Sexual harassment: Cuts down on babies and mutations in flies

This image came up in an image search for “sexual harassment”. I couldn’t resist (sorry).

Hello world. It’s been a while (again) since I’ve posted on here. Since my last post there’s been a hurricane, followed by me moving to New York City. Talk about bad timing. I also started a new job. Again, pretty bad timing.

But now that things have settled down a bit, it’s time to get back to exposing the sex lives of creatures that at least some people have probably never even thought about having sex. Like fruit flies. I’m always amazed when people ask me “you mean they have sex? Like with a penis?!” Of course they do. Because fruit flies (in this case, Drosophila melanogaster) are basically just smaller, even more freeloading versions of ourselves.

Case in point: female flies experience sexual harassment. They haven’t gotten around to inventing their own version of lawyers yet, but they do suffer negative consequences from the harassment. And, strangely, some positive consequences as well.

I am a fruit fly, and I’m here to harass you. (Image via Wikipedia)

Alexei Maklakov and colleagues in Sweden recently performed an experiment to see what effect sexual harassment (ie: constant, unwanted efforts by males to gain sex) had on the mutation rate in the next generation. The results were published online last week in the Proceedings of the Royal Society (see citation below).

Okay, so why look at mutation rate? The rate at which new mutations are introduced into the next generation is–theoretically–constant in a population. Meaning each individual should have the same number of mutations in their sperm or eggs. But in reality, things are much more complicated. Mutation rate can be altered by environmental conditions. Some experiments have shown that “low condition” individuals (of many different species) have higher mutation rates. “Low condition” here means whatever sub-optimal conditions the experimenters have imposed: starvation, bad nutrition, the wrong temperature for that species, already having a bunch of crappy mutations.

That makes sense, kind of. Crappy individuals give rise to even crappier babies. But there are other experiments that show an opposite trend: environmental stress can lead to lower mutation rates. The types of stress that have so far been tested are things like: starvation, too hot/too cold environment (in other words, things that have also been shown to have the opposite result on mutation rate).

These semi-conflicting results lead to two competing theories (they’re only semi-conflicting, because none of these experiments are really identical. There’s always tons of variables that change, unfortunately).

First theory: individuals in poor condition have less resources to devote to fixing their DNA. This one acts like a feedback loop, creating more and more mutations until, presumably, everyone grows two heads and dies.

Second theory: individuals in poor conditions have less resources to devote to making babies, because their using up all their crappy resources to fix their DNA. This makes some sense, because if they can stay alive long enough, without accumulating too many mutations, maybe things will get better and they’ll have enough resources for everything.

So…right, sex. For this experiment, the researchers decided to look at a form of social stress: sexual harassment. They found that females constantly harassed by males for sex had fewer babies overall, but those babies were top-notch. Somehow, all that harassment really forced them to get to work fixing the DNA that was going to be used to make their offspring. Maybe they were motivated to make some sons that wouldn’t grow up into harassing a-holes.

That’s all well and good, but how do you even measure something like that? There are so many things that can make the data difficult to measure, so the researchers went to great lengths to cover their bases.

First, measuring mutation rate is hard. Even when the natural mutation rate is very high, it’s still really really low. There just aren’t that many new mutations every generation–at least not ones you can easily see. So, the researchers decided to measure how effective the females were at repairing mutations after artificially raising the number of mutations.

Making new mutations is easy: you blast the flies with some X-rays. But if you X-rayed the females, you wouldn’t be able to tell whether the resulting mutation rate in the offspring was because of sexual harassment or the stress of mommy having been blasted with X-rays. So, they only blasted the males (giving lots of new mutations to their sperm) and made use of a fun fact of fruit fly biology: females can fix sperm DNA once it’s inside the egg, but before fertilization. Males can’t fix their own sperm’s DNA.

Females that mate to males with X-rayed sperm will lay many more eggs that never become adult flies. With this set up, the experimenters could actually calculate the mutation rate.

Next problem: males that harass females often succeed in getting some. Sex leads to babies. And females that have more babies are more stressed, because they’re using more resources for reproduction–another factor that could affect mutation rate. To separate sexual harassment (and sex) from reproduction, the harassing males were made sterile. Females were either harassed (11 females to 33 sterile males in a vial) or not (44 females in a vial) for 12 days. After that, each female was mated to either a normal or an X-rayed male.

The variables tested in the experiment. The effect of harassment on mutation rate was tested in females who mated to males with mutated sperm. The effect of harassment on the number of eggs laid was tested in females mated to normal males.

While this sounds really complicated, and probably really obnoxious for whoever had to actually set up these experiments, it let the researchers get some pretty sweet data. From females that only mated normal males, they could calculate the effect of harassment on fertility. Turns out, being harassed constantly by horny males makes female fruit flies lay less eggs. This is true, even though by the time the females met their future baby daddy, the harassment had already stopped.

From females that mated to the X-rayed males, they could calculate the effect of harassment on the female’s DNA repair machinery. This was the weirdest result: getting constantly harassed, as I mentioned earlier, amps up the DNA repair, so that the few eggs that do get laid end up turning into higher quality flies with fewer mutations. The difference was pretty big, too: harassment led to about a 35% lethal mutation rate, compared to over 50% when females weren’t harassed.

What does all this mean for your life? Basically nothing. But it is pretty cool to think about how everything around us–including stress–affects even the ability of our cells to fix damaged DNA.

Maklakov AA, Immler S, Løvlie H, Flis I, & Friberg U (2013). The effect of sexual harassment on lethal mutation rate in female Drosophila melanogaster. Proceedings. Biological sciences / The Royal Society, 280 (1750) PMID: 23173200

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