Scent of a maggot

06 February 2014

Interview with

Matthew Cobb, University of Manchester

Kat - It's not just mammals like humans and dogs that need to sniff out the world around us. At the University of Manchester, Professor Matthew Cobb and his team are studying how maggots smell things. But what does a maggot need to smell anyway?

Matthew -   If you are studying houseflies, then they are probably going to want to smell things like ammonia that are from decaying meet or poo and stuff like that.  I study fruit flies and so the clue's in the name, they're interested in decaying fruits. So they are very sensitive to things like alcohols, which are produced by the yeast that's living on the fruit and to the various compounds you get coming off for both decaying and living fruit.

Kat -   And what do we know so far about how maggots actually do smell things?

Matthew -   Well, we know in principle their sense of smell is like ours and that each of their smell cells has got just one kind of receptor on it.  We would generally argue that the sense of smell is the oldest sense evolutionarily, so it goes way, way back to when we were just blobs of cells, small multi-cellular organisms swimming around the sea.  Organisms would need to be able to detect chemical gradients to need to be able to orient themselves, to detect prey or to detect predators. 

Now, we know that much and then it all starts to get very hand-wavy and people aren't quite so sure about how it exactly works.  What we know is that each smell can activate more than one kind of receptor.

Kat -   And there were loads of different smell receptors, aren't there, in many species.

Matthew -   The reason why we use maggots is they only got 21.  They've got 21 smell cells.  You've got about 4 million smell cells, maggots got 21.  A dog, despite what people think about dogs having very, very good sense of smell in terms of the number of cells is very, very similar.  The number of kinds of cells is very, very similar - about 400, 500.  A chicken has got about the same.  The question is how sensitive is our cells and how many copies of each kind of cell do you have.  My maggots have got just 21 and each of those cells is unique - each one is different. 

So you've got a very, very low level of organization.  They're not terribly sensitive to the amount of smell. They're very poor at smelling because they've just got one kind of cell that's able to detect each range of odour.  So, each cell can detect more than one odour and each other can be detected by more than one kind of cell.  So you've got this complicated kind of interaction between a number of cells and a number of odours, which means that each odour will produce a specific pattern of activity in the neurons in the smell cells and then in the brain where it's actually start - gets turned into reception.

Kat -   How do you go about in the lab testing what your maggots are attracted to, what they can smell and then what you can find out about their smelling system?

Matthew -   We've got two ways of doing it.  The simple way, which is the way I like because it's easy, is you get a load of maggots, you put them on a plate of agar, which is a hard jelly.  You put the maggots in the middle.  You put a smell on one side.  You put the lid on.  And you wait five minutes.  And you see where they've gone because the maggots are going to wriggle about.  And if they like the smell, they'll go towards it and you can simply count how many are on each side and you come up with a little index.  So that's the simple way, simply to watch them. 

The complicated way and incredibly delicate way is to get an electrode and put it into the maggot's nose.  These very, very thin glass electrodes are put inside the maggot's nose and then you can blow a smell over it and you get an electrical signal, which corresponds to what that particular cell is doing.  And because we're studying Drosophila, the maggots we're looking at are Drosophila, which is a geneticist's friend and really helps to establish the science of genetics in the 20th century.  You can do all sorts of very strange things with them, so we can make a maggot which has no smell cells at all, no functioning smell cells and then by using the power of Drosophila genetics, it's relatively easy to allow just one particular kind of the - one of the 21 cells to express all the things that it needs and then to work. 

So we end up with a system, which has got one cell working and 20 other cells that don't respond to any odour at all.  So we can then, one see what the electrophysiological response is of what the neuron is actually doing and then get some insight into what the brain thinks that means.  So for example, often, if you got a receptor that has got a very broad spectrum, so this is a neuron that can be activated by lots of different odours, if you blow an odour over that receptor, you get a very strong electrical signal.  But when you put that maggot onto our little arena, and you give it the smell, it's not actually interested.  It doesn't do anything. 

So, you're getting a very strong signal in the brain, the brain can "see something", but the maggot doesn't show any behavioural response.  It must have in its brain and therefore ultimately in its genes, a pattern of activity that it expects to have meaning.  And if we can fool it by giving it something that has no biological meaning then it - well, if it had shoulders, it would shrug them and it just potters about.

Kat -   Why the maggots actually need to smell?  Because you'd think they'd be laid onto a piece of rotting banana by the fruit fly.  Why do they actually need to smell anything?  They don't go that far.

Matthew -   Well, we don't know.  A simple answer, we don't know.  The vague explanation we have is well, what if they fell off?  So, exactly as you say a maggot is laid as an egg on to a piece of rotting fruit or whatever it lives on.  Why would they want to be able to smell?  But what if they fell off the fruit, how do they find their way back?  I have no idea if that's actually real.  We know that maggots that don't have a sense of smell are much less likely to survive, which is kind of obvious because they can wander off the food and then they can't find their way back. 

To be more serious, perhaps the reason why a maggot has a sense of smell is that the adult has a sense of smell.  So, one of the things that we're interested in is whether a fly can remember what it knew when it was a maggot.  And there've been a number of contradictory results about this.  In ecological terms it's known that many insects will either go on to the food on which they were reared or in some cases, in the case of parasitoid wasps, will avoid the food upon which they were reared.  So, there's plenty of evidence that there can be some kind of transfer of information. 

Where that information is encoded is a big debate.  It may be in the brain.  It may be in the outside of the pupa that the insect forms before it turns into an adult insect.  But there is certainly a lot of ecological evidence that insects are actually remembering in some way what they knew as a maggot, so maybe that's why maggots have a sense of smell because flies need to have a sense of smell and they need to be able recall stuff that they've learned at an earlier stage.

Kat -   That was Professor Matthew Cobb from the University of Manchester. 

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