Science Interviews


Sun, 7th Oct 2007

Hot, Smelly Sex - In Primative Plants, that is.

Irene Terry, Univeristy of Utah

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Chris -   Now also in the news this week is a story about hot, smelly sex. Well, not in humans but in primitive plants.  Now Irene Terry is at the University of Utah and she’s found that there is a special adaptation in cycad plants that forces insects to move from a male plant to a female one during pollination.  They do it by getting very hot and then releasing a very powerful, even toxic smell. She joins us now from Utah. Hello Irene.

Irene -   Hello!

Chris -   Thank you for joining us on the Naked Scientists.  So tell us about these plants, what actually are they?

Irene -   They’re very primitive plants.  Their lineage goes back well before the dinosaurs, called the Permian.  That’s when the fossils have been found.  And so they made their heyday during the Mesozoic during the age of the dinosaurs.  It’s also gone through a lot of bottlenecks, a lot of other extinctions between the cretaceous and the modern period.

Chris -   So what was the big unknown with this plant, then?

A Cycad cone & the Thrips which fertilise themIrene -   For a while we thought these plants were wind pollinated but many researchers in the 1980s discovered there were beetles that were associated with the pollination.  On sabbatical in 1999 I worked with a taxonomist who used to be the head of the British Museum actually, living in Canberra now, in Australia.  I worked with him on some small insects that he found on male cycad cones.  He had seen them go to female cones but he hadn’t done definitive studies to show they were actually pollinating.

Chris -   So why is this such a big question?  It stands to reason that if you’ve got a cone which attracts these beetles or these insects, why should there be a problem of them migrating between the male and female parts of the plant?

Irene -   Well because most of the insects that are associated with these live only on the male cone.  They lay their eggs and the larvae develop on these male cones.  The question arises, why would they want to leave this nice home where they can mate, lay eggs and complete their life cycle?  The male plants have to get rid of these insects to spread their pollen to the female cone.  That’s where the question of, ‘how did they do that?’ came.  With the system that I’ve been working with in Australia, the cones start to heat up and start producing these really strong odours.  If I’m standing over one of these cones it almost makes me choke because it becomes so strong.

Chris -   When you say the cones warm up, how much warmer do they get?

Irene -   We’ve measured them up to 12 degrees Celsius above the ambient temperature.

Chris -   That’s a lot!  This is not just the sun warming them up?

Irene -   No, it’s not just the sun.  We’ve actually put some little tents over them so it isn’t the sun.  We’ve measured them that way.

Chris -   So how are they doing that?

Irene -   They have a process whereby they’re burning a lot of the sugars and starches that they’ve stored within the cone to increase their metabolism.  Instead of making certain cellular compounds that run the cellular functions, they’re losing a lot of that energy in heat.

Chris -   What effect does this have on the cone when they get hot like that?

Irene -   It makes the insects actually more active so we know the heat is part of the process.  In addition there’s this chemistry that’s taking place where they’re ramping up a lot of the chemistry that becomes very noxious to them.  Actually in studies where we’ve tried to look at the behaviour of the insects, just with the chemistry alone, it can actually kill them if they’re stuck inside.

Chris -   So the cones are getting very, very warm and this is causing them to produce lots of these smelly chemicals which drive the insects away…

Irene -   Yes

Chris -   So then what happens?

Irene -   Once they get to the outside of the cone all of a sudden they begin to take off in flight and they drift around in the air for a while.  After these cones heat up later in the day they begin to cool off.  The odours diminish quite a bit.  So the insects are attracted back to the cones.  Some of those may be males and the females, as well, have the same odours.

Chris -   So the male cones and the female cones look very alike so the insects come back to both inevitably?

Irene -   We think it’s mostly the odours they’re attracted to, we think there are some visual cues but we haven’t actually been able to isolate them.

Chris -   And the insects are presumably still covered in pollen from the male cone previously so they go into the female cone and pollinate it?

Irene -   Yes, that’s exactly right.  That’s basically how it works.

Chris -   Evolution’s an amazing thing, isn’t it?

Irene -   It is and through natural history I think we have a lot more to learn out there: just by making observations and then doing some follow-up studies.

Chris -   Irene, thank you very much.

Irene -   OK, Thank you!

Chris -   That’s Irene Terry, she’s from the University of Utah – proving that the world of plants and sex is often a very hot topic!


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