Texas researchers discover fossil chromosomes of 52,000-year-old woolly mammoth

Texas-based researchers have discovered something never found before: fossil chromosomes from a woolly mammoth that died 52,000 years ago.

This finding, made by an international team led by researchers at Baylor, involved the study of a remarkably preserved piece of woolly mammoth skin found in Siberia in 2018 – and where they’re taking these findings sounds equally remarkable.

For more on the discovery, Olga Dudchenko, one of the team’s leaders and an assistant professor of molecular and human genetics at Baylor College of Medicine, spoke to the Texas Standard.

This transcript has been edited lightly for clarity:

Texas Standard: I understand DNA fragments have been found before. What makes this discovery different? It’s the chromosomes, right?

Olga Dudchenko: Yes, so, indeed, fragments have been discovered, and we’ve already known for about 40 years or so that DNA can survive in short fragments and samples; however, usually the mental view that people have is that these fragments are just kind of scattered across the ancient site, and they have lost really everything about the original arrangement.

In this particular case, we see that those fragments of DNA are in the original conformation just the same way they were 52,000 years ago.

Tell us a little bit about the team behind this. Who all was involved?

I would say it’s an international collaboration at its finest. This study was co-led by three different centers in three different countries.

We’re here at Baylor College of Medicine in the U.S. There was also the University of Copenhagen and the University of Barcelona, and there were 56 authors overall across many, many countries. So, we’re really proud of, you know, in the end how it all came together.

Let me ask maybe a silly question: How is it possible that these chromosomes would survive over 52,000 years?

It’s a question that we kept asking ourselves with more and more urgency as we looked closer and closer at the sample and saw the degree of the preservation of the original conformation. And, you know, we’re in Texas. Jerky is a big thing here, and in general, shelf-stable food. So, we started thinking in that direction and viewing the sample in some way as this extreme case of a very shelf-stable material.

So, what happens is you dehydrate, or with a combination of cooling and dehydration, you force the piece of material to lose a lot of its water, which leads to something that looks like a traffic jam. Things can’t really move away from each other, even if they wanted to, and so we think that something similar happened here.

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I understand that your team was able to replicate the DNA as well for the first time ever. We’re talking about an ancient animal, right?

The matter of debate to some degree is what it means to sequence something like an ancient animal. So, what people have been doing before is, as we mentioned, for 40 years now, we’ve known that we can go and potentially extract pieces of DNA from ancient species, long-extinct ones.

And what we can do with those DNA fragments is to compare them to the DNA of modern species, such as the Asian elephant and the African elephant, and we can annotate the differences; however, up to this study, we, for example, didn’t know how many chromosomes overall the mammoth had.

Annotating differences is a little bit like looking through a book and marking that a letter has changed or maybe a word has changed, but we didn’t really know anything about the overall arrangement of chapters in this big blueprint that the DNA is. Chromosomes can be viewed as these chapters.

So, now we have a bigger context. It happens that the number of chapters in their overall order matches that of modern elephants. You may consider this not surprising, but it could have been different. There are plenty of species out there with very, very, different arrangements.

That sounds like a huge takeaway, given that now there’s no more guesswork. You have a very clear picture of what the DNA sequence would be, it sounds like. Was their other information you were able to draw out of this research?

Yes, indeed. We’re really interested in how genomes fold in 3D inside nuclei. It turns out that the way the genomes fold in 3D is not at all random. There are a lot of biological insights that one can glean from if one knows how a genome of a particular species falls.

In fact, it differs from cell type to cell type, and this is to a degree responsible for what makes a particular cell type different from anything else. What makes a cell in the skin different, for example, from a cell in a retina? So, we know how to interpret this folding information, the way that the chromosomes exactly appear, into what exactly the genome does.

So, that’s what we do in addition to just counting the chromosomes in this work. We look into the activity pattern for the first time for the mammoth skin and compare it to that in the Asian elephant skin.

And, interestingly enough, we see a lot of signals that are associated with woolliness, you know, spoiler alert. So, we’re quite excited about this, of course, because people really want to know what makes the mammoth woolly, how exactly it adapted to the cold climates, and to see what we can learn from that adaptation process.

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