A Mexican-European team targeted the Basin of Mexico to search for Columbian mammoth DNA despite warmer conditions that tend to degrade DNA. The Basin yielded abundant mammoth remains, including over 100 individuals found during Mexico City's airport construction. The team sequenced mitochondrial genomes because mitochondria exist in high copy numbers per cell and therefore preserve better. From 83 samples, 61 mitochondrial genomes were recovered and 28 met quality thresholds for analysis. A phylogenetic tree combining these genomes with other Columbian and woolly mammoth samples placed the Mexican mammoth genomes together in a single block with three subgroups and an unusual placement adjacent to woolly mammoth lineages.
The researchers focused on the Basin of Mexico, which is well south of where any woolly mammoths were likely to be found. To begin with, there were a lot of bones. The Basin of Mexico has been heavily built up over the centuries, and a lot of mammoth remains have been discovered, including over 100 individuals during the construction of Mexico City's international airport.
In addition, the team focused entirely on the mitochondrial genome. In contrast to the two sets of chromosomes in each cell, a typical cell might have hundreds of mitochondria, each of which could have dozens of copies of its genome. So, while the much smaller mitochondria don't provide as much detail about ancestry, they're at least likely to survive at high enough levels to provide something to work with.
Altogether, the researchers obtained 61 new mitochondrial genomes from the mammoths of Mexico from the 83 samples they tested. Of these, 28 were considered high enough quality to perform an analysis. By building a family tree using this genetic data, along with that from other Colombian and woolly mammoth samples, the researchers could potentially determine how different populations were related. And one thing became very clear almost immediately: They were in a very weird location on that tree.
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