Many snakes make meals of frogs, but some appear to be transferring their DNA into the amphibians as well. A genetic analysis suggests that parasites shared between snakes and frogs may facilitate the movement of genetic material from one species to another.
The “horizontal” transfer of DNA between species was long considered a rare event that took place only between microbes, but there is growing evidence that the process has been going on all over the tree of life.
Genetic sequences called transposons – also known as “jumping genes” and sometimes described as selfish or parasitic DNA – can move around in a genome. Some of these, called retrotransposons, expand in the genome by copying and pasting themselves.
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One known as Bovine-B (BovB) now makes up more than 18 per cent of the cow genome, but originally came from a snake through horizontal DNA transfer 40 to 50 million years ago.
Almost 10 years ago, at the Nagahama Institute of Bio-Science and Technology in Japan found BovB in the genomes of frogs from Madagascar. The sequence was a 95 per cent match with viper BovB.
Suspecting that it had jumped from snakes to frogs, Kurabayashi and his colleagues have now analysed the DNA from 106 snake species, 149 frog species and 42 species of their shared parasites – like leeches and ticks – from around the world. The researchers screened the samples for BovB and charted its history of hopping between species.
They estimate that BovB jumped from snakes to frogs at least 54 times between 85 million and 1.3 million years ago. Madagascar seems to be a hotspot for the transfers: 91 per cent of the island’s frogs have BovB and it has invaded their genomes 14 times within the past 50 million years. In that same period, this only happened once in mainland Africa.
Various parasites may be aiding the process, since several of the parasite genomes contained BovB sequences close to those in snakes. Half the parasite species in Madagascar had BovB in their genomes, compared with less than 3 per cent in East Asia.
The findings hint that the horizontal transfer of DNA can be affected by the prevalence of parasites in a region, just like some diseases, says Kurabayashi.
at the University of Exeter in the UK says he is “blown away” by the findings and is interested in whether other biodiversity hotspots also show high rates of horizontal transfer or if there is something special about Madagascar.
at the University of Colorado Boulder cautions that key intermediate species in the BovB chain could still be missing. “All we see is the end result of BovB presence in a snake and frog, or leech and frog, or tick and snake, but it’s very hard to track how BovB elements ended up in these species,” she says.
BovB doesn’t have a known function in any animal, but Ivancevic is curious about whether it could have been co-opted to serve a purpose in snakes and frogs.
To be passed on to the next generation, BovB must have got into the frogs’ reproductive cells, but the parasites are chiefly found in the digestive tract.
Kurabayashi and his team think viruses probably carried the DNA into the frogs’ cells and helped it spread to other tissues. They hope to survey viruses in Madagascar that implant snake BovB into their genomes.
One snake species carrying BovB – the brahminy blind snake (Indotyphlops braminus) – is currently spreading across the globe thanks to human activity.
“This has the potential to present the world with a new invasive species problem, in which alien species disturb the genome structure of vertebrates in the invaded area,” says Kurabayashi.
Molecular Biology and Evolution
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