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Cane toad DNA puzzle finally cracked – Science News


It’s hard to love a cane toad, but this iconic pest is the face of evolutionary success in Australia.

Toad Key points

Key points

  • Scientists have sequenced and assembled a draft cane toad genome for the first time
  • The genome will help scientists find out how the toad has evolved to be such a successful invader
  • The genome also contains viruses, which could be potential biocontrols

Now an international team of scientists is one step closer in working out how the rainforest toad morphed into an invasive machine in just 80 years.

They’ve decoded the cane toad’s genome, they report in the journal GigaScience.

The genome project, led by virologist Peter White of the University of NSW, started out as a conversation with colleagues about developing a virus to stop the toad in its tracks.

“We have few genomes of amphibians and we know very little about the viruses in them,” Professor White said.

“When the DNA wasn’t out there I decided to do it.”

Cane toads (Rhinella marina), which are native to French Guiana, spread around the world as the sugar cane industry expanded.

They were introduced into Queensland in 1935 from Hawaii to control a sugar cane beetle.

Today, they are pushing through the Kimberley in the west, and slowly moving south to northern New South Wales, with the odd hitchhiker making it to Sydney and beyond.

The toads on the western front are nothing like their sedate Queensland cousins, said study co-author Rick Shine, an environmental scientist from the University of Sydney.

The westies have changed the way they move and are much bolder. They have longer legs and bigger heads.

“The toads in Queensland sit on their bums and don’t really go anywhere from one day to the next,” Professor Shine said.

“The toads on the invasion front are these long-distance athletes that go further than any amphibian has ever been recorded to in the world.

“We know that 80 years ago this was a single gene pool.”

A hard case to crack

While a handful of amphibian genomes have been sequenced in the past, this is the first genome from the toad (Bufonidae) family.

It is not the first attempt to decode the cane toad’s genome.

A team of West Australian scientists had a crack at it a decade ago, Professor White said.

While the WA team were able to sequence the genome — identify base pairs of amino acids that make up DNA — they were not able to put the notoriously complex jigsaw back together, he said.

Ten years later and armed with more sophisticated technology, Professor White’s team sequenced and assembled the draft genome from tissue samples taken from four female toads from the western front.

These toads are the furthest from their genetic roots in French Guiana.

Professor White said the draft genome is one of the best amphibian genomes to date.

“We’ve found over 90 per cent of the genes,” he said.

The next step will be to work out what those genes do.

“The excitement for me is not so much working out how a frog is constructed, but it’s being able to look at the differences in these populations,” Professor Shine said.

“We have an extraordinarily powerful opportunity to look at the process of evolution.”

Professor Shine said analysing the genome will provide clues about how the cane toad went from being a “pretty average” rainforest toad to romp across the driest continent on Earth, and why their toxin is so deadly.

“We don’t know how much of that is going to be changes in gene frequencies, or changes in regulation, or it could be epigenetics.”

Professor White and his team are now sequencing the genomes of toads from all stops along the sugar cane trail between Australia and French Guiana to see how much they’ve evolved.

Viruses may point to biocontrol

Along with the genes, the team also identified two viruses embedded in the genome.

Additional sequencing research on a further 16 cane toads has also identified a third virus lurking in the animal’s RNA, the material that carries DNA information to the building blocks of cells.

Professor White said the discovery of toad specific viruses could lead to the development of a biocontrol.

Around 15 years ago, the CSIRO looked at whether a genetic virus could kill cane toads.

“When they started using it to see if it was going to work, they found it killed the native frogs and newts as well.”

But, he said, much more work would be needed to see if the new viruses were toad specific.

“Future studies would have to grow that virus and check that it can’t infect related native animals,” he said.

Professor Shine, however, is not as comfortable with the idea of a biocontrol.

“I think it would take a lot of work to convince the public, and to convince me that this was really going to be a safe route to travel.

Although the toads are extremely destructive when they move into an area, native wildlife has adapted to them much better than expected, he said, adding that programs teaching predators to avoid toads were promising.

“I’ve actually transitioned to a much more positive view of toads,” Professor Shine said.

“It’s not their fault. We brought them here. They kill a lot of native animals, but they do it in self-defence.

“We’re going to have to learn to live with the toads.”

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