Volcanic blast recorded in DNA

By Jonathan Amos
BBC News Online science staff

The tortoise population is the largest on the Galapagos

The tortoises on the slopes of Alcedo Volcano in the Galapagos Islands have the signature of an ancient eruption written in their DNA, scientists say.

The animals have comparatively little genetic diversity compared with other tortoise groups on the archipelago.

This suggests a dramatic reduction in their numbers occurred in the past.

The scientists say their DNA studies time this near-extinction event to 100,000 years ago - exactly the time that Alcedo is known to have blown its top.

Clocked event

The explosive past of the Geochelone nigra vandenburghi tortoises is reported in the journal Science by Luciano Beheregaray, at Yale University, Connecticut, US, and colleagues.

Beheregaray had been engaged in a programme to measure the genetic variability of animals across the archipelago when he arrived at Alcedo to investigate its residents, which numbered 3,000 to 5,000 at the last official count.

"By the time I got to Alcedo I was expecting a lot of variability because it is by far the largest population and it was the other way around - there was three to fives times less variability than in other groups," he told BBC News Online.

GALAPAGIAN GIANT TORTOISES 11 subspecies of giant tortoise Can weigh up to 300 kilograms Many culled in 19th Century

By studying the DNA found in the nuclei of the tortoise cells, the team was able to establish that the lack of variability was due to a contraction in the population some time in the distant past.

And by studying the DNA found outside the nuclei, in the cell's "power units" called mitochondria, the scientists were able to put a time to this contraction, or "bottleneck".

This is possible because mitochondrial DNA changes regularly over time, allowing the emergence of patterns in its sequence to be clocked.

Evolution study

Dr Beheregaray said it was unlikely the collapse could be attributed to the hunting that once occurred on the archipelago to provide whalers with food.

"It's unlikely because Isabella is not as accessible as the other islands," he said.

Numbers are now falling because of competition from feral goats

What is unique about Alcedo, however, is that it experienced a major explosive eruption 100,000 years ago - a moment in prehistory that coincides with the emergence of the patterns Dr Beheregaray sees in the G. n. vandenburghi DNA.

Geologists say this violent event deposited almost 3.5 cubic kilometres of rock and ash on the mountain's slopes. It would have wiped out almost all the animals living on the volcano's slopes.

"It is difficult to say just how few animals were left.

"But it must have been a dramatic reduction in the population - to just a handful or maybe even just one female that recolonised Alcedo - otherwise the bottleneck signal would have disappeared by now. After many generations it will eventually be lost."

Dr Beheregaray said his team's approach would be useful for studying questions of evolution on other volcanic islands such as Hawaii.

Investigating extinctions and recolonisations that were associated with eruptions on these islands would improve our knowledge of how different species emerged, he added.