Ancient whales, secret passages and diversity

Getting to see the relatives can be a chore for most, especially if they’re a long way away. Now imagine your relatives live in a different ocean, separated by a continent, and the only way there is frozen solid all year long.

This is the problem that the Atlantic Bowhead whale populations face if they wish to meet their Pacific dwelling relatives; presumably separated millennia ago as the arctic froze, slamming shut the door of the Northwest Passage until recently. Or so it was thought.

Sadly, the whale’s problems didn’t stop there; subsistence hunting by man resulted in some loss of numbers, and then as commercial whaling began they were put on the track of an endangered species. Fortunately, now commercial whaling has (mostly) stopped, they are recovering.

Both the separation of populations and hunting have forced the whales through something of bottleneck; an event thought to impact on the genetic diversity of Bowhead whales, but the extent was unknown. To see how drastically the existing whales were affected Elizabeth Atler and a team of colleagues from many organisations, have probed this diversity with some surprising tools; found in 500-800 year old house ruins on Somerset Island (Nunavut).

Bowhead whale skull

Bowhead whales are true leviathans; up to 20 m long and weighting up to 100 tonnes. They have two major adaptations for living in the Arctic. The first is a huge bony skull they use to smash through up to 60 cm of ice to reach the surface and breathe. The second is their huge amount of blubber, the thickest of any animal, which can be 40-50 cm thick. They are baleen whales—filter feeders—and with the biggest mouth of any living animal, they’re good at it; gulping in sea water, squeezing it out through long, thick, stiff hair-like filters (baleens) and swallowing what remains, usually krill.

Sadly, it was these very adaptations that made them prized during the years of commercial whaling: their baleens were used to stiffen corsets, their blubber to make clean burning lamp oil, among other uses. The ancient Thule hunters of Somerset Island used their bones as frames for their sod houses, as well as to make tools and other artefacts. Those same bones and baleen artefacts once used to shelter, mend and entertain now have a new use: to help probe the genetic diversity of ancient and current Bowhead whales.

Remains of a Thule whale bone house from around 1600-1800

The researchers painstakingly extracted ‘ancient’ DNA from these bones; in this case a type of DNA called ‘mitochondrial DNA’ (mtDNA); each child receives only its mothers mtDNA. This makes it useful as it’s not subject to the shuffling and rearrangements of chromosomal DNA which comes from both parents; so it’s easier to tell related individuals and populations.

However, one region of mtDNA, the D-loop, is ‘hypervariable’: it accumulates changes faster than other areas. These changes propagate through populations as the mtDNA is passed on to each generation. By sequencing the mtDNA and looking for changes in different parts of the D-loop, you can start to infer how diverse or related the population is.

One way to think of it is like an old manuscript, copied by a monk who makes a small error—maybe writing a ‘b’ instead of a ‘d’—another monk, then copies this mistake faithfully, but might also make a mistake of their own elsewhere, which is copied by another monk, and so on. Comparing old manuscripts, by aligning the texts and seeing where and which errors are present, you can see which ones are related (copied from which source). You can tell which copy was likely first, second and so on, by comparing the ‘errors’. Different errors that aren’t related (started with a different monk, say) will suggest different populations, and therefore increased diversity of a population as a whole.

The researchers complimented their own ancient mtDNA data with data from other researchers; samples taken from Spitsbergen, including some up to 3,000 years old. Current populations were investigated in the same way using existing biopsy samples taken over the last 20 years or so. The contemporary and ancient mtDNA sequences—a total of 759 samples— were crunched through a computer programme that aligns all the individual base pairs of the mtDNA based on their similarities and differences. And the results were not all quite in line with what they expected to find.

In terms of diversity, they found that modern populations are indeed lacking compared to their historical ancestors; in fact, they found entire maternal lineages seem to have disappeared as recently as 500 years ago, most likely due to the whaling activities of man.

What was unexpected was that the ice-plugged Northwest Passage appears to be no barrier for the whales. The Pacific and Atlantic populations were so closely related, that the only answer was that some whales must be able to make the journey through, and have been doing so for some time. It may be that Bowheads are far better adapted for life among the ice than we gave them credit for. Though we certainly expect more Bowheads to start doing this; it’s getting easier.

Chart showing historical and 2012 Arctic sea ice loss by month. Chart from the National Snow and Ice Data Center (http://nsidc.org)

2012 was a record year for loss of arctic sea ice. With increasing human activity in the arctic–both shipping and our continued desire to find and exploit new reservoirs of fossil fuels–this study could help us to understand how these activities will impact Bowhead whale populations throughout the arctic. And, hopefully, help us to manage and protect these impressive mammals.

We don’t know what effect loss of the arctic ice cap might have, but we can expect to see some pacific species finding their way through to the arctic. It may already be happening. In 2010 there were reports of a grey whale, extinct in the Atlantic, in the Mediterranean sea, off the coast of Israel, presumably having found it’s way through the Northwest Passage following its food. How such events might imact the existing ecosystems in the Atlantic and Pacific is a big unknown.

Reference: Elizabeth Alter, S. et al., 2012. Gene flow on ice: the role of sea ice and whaling in shaping Holarctic genetic diversity and population differentiation in bowhead whales (Balaena mysticetus). Ecology and Evolution, p.n/a–n/a. Available at: http://dx.doi.org/10.1002/ece3.397.