Tortoise DNA shows volcanic past



While tortoises might be the less popular cousin of the turtle, a new study has found that their DNA can be used to date geological events.

In a report in the Oct. 3 issue of Science magazine, researchers revealed that they could date a volcanic explosion in the Galapagos Islands based on the genetic make-up of a species of tortoise living there. Yale researchers Luciano Beheregaray, Claudio Ciofi, Gisella Caccone and Jeffrey Powell, all members of the Department of Ecology and Evolutionary Biology, wrote the report along with other scientists.

“Our study shows that we can infer specific historical events in a population using DNA markers from [existing] individuals,” Beheregaray said in an e-mail. “Our results emphasize the value of modern molecular approaches in uncovering hidden twists and turns in the demographic histories of populations.”

Beheregaray is currently an assistant professor in the Department of Biological Sciences at Macquarie University in Sydney, Australia.

Caccone said they found a surprising lack of genetic variation among the most populous tortoise species. He said this finding was surprising because large populations usually have greater genetic variability. This lack of genetic variation indicates a large reduction in genetic variability — or bottleneck — probably due to a drastic reduction in population size.

“We found that analyzing the DNA through time, the genetic data tells us that the bottleneck happened 100,000 years ago. We then wanted to look at the geological records and see what happened at that time on the island,” Caccone said. “We asked for some help from the geologist Dennis Geist [another author of the article]. He said that 88,000 years ago one of the biggest explosions on the Galapagos happened and the lava probably destroyed everything.”

That volcanic explosion occurred on Alcedo, the habitat of the g. n. vandenburghi, the tortoise being studied. After the explosion, the volcano was recolonized by species from nearby populations. Estimates for age of the tortoise population suggested that the population began living on Alcedo shortly after the volcanic explosion. Caccone said this data suggests that the g. n. vandenburghi are most likely the descendants of these recolonizing tortoises.

Caccone said these results were particularly noteworthy because of the correlation between the estimated dates of the volcanic eruption and of the arrival of the tortoises.

Powell said he was excited about this connection as well.

“It was really cool putting geology and genetics together and coming up with a consistent and clear answer,” Powell said.

Caccone, the director of the YIBS-Molecular Systematics and Conservation Genetics Lab at Yale, and Powell, a professor in the Department of Ecology and Evolutionary Biology, have studied tortoise populations in the Galapagos Islands for the past 10 years. They collected most of the samples used in this study in 1995 and 1996, and Beheregaray analyzed most of the DNA using relatively new techniques.

“In the past three or four years most of [the techniques] have been developed, but they haven’t been used a lot,” Powell said. “People write theoretical papers on how you can use these theories, but relatively few people have used them.”

Stephen Stearns, chair of the Department of Ecology and Evolutionary Biology, said the publication of this study has positive implications for the department.

“They’ve published a good paper,” Stearns said. “A part of the department has good prospects and probably will have increased support for undergraduates and graduates. We bask in their reflective glory.”

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