Mimicry: sometimes it’s clever to be a copycat

14 02 2010

To take your mind off this frosty winter weather I want you to think back to the summer, when you were relaxing outside in a pub garden with a crisp, cool pint of beer. A yellow and black winged beastie buzzes its way over to your drink, and you jump up screaming and wildly flailing your arms (yes, you, with the pint). You then realise that it’s only a hoverfly… panic over. Have you ever wondered why a hoverfly, with its yellow and black stripes, looks so much like a bee or a wasp?

What is a mimic?

A mimic is a species that physically resembles another species. Mimicry is rife right across the tree of life. For example, several species of butterfly look remarkably similar to the pipevine swallowtail (below left), and in the aquatic world it is all the rage to look like a cleaner fish (below right). Plants even get in on the act, as insect-eating species produce glistening droplets that look just like nectar, or patterns on their leaves that resemble flowers. And cunning fungi are able to grow parts that look like the pollen grains and pollen tubes of their host plant.

Mimics and their models. Can you tell which species is copying which?

What’s it all about?

Mimics do not appear simply because it’s fun to look like a bee (although clearly it is); their copycat characteristics have come about for very important evolutionary reasons. Animals wear the warning stripes of other creatures that actually have something to back it up, such as a nasty sting or bite, or chemicals that are toxic or distasteful. This way they should get the same protection from predators, who have learnt to steer clear of the bright yellow stripes or bold coloured spots.

Of course, protection from predators is not the only reason to mimic. A fish called the sabertoothed blenny (above) copies the colour and body shape of a cleaner fish, and even dances like it. Other fish are happy to get up close to the cleaner fish as it picks off parasites from their scales and eats them (yum), so the disguise allows the blenny to get in close and take a bite out of its unsuspecting prey.

In the case of the fungi, faking it as a pollen grain gives them unrestricted access to the ovary of their chosen plant, providing an ideal route of infection. And the plants mimics? They use their pretend nectar or flowers to lure in their insect prey.

This phenomenon is called Batesian mimicry. It is named after the English naturalist Henry Walter Bates (right), who came up with his explanation for copycats after observing butterflies in the rainforests of Brazil.

When all copycats are equal.

Brazil is clearly the place to be if you want to have a biological theory named after you. Another adventurer who spent a lot of time there was German biologist Johann Friedrich Theodor Müller (right). He discovered a different type of mimicry called… wait for it… Müllerian mimicry.

He set out to answer a big question: how come there are plenty of species that resemble each other and have distinctive markings, yet which do have the dangerous sting or the crafty lifestyle to back it up? For example, why is yellow and black so popular with poisonous stuff?

Let’s say you start out as a big bunch of species, who all use toxic poison or venom as your defence against predators. It makes sense that you would all evolve the same visible markings to warn of this. If you all use the same (or a similar) system, predators will learn to avoid it whatever type of creature you are, and those of you who match that critical signal more closely are more likely to survive. You survive, you have offspring, and so the cycle continues; this is natural selection at its best.

Back to the humble hoverfly.

So, next summer when your picnic is disturbed by a hoverfly, before you dismiss it as a harmless insect, perhaps you could think of all the evolution it had to go through to get its stripes. And give it a pat on the back for doing such a good impression.





How Super are Superbugs?

2 02 2010

Unless you have been locked in a darkened room for the past decade you’re sure to have heard of the dreaded superbug MRSA.   But what is it, what can it do to us, and is it really very super?

First things first, what is MRSA?  Those four ominous initials stand for Methicillin Resistant Staphylococcus Aureus.  It is a type of bacterium, with the scientific name Staphylococcus aureus, that causes a whole host of life threatening diseases but can’t be killed off by the antibiotic methicillin.  Or, it has to be said, by pretty much any of the antibiotics we throw at it.

And what can it do to us if we catch it?  You may simply have a superficial infection of the skin or the soft tissue beneath it, perhaps a boil or impetigo, and make a full recovery.  However, in some cases bacteria spread into your bloodstream and to other organs in the body.

The list of ensuing diseases does not make for very pleasant reading:

  • Septicemia – blood poisoning
  • Endocarditis – infection of the heart valves
  • Necrotising pneumonia – infection of lung tissue
  • Toxic Shock Syndrome – bacteria can release a potent toxin into the body and cause fever, sickness and organ failure

Tragically, many people will know a friend or relative who caught MRSA during a hospital stay as the bug infects around 2% of all patients.  But where does it come from in the first place?  And how has it become an antibiotic resistant superbug?

A body full of bacteria

It may not be a very nice thought (my apologies to anyone reading whilst eating their breakfast), but the human body is covered inside and out with millions of microscopic bugs.  This includes things like bacteria, viruses and fungi.  Amazingly, there are actually 10 times more bacterial cells in your body than human cells.  They live on your skin, up your nose, in your mouth, throughout your gut, and most of the time they are completely harmless.  In fact, we could not survive without them.

Among these millions of germs is one particular type of bacteria called – you guessed it – Staphylococcus aureus.  It makes its home on the skin and inside the noses of 20% of people, but most of them will never know.  In a few people, however, the bacteria find a way to get inside the body.  This might be via a cut such as a surgical wound, or where a medical device is inserted – perhaps a drip or catheter.  When this happens it is bad news; devastating news if there are no antibiotics around to combat the infection.

DNA: the key to antibiotic resistance

One of the amazing things about bacteria is how they get their DNA.  Whereas we, like other animals, inherit all of our genes from our parents, bacteria can also pass useful segments of DNA from one to another, even to bugs of a different species.

Scientists discovered one such segment, called a cassette, containing several very important genes that protect the bug from antibiotics.  If you are a bacterium and a neighbour sends over the cassette, you can incorporate it into your genome and – voila – you are resistant.  You are now a superbug.

How super is MRSA?

Humans and bacteria seem to be locked in an arms race. As fast as we can develop new treatments or prevention measures, the bugs find a way to get round them and keep infecting us.  But as long as scientists keep up the research into new treatments, and continue to unravel the story of how the bugs are doing such a good job of making us ill, there is certainly hope.  MRSA may be super at getting round our defences, but we are fighting back with the product of our human brain cells.