When the brown tree snake (Boiga irregularis) arrived on the island of Guam shortly after WWII, likely having hitched a ride on a cargo military ship, no one suspected it would become one of the world’s best-known examples of an invasive species.
- "One bite contains enough venom to kill at least 100 adult humans" 13 deadliest, most venomous, snakes on the planet
- Trap tentacles, spider-tails and faking death: Meet the world's most cunning snakes
Now a study published in the journal Science Advances, led by scientists from the University at Buffalo, New York, has revealed how the brown tree snake managed to flourish on Guam, an isolated island in the middle of the western Pacific Ocean.
This unassuming-looking snake is medium-sized with reddish-brown or yellowish-brown skin marked with darker crossbands, which helps it blend into tree bark and forest floors. It’s only mildly venomous and poses very little threat to humans (the bite might cause swelling and discomfort but not much more).
Once the brown tree snake landed on Guam, it spread quickly. Its population was estimated to be two million in the early 1980s. In some parts, the numbers have reached 30,000 snakes per square mile, which has taken a toll both on the biodiversity and economy of the island.
Out of the 12 native forest bird species found on Guam, 10 have gone extinct. With pollinators in decline, plant diversity has followed. Because brown tree snakes are exceptional climbers, they often cause power outages, severely impacting the island’s energy sector.
A genetic advantage
The impact of the brown tree snake on Guam is undisputed. But what’s puzzled scientists is how a small number of snakes managed to get this far. A limited founding population usually creates something called a genetic bottleneck: a drastic reduction in numbers limits the gene pool, causing loss of genetic diversity and the need for inbreeding. Normally, this makes a species more vulnerable to diseases and less adaptable.
So how did the brown tree snake overcome these hurdles and so severely impact Guam’s ecosystem? Researchers have discovered that they have been carrying a genetic advantage all along.
“The brown tree snake is maybe not wildly diverse, but it has important sources of genetic diversity that have been underappreciated,” said the study’s corresponding author, Trevor Krabbenhoft, who is an associate professor at the University at Buffalo.
Early DNA sequencing tools were designed to detect small changes in individual DNA base pairs, making it harder to identify anomalies and changes in longer stretches of DNA.
This is where long-read sequencing comes in – it allows researchers to examine continuous pieces of genome, making it possible to detect mutations affecting 50 base pairs or more.
“It’s like looking at portions of two books letter by letter with a magnifying glass and thinking they’re the same but not realising entire paragraphs have been moved around or duplicated,” said Levi Gray, a postdoctoral researcher in Krabbenhoft’s lab.
“Older sequencing technology didn’t allow us to easily see that DNA in one individual might be in a completely different place on the chromosome than in another. How we define genetic diversity and how we actually measure it is shaped largely by the technology of the day.”
By using this technology to examine the brown tree snake’s genome, scientists have discovered nearly 19,000 genomic structural variations – mostly concentrated in genes vital for adaptive immunity and olfaction (sense of smell).
Smell is extremely important to brown tree snakes as they use it to navigate and locate prey. A better immune system, meanwhile, would have helped the snakes cope with being exposed to new and unknown pathogens in Guam.
The combination of the two could have therefore tipped the odds in the snake’s favour.
Scientists can’t yet confirm whether the snakes formed these genetic variations before arriving in Guam, (perhaps during dispersal from native Papua New Guinea to nearby Manus Island) or whether they evolved after the invasion began.
“Is it possible some of this diversity emerged after the invasion? It is but we would have to sequence snakes from the native populations to know for sure,” said Gray.
Wider implications
For agencies that have spent decades trying to control Guam’s brown tree snake population, as well as invasive species management more generally, this could be bad news. These snakes might simply be better equipped for colonisation – and other species just like this one may be out there.
On the flip side, this might be good news for endangered species with small, fragmented populations.
“It’s possible that endangered species may have more flexibility in their genes than we realise,” said first author Christopher Osborne, a former PhD student in Krabbenhoft’s lab.
“We’re now getting a better understanding of unappreciated sources of genetic diversity that may explain how some inbred species can still respond to their environment.”
The researchers frame this as a methodological wake-up call: examining larger sections of the genome through long-read sequencing might allow scientists to look at the bigger picture and potentially reveal hidden genetic resilience in endangered species previously written off as genetically doomed.
Read the full findings here.
Top image credit: Ken Griffiths/Getty Images
