New & Noteworthy

The Benefits of Sex

March 3, 2016


It is a good thing for lion-kind that these cubs weren’t budded off their mom asexually! Image from Stephanie Cornell via Pinterest.

While you doggedly swipe right and left or wait night after night at that club, you may be wondering whether it is all worth it. Biologists have been wondering something similar.

Now they haven’t been wondering about the value of sex…since everything from amoebas to zebras has sex, it must be pretty important. No, the hard part has been figuring out why it is so beneficial.

On balance it can seem that the minuses of disease risk and passing on only half of your DNA outweighs the benefits of the combining two individual sets of DNA for some brand new combination. A new study by McDonald and coworkers in Nature using our old friend S. cerevisiae provides compelling evidence for a couple of ways that sex is good for a species.

First it is a way of combining individual beneficial mutations into a single individual. Now rather than having a couple of well adapted individuals battling for supremacy, the mutations can merge into one super beast that can outcompete everyone else.

This benefit, recombination speeds adaptation by eliminating competition among beneficial mutations, had been predicted and goes by the name of the Fisher-Muller effect. But this is the first time scientists have actually seen it playing out at the DNA level.

The second big benefit of sex is freeing good mutations from a bad genetic background. Now the beneficial mutation is not weighed down by other negative mutations. It’s like finally getting rid of that concrete block tied around your ankle.

Yeast is an ideal system for studying the benefits of sex because it can happily exist as a sexual or asexual creature. This means that researchers can directly compare the two in the same experiment. Which is just what McDonald and coworkers did.

They followed 6 sexual and 12 asexual populations through about 1000 generations of adaptation. The only difference between the asexual and sexual populations was, as you might have guessed, sex.

The sexual populations included 11 bouts of sex. In other words, every 90 generations or so, an ‘alpha’ cell would swipe left and find an ‘a’ cell to hook up with.

As expected and has been seen before, the sexual populations were much better adapted to their environment than were the asexual populations. Sex is clearly a good thing! The next step was to tally up the mutations in each population to try to figure out why.

What McDonald and coworkers found was that there wasn’t a lot of difference in the mutations that crop up in each. Over time, both groups had about the same number and ratio of intergenic, synonymous, and nonsynonymous mutations.

The big difference between the asexual and the sexual populations was in the mutations that became fixed. In the sexual group, most mutations were weeded out over time. In their experiment, 78% of mutations became fixed in the asexual population while only 16% hung around in the sexual population.

Even the birds and the bees do it! Image from blog.beeminder.com.

Sheer numbers wasn’t the only difference between the two either. The kinds of mutations that became fixed differed significantly in both as well.

In the asexual population, each of the three kinds of mutations fixed at around the same rate. Around 75-80% of intergenic, synonymous and nonsynonymous mutations became established in this population.

It was a different story in the sexual population. Here, 22% of the nonsynonymous, 11% of the intergenic and none of the synonymous mutations became fixed. It seems like only mutations that make a difference end up getting selected for.

Further analysis revealed two big reasons why the two populations differed. First, good mutations ended up getting stuck with other bad mutations in the asexual population. This blunted the positive effects of the beneficial mutation.

And second, the various good mutations tended to be spread out among different groups in the asexual population. The end result was that instead of working together, these groups battled each other for supremacy resulting in some beneficial mutations being lost.

So no need to wonder anymore about the benefits of sex to a species. It is a strong purifier, weeding out unimportant or damaging mutations and a powerful aggregator, squirrelling all the good ones into one group. No wonder most every beast does it!

by Barry Starr, Ph.D., Director of Outreach Activities, Stanford Genetics