Digital reconstruction of ancient chromosomes reveals surprises about mammalian evolution | Science

Among all mammals studied thus far, the orangutan’s chromosomes are the most like those of the first placental mammal. 

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Humans have 46 chromosomes. Dogs have 78. And a small South American rodent called the red viscacha has a whopping 104. Geneticists have marveled at the chromosomal diversity among mammals for decades, and now, they may know how it happened. A new digital reconstruction of the chromosomes of the ancestor of all placental mammals reveals that these tightly packed structures of DNA and proteins have become scrambled over time—a finding that may help pinpoint possible problem sites in our genomes that underlie cancer and other disease.

The work “helps us to understand how chromosomes have changed over time, which chromosome rearrangements may have led to the formation of new species, and what might be driving chromosomal rearrangements,” says Janine Deakin, a geneticist at the University of Canberra who was not involved with the work. “This was a very elegant study.”

There are three kinds of mammals: egg-laying monotremes such as the platypus, marsupials like kangaroos and opossums, and the majority—placental, or eutherian, mammals—including humans and about 4400 other mammal species. The earliest members of this larger group were mouse-sized, lived in trees, and ate insects about 105 million years ago. To figure out how chromosomes of placental mammals have changed over time, researchers need to know what those early eutherians started with. And that required putting some complicated puzzle pieces back together.

To do that, Harris Lewin, an evolutionary geneticist at the University of California, Davis, and colleagues compared 19 genomes of various mammals at different spots in the eutherian family tree, including several primates. But genomes usually…

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