Chromosomal fossils of a mammoth that died 52,000 years ago

Diana Ilya Cruz |

Barcelona (EFE). An international team led by scientists from the National Centre for Genomic Analysis (CNAG) has discovered fossils of ancient chromosomes in the remains of a woolly mammoth, locked in the Siberian permafrost for 52,000 years, and has managed to assemble, for the first time, the genome of an extinct species.

The study, published in the scientific journal Cell nine years after its launch, was conducted by more than 50 scientists from teams at CNAG and the Center for Genomic Regulation in Barcelona, ​​Baylor College of Medicine (USA) and the University of Copenhagen (Denmark).

The study represents a before and after look at these species and their ability to adapt to the climatic adversities of their time.

According to study co-author Olga Dudchenko, the survival of these fragments of ancient DNA for tens of millennia is a “mystery of physics” because, as Albert Einstein predicted in 1905, they should not exist under normal circumstances.

Creation of fossil chromosomes

However, researchers believe that the permafrost (permafrost) in which the mammoth was found in Siberia led to the creation of fossil chromosomes through a process similar to the production of dried meat or jerky.

For this reason, having been preserved in a state similar to glass molecules, the structure of the chromosome has been preserved to this day.

A 1984 study already suggested that DNA was preserved in these ancient samples, but given the limitations of its analysis due to its low quality, no one had been able to study its structure. Until now.

“Fossil chromosomes are a game changer because knowing the shape of an organism’s chromosomes allows us to assemble the complete DNA sequence of extinct creatures. This gives us information that we didn’t have before,” explains Dr. Dudchenko from Baylor College of Medicine in the US.

Compared to ancient DNA samples that have survived to this day, the new fossils of ancient chromosomes contain a million more genetic information; very important data that allows us to know the genes that were active in this species and its evolution.

Separation of chromosomes

The phenomenon of chromosome compartmentalization present in the mammoth skin sample separates active and inactive DNA into adjacent spaces within the cell nucleus and allowed the researchers to determine which genes were active at the time of the mammoth’s death.

Comparing the genome (the complete set of DNA) of the mammoth and its closest living relative, the elephant, they found many similarities, but were not completely identical.

In particular, they found about 1,000 regions with differential activity in the genome; for example, a growth gene, an immune adaptation gene, and a cold adaptation gene.

“With this type of data, we not only know which genes the mammoth has versus the elephant, but also which ones helped it adapt and can explain why it went extinct,” whether due to climate change or an inability to adapt to other phenomena, says CNAG team leader and co-lead author of the study Marc Marty-Renom.

In addition to compartmentalization, they found structural similarities with modern chromosomes: chromatin loops, small structures about 50 nanometers in size.

“DNA loops are important because they bring activating DNA sequences closer to their genetic targets. So these fossils not only tell us which genes were active, but also why,” says Marty-Renom.

To confirm the study, they also worked with a second sample, taken from a mammoth that died 39,000 years ago, also in Siberia, which was very well preserved in a museum in Russia.

A Milestone in Paleogenomics

The scientists say the discovery opens the door to working with other types of specimens in the future, particularly from natural history museums, where there are many specimens preserved in the same way as the animal analyzed, as well as endangered specimens and even dried mummies.

“We can now study the ancient specimen and its gene activity, which shows how it adapted to the environment at the time,” adds the CNAG researcher.

For her part, Marcela Sandoval-Velasco, a researcher at the University of Copenhagen’s Centre for Evolutionary Hologenomics and another co-author of the paper, considers it “a milestone in the field of paleogenomics.”

“We’re taking a step forward in studying the past,” says Sandoval, who argues that these small fragments of ancient DNA “speak to us and allow us to study the evolutionary history of different organisms.”

But experts are not just looking back, but also seeing the discovery as an opportunity to answer questions about how changes in biodiversity – as a result of climate change – might affect our own existence on the planet.

“Having this type of molecular material from 52,000 years ago, when our planet’s climate was very different, can help us make correlations between our past, our present, and our future,” said Baylor College of Medicine researcher and study co-author Cynthia Perez.