Australasian Science: Australia's authority on science since 1938

What’s Jumped Into Your DNA?

Credit: k_e_n/Adobe

Credit: k_e_n/Adobe

By Atma Ivancevic

DNA elements that can transfer between species make up an astonishing 17% of the human genome, and have been associated with schizophrenia and cancer.

A tick is on the hunt for prey. It spots a resting snake: it’s first victim. The tick feeds hungrily, sucking in blood and DNA. Later, it spots another delicacy in the form of grazing cattle. The tick hops from animal to animal, latching on tightly to consume a second blood meal. The cow squirms uncomfortably at the sudden intrusion, but quickly forgets and goes back to grazing.

Unbeknown to all, the tick has just transferred DNA from the snake to the cow. This process is called horizontal transfer, and the DNA is a transposable element.

Transposable elements are mobile DNA sequences. Unlike genes, they have no known function but they do have the ability to copy and paste themselves to new locations within the genome.

Sometimes these elements can jump into new organisms by hitching a ride on parasites such as ticks. Transferred elements can then start replicating inside the new host, bloating out the genome and interrupting genes.

If they get into reproductive cells like eggs and sperm, they can be passed on to the next generation. Sometimes this leads to the evolution of new species traits. Other times it can lead to disease.

In new research published in Genome Biology (https://tinyurl.com/ycabdnew), we looked at two transposable elements: BovB and L1. BovB is the example that jumped from snakes to cows. Nowadays, cows have so many BovB copies that they make up more than a quarter of the cow genome sequence.

BovB is found in such a bizarre and sporadic assortment of animals that it contradicts any normal inheritance pattern. It’s jumped between elephants, marsupials, sea urchins, and even twice between frogs and bats. We have evidence that BovB uses blood-sucking and flying parasites to get around, such as reptile ticks, leeches, bed bugs and locusts. BovB is a clear-cut example of recent DNA transfer between very different animals.

L1, on the other hand, is an ancient transposable element. We know that L1s are present in mammals. In humans, L1 elements occupy more than 17% of the genome (https://tinyurl.com/yakn6kjh). L1 activity has been linked to severe side-effects like schizophrenia and cancer progression (https://tinyurl.com/yc8lgsty).

For a long time we believed that L1 could only be inherited vertically, passing down generations from parent to child. However, our recent evidence suggests that this isn’t true; like BovB, L1 can also jump between species.

Biologists study horizontal transfer by looking for patterns. We survey organisms across the tree of life to find similar segments of DNA in distant species. If a reptile (e.g. a snake) and a mammal (e.g. a cow) share identical BovB sequences, yet these sequences are missing in other mammals (e.g. mice and humans), then the sequence likely did not come from a common ancestor. Using public domain data and specimens from the South Australian Museum, we tracked BovB across 759 species. Then we did the same for L1.

L1s are found in more than 500 species. They have had millions of years to colonise genomes, and in some cases, like mammals, they’ve run rampant, expanding to tens of thousands of copies. But they are surprisingly absent in two key mammals: the Australian monotremes, the platypus and echidna. This complete absence suggests that L1s entered the mammalian lineage after they diverged from their odd-looking, egg-laying relatives 160–191 million years ago.

If you go further back in time, L1’s history becomes even more tangled. Unlike BovB, L1 seems to prefer marine parasites, sea worms and oysters, fish and jellyfish. L1 elements are also found in plants and fungi, but their presence across these lineages is patchy. Most fungi don’t have L1s at all. In the few that do, the L1 copy number is very low. In these genomes, L1 barely left a mark.

This begs the question: are some genomes more vulnerable to foreign DNA?

When BovB infiltrated cows, it replicated to such a degree that it is now one of the dominant features of cattle DNA. Likewise, when L1 entered non-egg-laying mammals, it drastically changed our genetic make-up forever.

We still feel the repercussions of L1 activity today. For whatever reason, mammals are particularly susceptible to rapid expansions of transposable elements.

Of course, genomes are equipped with safety mechanisms to prevent jumping DNA from wreaking havoc. In humans, only a small portion of L1s are still capable of moving around, and even these are silenced in healthy adult cells. But the silencing process is not absolute. When things go wrong, reactivation of L1s can cause DNA breaks and genetic instability, leading to diseases such as cancer (https://tinyurl.com/ya3dtn5w).

Some mammals are even more efficient at silencing mobile DNA. Our research found that fruit bats contain both BovB and L1 elements. However, these elements have been silenced so effectively in bat genomes that they are essentially “dead”. All that remain are degraded remnants that can never be reactivated.

Perhaps these heightened silencing mechanisms reflect a necessary adaptation. Bats, including fruit bats, often prey on insects. Some researchers believe this increases their exposure to horizontal DNA transfer (https://tinyurl.com/ybsu69vx). Bats also frequently transmit diseases, such as the rabies virus. Perhaps bats have evolved to act as carriers, facilitating the exchange of DNA while minimising impact to themselves.

Maybe this is what prompts transposable elements to transfer between species. Facing extinction in their current host, they escape to survive and replicate again. Under threat, the new host adapts stronger silencing mechanisms. And so the process repeats itself.

Not that transposable elements are inherently bad. They were first discovered in the 1940s by pioneer Barbara McClintock, who found them in multicoloured corn kernels and declared them “controlling elements” of the genome (https://tinyurl.com/yao3r6dx). Decades later, we’re finally starting to see this is true. Yes, they have the potential to cause disease, but host genomes can also domesticate transferred elements for their own benefit.

Jumping DNA machinery has been co-opted to strengthen our innate immunity (https://tinyurl.com/ztyg5ua) and facilitate rapid evolution of the mammalian placenta (https://tinyurl.com/ybe3uq7o). Exposure to foreign DNA continually helps us reshape genetic networks, acquire new and diverse structural genes, and evolve.

For better or for worse, jumping DNA is a potent source of genetic change. It introduces a level of randomness to the traditional notion of inheritance.

Our research aims to understand how transposable elements have shaped the genomes we see today, and their potential to cause further changes. In the words of renowned chemist Marie Curie: “Nothing in life is to be feared, it is only to be understood. Now is the time to understand more, so that we may fear less.”


Atma Ivancevic is a postdoctoral researcher at the University of Colorado Boulder. The study described in Genome Biology was her PhD project undertaken at the University of Adelaide and the South Australian Museum.