Friday, October 31, 2008

Sorting out the bears

In today's issue of Science, I write about two papers--one published this week in the Proceedings of the National Academy of Sciences, the other published last July in BMC Evolutionary Biology--that report the complete sequencing of the mitochondrial DNA of the extinct cave bear. Why is this a big deal? Two reasons: First, researchers have been unsure about the structure of the bear family tree, how many genera to put the bears in, and how each of the 8 living species of bears is related to each other, as well as to extinct bears that lived in the past. The new papers appear to resolve at least some of these issues to everyone's satisfaction, although there are remaining questions, such as when the explosive radiation of the genus Ursus took place. And second, the researchers have, for the first time, sequenced mitochondrial DNA from animals living during the Pleistocene period (1.8 million to about 10,000 years ago) that were not preserved in permafrost, thus overcoming an important ancient DNA technical hurdle.

The link to the story requires a subscription to Science, but here are a few tidbits from the story--which includes details about a priority fight between the teams that published the two papers.

What kind of bear was Winnie-the-Pooh? Author A. A. Milne christened the fictional character after the teddy bear of his son, who in turn had borrowed the name from an American black bear in the London Zoo called Winnipeg. Yet for decades, researchers have argued about whether Winnipeg's scientific name should be Ursus americanus or Euarctos americanus. Indeed, although there are only eight species of living bears, scientists have come up with at least half a dozen versions of the bear family tree.

Now a paper published by the Proceedings of the National Academy of Sciences (PNAS) online this week helps untangle bear phylogeny by presenting "the first mitochondrial genome" from the extinct cave bear, Ursus spelaeus. But another paper, published with little fanfare last July, also reported the complete mitochondrial DNA (mtDNA) of the cave bear, as well as that of the extinct American short-faced bear, Arctodus simus. The two teams are arguing about scientific priority. But for the bears, this means that two sets of data now illuminate their family tree, although the studies disagree about the timing of bear evolution.

Here's more:

... both groups agree on the outline of the bear family tree. They confirm that the giant panda was the first species to split off from the lineage leading to later bears, and both conclude that the cave bear shared a common ancestor with the brown bear and the polar bear, which turn out to be closely related to each other. Moreover, both teams slash the number of genera of living bears from seven in some schemes, to three for the Hofreiter group and four for the Elalouf group. They assign most species--including Winnipeg's--to the genus Ursus.

But in addition to the disagreement over priority, they also differ on when this all happened:

Yet when it comes to the timing of the recent bear radiation, the two groups part company. Elalouf concludes that it was only about 2 million to 3 million years ago, using a previous estimate of the giant panda's divergence at 12 million years ago as a chronological anchor point. Hofreiter's team anchors its tree with the much earlier divergence of the harbor seal and finds that the panda split off earlier, about 19 million years ago, and that the rest of the bears radiated about 5 million years ago. He notes that some aspects of climate changed dramatically about that time, when the Bering Strait opened and the Mediterranean Sea became drier. Other mammals also showed dramatic changes at this time, such as the split between the human and ape lineages.

Nevertheless, as I point out in the story, at least Winnie-the-Pooh finally has his scientific name.

Illustration: Artist's drawing of the extinct cave bear


jqb said...

Nature endorses Obama.

Anne Gilbert said...

I remember "black" bears being listed as Euarctos americanus and polar bearts being listed as Thalarctos maritimus in some older guidebooks and the like, but since adulthood(and I've supposedly been adult for a long time now), these two bear species have been listed in any reference guide's I've seen, as Ursus americanus and Ursus maritimus, respectively. This despite the fact that, apparently, polar and brown bears are more closely related to each other than they are to "black" bears. Still, the fact that they are all now in the genus Ursus, which is where I would have thought they ought to have been in the first place, which suggests that these Northern Hemisphere bears' relationships to one another are quite close to begin with.
Anne G

Joseph Caputo said...

Great Winnie the Pooh lead! How did you come up with the idea?

Michael Balter said...

Thanks, Joe the Blogger! Good question, it just came to me I guess. Then I checked to see what kind of bear Winnie was and found out he was named after an American black bear, and I knew it would work.