Anti-aphrodisiacs, for flies

Pheromones are chemicals that can alter behaviours like mating. Now researchers have discovered the pheromone equivalent of an anti-aphrodisiac.

National University of Singapore (NUS) scientist Joanne Yew has found that male fruit flies impregnate a female fly with a cocktail of fats, which puts other males off wanting to mate with her. She spoke to Chris Smith...

Joanne -   Pheromones are chemicals that animals use to communicate with each other and we wanted to see what kinds of different chemicals are used by different species of fruit flies.  So, the reason we thought we might be able to see something that people haven't seen before is because of a method we worked out a couple of years ago where we use laser to scan the surface of the insects.  By doing so, we're able to detect different kinds of molecules then the kind of classical methods that are used to identify these molecules.

Chris -   So that laser technique would enable you to study not just what the insect itself is producing, but, potentially, what another insect it's been knocking around with may have left on it?

Joanne -   Yes.  So, by using a really small laser, compared to the size of the insect, we get really good spatial specificity as to where these different molecules are expressed on a surface and also where they get transferred following some kind of behavioural interactions, like mating.

Chris -   And that particular laser technique doesn't just tell you where on the insect.  It will actually give you the chemical identity of the substance, will it?

Joanne -   Well, it gives you a good guess.  So, based on that guess, you know what is the elemental composition, what kinds of different atoms are there and, probably, number of double bonds there.

Chris -   When you started doing this with these fruit flies, what did you see that stood out as clearly different or exceptional compared to what previously people had managed to spot using traditional techniques to study pheromones?

Joanne -   So, we had an expectation of the kinds of molecules we would see based on their mass.  So, we had a laundry list of things to look for; but what we saw that was novel was that, in males only and not females, there were a series of molecules that looked a lot heavier than what had been reported before.  And then, when we looked at things like the elemental composition and number of double bonds, it seem to fit the characteristics of what we would expect from a particular kind of fat.

Chris -   So, these male flies are depositing into the females' fats which appear to be linked to mating, or influencing mating behaviour?

Joanne -   That's right, yes.

Chris -   So, what are these fats and how do they change the behaviour of the female and when does the male put them on the female?

Joanne -   They are transferring a particular kind of fat called a triglyceride.  This is a common kind of fat molecule and the males express it in a region close to their genitals.  And so, when they mate with a female, they're transferring sperm as well as these particular fats.

Chris -   The triglycerides go onto the female, but do they affect behaviour of the female or do they affect behaviour of other males?

Joanne -   We looked at both aspects of that.  Females, after they mate, they exhibit a series of rejection behaviours where they really are not interested in mating anymore for some reason.  And so, the male who tries to mate with them, she will kick them in the head and run away.  And so, we tried to see, if we perfumed the female purely with these triglycerides and not have her mate with the male, would it cause her to do any of these behaviours? But it doesn't seem to be the case.  So, it seems that it's more to tell other males to avoid this female.

Chris -   Let's just run over that.  The female that you coat in this material, in these triglycerides, does not appear to be less receptive to mating, but she is actively avoided by males that could potentially mate with her?

Joanne -   Yes, that's right.

Chris -   Do you know how the males are picking it up or how they're responding to it? And if they do respond like this, why don't they get confused with their own smells and therefore just go into a non-mating state?

Joanne -   Yes, we speculate because its context-dependent and so they're smelling this smell on top of the female's natural scent.  That whole bouquet of the female scent, plus this added male fat, is probably the signal that tells other males to avoid this female.

Chris -   Now, you've looked just in these insects in this first instance, but do you think that this whole concept - for want of a better phrase, an anti-aphrodisiac, do you think this applies in other animals too and dare I say, even in humans?

Joanne -   I think so.  I mean, I know so because there is actually large body of literature showing this phenomenon in other insects.  In humans, this is hard to say, especially in terms of chemical signals, but it benefits both the male and female to have some kind of signal indicating a period where the female is not interested and when males won't be successful.  So, it seems reasonable that it would be a conserved mechanism...