History Revealed

(September 21st, 2017) By analysing 10,000-year old DNA, Swedish researchers show when fish colonialised Swedish lakes.

DNA in lake sediment forms a natural archive, displaying when various fish species colonised lakes after the glacial period. Analysing the prevalence of whitefish DNA in the sediment, researchers at the Department of Ecology and Environmental Science at Umeå University revealed that whitefish came to Lake Stora Lögdasjön in Northern Sweden already 10,000 years ago, whereas they colonised Lake Hotagen in Central Sweden only 2,200 years ago.

“It's fantastic news that DNA can be stored for so long in lake sediment. Normally, free DNA molecules break down within days, but certain DNA fragments are preserved because they bind to clay particles,” says Göran Englund, one of the researchers behind the study. Bound DNA molecules in the lake sediment, however, resulted in challenging analyses and required the development of new methods, both for extracting sufficiently clean DNA and for the statistical analysis of data.

“Being able to map the prevalence of DNA in lake sediments is now opening up a new window into history, which lets us see how nature has developed over a long period of time,” says Göran Englund. “We have already started a project aiming to study how lake ecosystems are affected by historical climate changes. That can provide important clues to a better understanding of how the current global warming will affect ecosystems.”

The researchers chose lakes Stora Lögdasjön and Hotagen for the study since they expected the whitefish to have colonised these lakes at different points in time. Stora Lögdasjön was connected to the Baltic Sea, when the inland ice melted around 10,000 years ago. The connection was cut off 9,200 years ago when the land uplift created a waterfall, which the whitefish were unable to travel up. “Our hypothesis was that the whitefish colonised Stora Lögdasjön immediately after the ice-melt, which turned out to be accurate”, says Göran Englund.

Historic, written sources, however, show that whitefish have been found in Hotagen at least since the 18th century. The researchers were also able to see that the speciation process that occurs in many lakes - namely that whitefish populations diverge into large-bodied and small-bodied species - had not happened in Hotagen.

“Based on this information, we assumed that the whitefish had colonised the lake long after the ice-melt but before the 18th century. That it already happened 2,200 years ago was, however, a slight surprise,” says Göran Englund.

“Naturally, we can't know for certain how the whitefish spread to Hotagen. Fish-eating animals like the otter, bear, osprey and dipper may have been involved, but the most likely theory is that hunters and fishermen, who resided in the area 2,000 years ago, played a part. We can see increasing evidence that fish species were introduced to new lakes by the humans who first colonised Scandinavia,” concludes Englund.

Adapted from the University of Umeå

Read more about how ancient DNA analysis has solved historic riddles in the upcoming issue of Lab Times.

Photo: Göran Englund

Last Changes: 10.12.2017