Centipedes have been borrowing proteins in their venom from bacteria and fungi

The use of venom by animals to subdue and kill prey or defend themselves against predators is incredibly common right across the animal kingdom, having evolved independently at least 100 times.

Usually, animals will co-opt proteins that do one function to use them in venom.

For example, one protein, usually used by sperm to break down the protective layer surrounding the egg and allow for fertilisation has been used by animals in their venom to loosen the matrix of tissues so that the venom can more easily spread through the body of prey.

But it turns out that centipedes have been using another way to get these toxic proteins into their arsenal.

Dr Ronald Jenner, a researcher of venom at the Museum, has been looking into the venoms produced by centipedes. He and his colleague Dr Eivind Undheim have discovered that several proteins found in centipede venoms did not evolve in the direct ancestors of the animals themselves, but in completely unrelated bacteria and fungi.  

'The interesting thing is that it is not just random proteins that the centipedes have borrowed from fungi and bacteria,' explains Ronald. 'They have used bacterial weapons that already kill cells. It's already an appropriate bullet for a venomous animal to use.

'The centipedes didn't have to wait for this protein to change or mutate to become toxic, it was already useful from the moment they could put it in their venom.'

The research also looks like that some genes coding for venom proteins may have passed in the other direction, from the centipedes into fungi.

The paper describing this discovery has been published in Nature Communications.

Centipede venom

Centipedes are a diverse group of arthropods found in most environments around the planet.

They are active predators, hunting down other invertebrates before injecting them with their venom. Like all venoms, theirs is typically made up of a cocktail of proteins and peptides.

Centipedes are thought to have evolved venom fairly early into their evolution, around 400 million years ago. This venom, and the proteins it contains have continued to evolve and change for hundreds of millions of years.

This has resulted in the five main groups of centipedes all evolving their own complex battery of toxins.

But Ronald's work is showing that not all of these proteins have their origins in the centipede's own DNA. Some of the active ingredients in their venom have been taken from bacteria and fungi. 

'Of the five different protein families which we know they have borrowed from these groups of organisms, three come from bacteria and they are all virulence vectors,' explains Ronald. 'What this means is that bacteria use these proteins to harm other organisms.'

Additionally, one of these venom proteins is closely related to a protein that is already well known to humans and is being exploited to help protect crops.

'This toxin is produced by bacteria to kill insects so they can then consume them,' says Ronald. 'So scientists have taken this gene and put it in genetically modified corn, for instance, so that when a caterpillar eats the plant it also eats this bacterial insecticidal protein and dies.

'Centipedes have a use for insecticidal toxins too because they like eating them, and so this gene has been transferred into a group of centipedes that you can find in London.'

It seems that humans have stumbled across something which the centipedes had already done millions of years ago through a process called horizontal gene transfer.

Borrowing genes

Horizonal gene transfer is a process in which two organisms pass genetic material, either partial or full genes, to each other. This is distinct from what occurs during reproduction, and can happen through a number of different ways.

Typically, horizontal gene transfer happens among unicellular life such as bacteria and archaea. Rather extraordinarily, it has also been found to occur in multicellular organisms such as fungi and even animals and plants.

This has allowed two organisms, from completely different branches of the tree of life and separated by hundreds of millions of years of evolution, to share their genes with each other. 

Ronald says, 'There are a few instances in which we already know that animals such as spiders and cnidarians, such as sea anemones and hydra, have taken venom components not from their own genomes but from proteins that have evolved elsewhere in the tree of life. They have taken bacterial or fungal proteins.'

But it is now clear that the spiders and cnidarians are not alone in this.

During a larger study looking at how the venoms produced by centipedes evolved, Ronald and Eivind noticed some curious proteins used by the animals that did not seem to come from their own DNA.

'It turns out that this process of borrowing genes via horizontal gene transfer from fungi and bacteria is very common, relatively speaking, in centipedes,' says Ronald. 'In their evolutionary history, they have repeatedly found it useful to borrow bullets for their venom gun from elsewhere.'

What's more, when they reconstructed what they think was the ancestral venom that evolved in centipedes over 400 million years ago, they narrowed it down to four proteins and peptides that were present from the start.

One of these four components already came from bacteria.

'So maybe this borrowing of components from other organisms was a very important step of putting centipedes on the road to the evolution of the very complex venoms that they have today,' says Ronald.

How this has occurred is still not known. It could possibly be something to do with the environments in which they live as another arthropod called springtails, for example, is known to have borrowed lots of genes from the bacteria and fungi which surround it.  

It is also possible that this process is much more common across many other venomous animals, from assassin bugs to octopuses.

'With the techniques that we use we can see that it has occurred,' says Ronald, 'the question now is just how and where else it has occurred'