing sons that will not directly give her any grandchildren. If we assume that the offspring produced by self-fertilization and sex are equal in terms of their ability to survive and have the capability to reproduce, then self-fertilization is a much better means of reproduction. But, therein lies the catch, if sex produces offspring that are better able to survive and reproduce, then the advantage of self-fertilization erodes away. Therefore, to solve the riddle of the evolution of sex, researches have tried to find instances when sex will produce better offspring than self-fertilization (or asexual reproduction).

How does the Red Queen hypothesis speak to these issues? Are there any other major theories put forward to account for the preponderance of sexual reproduction?

There are a multitude of theories put forth to explain the evolution and prevalence of sex. They all essentially identify cases in which sex produces offspring that are more fit than offspring produced by self-fertilization or asexual reproduction. These theories generally fall into two broad categories, either deleterious mutations or ecological factors. The idea behind the deleterious mutation theories is that sex may be useful for managing genomic errors, while the ecology based theories identify various environmental factors that may select for sex. The Red Queen hypothesis falls into the ecology group.

How did you go about demonstrating the hypothesis? How was this different from previous attempts by other researchers?

First of all, we did not demonstrate every aspect of the Red Queen hypothesis. The Red Queen makes several predictions about the underlying genetics of host-parasite coevolutionary dynamics, we have not yet tested those predictions in our system. But, we did test a key component of the Red Queen hypothesis, that coevolving parasites favor the maintenance of sex. We were able to conduct a controlled test showing that exposure to coevolving parasites led to extinction of populations that could only self-fertilize, while populations that could have sex were able to survive and even adapt to the coevolving parasites. This was a step forward in testing the Red Queen because we were also able to show that self-fertilization was favored by natural selection when no parasites were present and when parasites were present but not coevolving with the hosts. So, we were able to isolate coevolving parasites as a factor that maintain high levels of sex in populations that can either have sex or self-fertilize.

What do your results say about our broader evolutionary history, including that of humans?

Our current results confirm the Red Queen hypothesis prediction that coevolving parasites can select for sex over selfing, and do so on a long-term basis. Thanks to many other studies, we know that parasites are associated the existence of sex in nature and that host-parasite coevolution is capable of driving rapid evolutionary change and is a common phenomenon in nature. Taken together, these results indicate that the Red Queen hypothesis is a plausible explanation for the evolution and maintenance of sex. However, there are many possible explanations for the evolution of sex, so it is premature to attribute the evolution of sex in a broad range of species to host-parasite coevolution.

What are the main takeaway points from this research?

First and foremost, genetic variation within and among individuals is very important from an evolutionary perspective. Sex is a successful strategy against coevolving parasites because it mixes up the genome. But, a lack of genetic mixing can rapidly lead to extinction. As stewards of our environment, we need to be careful to maintain large and genetically diverse populations of plants and animals in both nature and agriculture to ensure that those species continue to exist.

Also, as odd as it may seem, it probably wouldn't hurt to thank a parasite for the existence of sex.

Read the scientific paper at Science. Image via.