Scientists reconstruct the genome of the common ancestor of all mammals 180 million years old

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The mammalian ancestor had 19 autosomal chromosomes and 2 sex chromosomes.

Scientists from the University of California, Davis help reveal the genome of the common ancestor of all mammals

From the platypus to the blue whale, all mammals alive today are descended from a common ancestor that existed around 180 million years ago. Although we don’t know much about this animal, a worldwide team of experts has recently reconstructed the organization of its genome by computer. The results were recently published in the journal Proceedings of the National Academy of Sciences.

“Our findings have important implications for understanding mammalian evolution and for conservation efforts,” said Harris Lewin, Distinguished Professor of Evolution and Ecology at the University of California, Davis and lead author of the item.

The researchers used high-quality genomic sequences from 32 living species, covering 23 of the 26 known orders of mammals. Humans and chimpanzees were among these species, as were wombats and rabbits, manatees, domestic cattle, rhinos, bats and pangolins. Chicken and Chinese alligator genomes were also used as comparison groups in the analysis. Some of these genomes are being produced as part of the Earth BioGenome project and other large-scale biodiversity genome sequencing initiatives. Lewin is the chair of the Earth BioGenome Project Working Group.

According to Joana Damas, first author of the study and postdoctoral researcher at the UC Davis Genome Center, the mammal’s ancestor had 19 autosomal chromosomes, which control the inheritance of characteristics of an organism other than those controlled by sex-linked chromosomes. (these are paired in most cells, 38 in total), plus two sex chromosomes. The researchers identified 1,215 blocks of genes that appear on the same chromosome in the same order across 32 genomes. Damas said these building blocks of all mammalian genomes include genes essential for the development of a normal embryo.

Stable chromosomes for over 300 million years

The researchers found nine whole chromosomes or chromosome fragments in the mammalian ancestor whose gene order is the same in the chromosomes of modern birds.

“This remarkable finding shows the evolutionary stability of gene order and orientation on chromosomes over an extended evolutionary time span of more than 320 million years,” Lewin said.

In contrast, the regions between these conserved blocks contained more repetitive sequences and were more prone to sequence breaks, rearrangements and duplications, which are the main drivers of genome evolution.

“Ancestral genome reconstructions are essential for interpreting where and why selective pressures vary across genomes. This study establishes a clear relationship between chromatin architecture, gene regulation and bond conservation,” said Professor William Murphy, Texas A&M University, who was not one of the authors of the paper. . “This provides the basis for assessing the role of natural selection in the evolution of chromosomes through the mammalian tree of life.”

The researchers were able to track the ancestral chromosomes back in time from the common ancestor. They found that the rate of chromosomal rearrangement differed between mammalian lines. For example, in the ruminant lineage (leading to modern cattle, sheep and deer) there was an acceleration of rearrangement 66 million years ago when an asteroid impact killed the dinosaurs and led to the rise of mammals.

The findings will help understand the genetics behind the adaptations that allowed mammals to thrive on a changing planet over the past 180 million years, the authors said.

Reference: “Evolution of Ancestral Mammalian Karyotype and Syntenic Regions” by Joana Damas, Marco Corbo, Jaebum Kim, Jason Turner-Maier, Marta Farré, Denis M. Larkin, Oliver A. Ryder, Cynthia Steiner, Marlys L. Houck, Shaune Hall, Lily Shiue, Stephen Thomas, Thomas Swale, Mark Daly, Jonas Korlach, Marcela Uliano-Silva, Camila J. Mazzoni, Bruce W. Birren, Diane P. Genereux, Jeremy Johnson, Kerstin Lindblad-Toh, Elinor K. Karlsson , Martin T. Nweeia, Rebecca N. Johnson, Zoonomia Consortium and Harris A. Lewin, September 26, 2022, Proceedings of the National Academy of Sciences.
DOI: 10.1073/pnas.2209139119

The study was funded by the National Institutes of Health and the US Department of Agriculture.

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