Do we really need two to tango?

It’s not easy finding an adequate male for reproduction. He needs to be manly, but not macho. He needs to be funny, but not immature. He needs to be romantic, but not needy. Ever wonder why we bother to go through the whole finding-a-mate dance when some species can just self-fertilize? Many animals and plants reproduce like us by outcrossing (which means two parents), but there are also a number of selfing species (meaning self-fertilizing or asexual reproduction). Oddly, when you look at the numbers, outcrossing doesn’t make a lot of sense. When selfing organisms (for example, aphids) reproduce, 100% of the offsprings can make more offsprings. When outcrossing organisms reproduce, we end up producing a variable proportion of those pesky boys (50% in the case of humans), who really are no good when it comes to having babies (except maybe for this guy). For many years, scientists have been speculating as to the evolutionary benefit of this numerical disadvantage. Recently, researchers tackled the question in a new way: by recreating evolution experimentally.

First, an important term to define: evolution. For the sake of this post, let’s use a simple definition: evolution is the change in the genetic material (genes, made from DNA) of a population of organisms from one generation to the next. Variations in the genetic material can occur in a few different ways, but a main one is mutations. Mutations can arise due to different factors: for example, a mistake can be made when the DNA is being copied during cell division, or the DNA can be damaged due to exposure to radiation or chemicals.

The study, published recently in the journal Nature, looks at a type of worm, C. elegans. Populations of this worm are composed of males and hermaphrodites, meaning this worm can reproduce both by selfing (hermaphrodites) or by outcrossing with males. The researchers were able to genetically engineer these worms to make two different populations: one that is only able to reproduce by selfing, and one that is only able to reproduce by outcrossing. This created a very valuable tool to look at how these populations deal with various evolutionary hurdles.

The researchers took both populations of worms (the selfing worms and the outcrossing worms) and exposed them to a chemical that increases the rate of mutations (a way to mimic a “sped up” evolution). They also created an environment where each population, in order to reach their food, needs to go over a worm-scale obstacle course. These two steps were important because they both impose a strong selection. Once the experiment was set-up, all the researchers did was let the worms reproduce through 50 generations and looked at which population did better.

Male readers, you’re safe! Even with all the hurdles, the outcrossing population of worms managed to maintain their fitness (or their evolutionary health) over the course of the experiment. The selfing populations of worms, however, showed a significant decline in fitness. To make sure this effect was not just a fluke, the researchers tried a different hurdle: they exposed both populations of worms to a disease-inducing bacteria. Initially, this bacteria caused an 80% mortality rate in both the outcrossing and the selfing worms. This means that the worms quickly had to learn to either avoid the bacteria or become resistant to it. This experiment confirmed what the researchers saw previously: the outcrossing population adapted rapidly to the bacteria and showed a significant increase in fitness over 40 generations, the selfing population did not manage to adapt.

This experiment may seem like a no-brainer (if we didn’t need males, they probably wouldn’t be around anymore, so they must be useful for *something*), it represents the first experimental test of the selective pressures that favor the evolution and maintenance of outcrossing. By digging into the genetics of the worms, the researchers were able to come up with two explanations for the usefulness of outcrossing. The first explanation is that outcrossing reduces the effect of harmful mutations. For example, if part of my DNA is damaged, it can be compensated for in my children if my partner’s DNA is intact. If I wasn’t mixing my DNA with someone else’s, my offspring would inevitably inherit my defective DNA, and this would weaken the population. The second reason is that in selfing organisms, mutations (good or bad) are trapped in a single genetic background. This means that a beneficial mutation can never combine with another that may have occurred in a different genetic background. Therefore, beneficial mutations can never add up or even synergize. This results in stalling evolutionary fitness.

So while it’s sometimes hard to find Mr. Dreamy, it seems like in the long run, it’s worth it.

Meet C. elegans, evolutionary tool extraordinaire

Reference: Mutation load and rapid adaptation favour outcrossing over self-fertilization. (2009) Morran LT, Parmenter MD, Phillips PC. Nature, 462(7271):350-2.