Winters residents in sex experiments by Richard Cowen

I found out recently that some Winters residents had been taking part in sex experiments in government-sponsored research labs, all expenses paid! My first reaction was jealousy that I had not managed to get on the list. My next reaction was that this was a great subject for investigative reporting. And now I'm glad I wasn't one of the chosen.

These experimental subjects were Winters fruit flies: the pesky little flies that gather round ripe fruit. They're not harmful, and they are not really a nuisance unless there are a lot of them. But clearly, there are so many of them that they must lead a fairly active sex life.

It turns out that this is rather exotic. Fruit flies do intricate mating dances and give off enticing scents to attract mates: rather like the senior prom. But when it comes to actual sex, habits vary in different cultures in the fruit fly world. Some fruit flies are reasonably monogamous, but others are rather, well, promiscuous. And just as a hell-fire and brimstone religious tract promises everlasting doom for promiscuous humans, promiscuity has its dark side for fruit flies as well.

Unlike humans, fruit fly females can store sperm for quite a while, and this sets up all kinds of trouble. If a female fruit fly has only one mate, there's no problem. She finds a partner with good genes (the best dancer with the best scent, a large one preferably, with wings of exactly the right shape and size), and flies off. When she finds a perfect ripe apricot that will feed lots of baby fruit fly maggots, she uses some of his sperm, fertilizes some of her eggs, and lays them. Then she flies off to find another good fruit.

But what about flirty females that mate with many partners? They pose a problem for a fruit fly male. Any partner he chooses is likely to be faithless. If his sperm is going to count, he has to do several things. He has to replace or kill any sperm she is already carrying. He has to stop her from mating again, or if she does, he has to try to poison any new sperm she may collect. And it would be good to persuade her to lay lots of eggs soon, while she is still carrying his sperm. Males in promiscuous fruit fly cultures have evolved clever ways to do all this.

Fruit fly sperm comes in a fluid glop that contains various proteins. In promiscuous fruit flies, males have evolved special versions of these proteins that have subtle effects on a female. Some of the proteins disable any stranger's sperm they come across: that takes care of the first problem. Some of the proteins are absorbed into the female's body, and turn off her libido, so that she loses interest in mating again. This solves the second problem. At the same time, the male's proteins kick the female's egg-laying hormones into high gear, solving problem three.

All this works well for the male fruit fly. But all this chemical warfare that goes on in the female's body also shortens her life, in ways that we don't yet understand. This doesn't matter to the male. The female is persuaded to lay lots of eggs that he has fertilized, and if she dies after that, why should he care? He simply flies off and works his chemical warfare on another female.

Is there no defense for the female? Yes: evolution works both ways. Fruit fly females in promiscuous cultures have evolved special proteins of their own that sit in their reproductive tracts. These female proteins resist the toxic chemicals in male sperm, so that female fruit flies can mate with as many partners as they like, without fear of toxic side effects.

Until, over time, the males come up with a new protein that messes with female hormones, and then females evolve yet another antidote, and so on...

In some places, the fruit flies are flirters, and the whole chemical warfare I've just described goes on all the time. But in places where the fruit flies are faithful partners, none of it does. A male fruit fly does better if his faithful mate is not harmed by his sperm, so that she will live a long time and lay lots of eggs, all fertilized by his sperm. Faithful males don't deliver toxic proteins with their sperm, so faithful females don't need to evolve defensive proteins.

This is exciting research for fruit fly scientists. They study fruit flies because they breed fast and live happily in laboratories. But how many other animals have subtle interactions like these? We don't know (yet), and it is going to be difficult to find out. Is human sperm toxic to females, for example? Nuns tend to live a long time, but many other factors may be involved with that. Humans are, on the whole, relatively monogamous, so in theory there is no advantage in evolving toxic sperm.

But what about our Winters fruit flies? Wouldn't you like to know whether you are swatting good faithful partners or fickle toxic fornicators? In the experiments I read about, scientists studied New York fruit flies, Texas fruit flies, and our very own, from Winters, California. Your intrepid reporter tracked down and asked the Winters fruit fly collector, Professor David Begun of UC Davis. Would you like to guess the astonishing answer?

THEY DON'T KNOW! (and they don't care)! Fruit fly scientists are a little strange. They don't care what they start with: they simply collect them and play evolutionary games with them. They allow flies to mate promiscuously, and the flies evolve toxic sperm and proteins. They restrict flies to single partners, and the flies evolve friendly sperm. They don't care where the flies were collected, or whether they were originally faithful or not.

So we have to remain ignorant for the moment about the ethics of our local little fruit flies, until someone cares enough to find out. The answer is in the DNA dripping from your fly-swatter.

Written July 24, 2002