Context-dependent smell in butterflies

Some male butterflies leave a strong smell behind after sex with females to ward off other males...
26 January 2021

Interview with 

Kelsey Byers, John Innes Centre


photo of Heliconius Melpomene butterflies


Now imagine strolling with your significant other through a park, and stopping a moment to gaze upon a beautiful butterfly…. Well I’m afraid that’s where the romance stops. Because news is out this week that male butterflies from a species called Heliconius melpomene, have evolved to produce a strongly scented chemical in their genitals, which they leave behind after sex with females, to deter other males from pursuing their mates. Lovely! And strangely, this same chemical is identical to one produced by plants to actually attract butterflies. So it’s either gross or great, depending on context. Katie Haylor spoke to Kelsey Byers from the John Innes Centre in Norwich, one of the scientists behind the study...

Kelsey - This is a chemical called beta-ocimene, and it smells kind of green, a bit medicinal, a little bit flowery. It's a little hard to describe.

Katie - You've been studying the genetics of this. So what experiments did you do?

Kelsey - We did a couple of different experiments. The first was, we understand a little bit about how this chemical is produced in plants, and also in some types of insects, or how similar chemicals are produced. And so we looked at the biosynthetic pathway to try to identify genes that might be important for the production of this chemical. We also took advantage of the fact that a related species, Heliconius cydno, doesn't produce the chemical, and used a genetic mapping technique to try to identify the genetic origin. So we found some candidate genes, very exciting. And then what we did is we looked at their expression. We're looking for genes that are expressed, as in many copies of the genes are transcribed, at high levels in the male abdomen, which is where the chemical is made. And we found a couple of candidates. When we put those candidates into bacteria, we found that one of those candidates in particular produce the chemical that we're looking for. And so that's how we identified the gene.

Katie - So what does it mean then?

Kelsey - The butterflies produced this chemical, and it's beneficial for male butterflies because it means that the females don't remate, and therefore all of the offspring of that female, probably for most of the rest of her life, will be from that male. It also benefits the females a little bit because it means they stop being harassed by other males, because the other males can smell and go, Oh, she's already mated. She's not going to be interested, but it also might cost the female if she's mated with a less good male. And now she would like to mate with a better male, but that better male is scared off by this chemical compound.

Katie - So is it the case that this same chemical is being made by these insects, but also by plants?

Kelsey - Absolutely. It's actually one of the most common volatiles that's produced by plants. And if you were to analyse the smell of different plant species, you would find this very frequently. So it's a little bit strange. We were curious, if it was produced by the same mechanism in both plants and animals. And it looks like it's actually produced by very different mechanisms between plants and animals. So between the flowering plants and the butterflies.

Katie - So hang on, if this chemical is attractive or repulsive, depending on context, you know, mated with a female or looking for a plant, how do the other males know whether to be grossed out, or think it's great?

Kelsey - The word you said, context, is absolutely the answer. So it depends on the context. If they smelled this chemical in combination with a female, they go: "Oh, the female has been mated." It's also at much higher doses in the butterflies than it is in the flower. So I can smell it a little bit in a flower. But if you have a female butterfly and a net, wow, you can really smell the amount of chemicals. But a lower dose of the chemical in combination with signals that look like a flower, would make the butterfly go: "Oh, this is food. This is great." So it's all about that context.

Katie - And so you're talking about butterflies in the net. Has someone done the behavioural experiments to back this up?

Kelsey - Absolutely. So it turns out that if you paint this compound onto the abdomens of female butterflies, the males will actually avoid them. So we, in this study, we looked at the genetics, but the behavioural effect of this chemical was actually already known.


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