The genome organization of the earliest mammalian common ancestor has been recreated by an international team. The reconstruction of the ancestral genome may aid in both the study of mammalian evolution and the preservation of contemporary animals. The fossilized animal “Morganucodon,” which lived about 200 million years ago, is thought to have resembled the earliest mammalian ancestor. The work was published in the Proceedings of the National Academy of Sciences, a scientific journal.
From a platypus to a blue whale, all living mammals today are descended from a single ancestor that existed around 180 million years ago. Although we don’t know a lot about this animal, a global team of researchers has now computationally reconstructed how its genome is organized.
According to Harris Lewin, distinguished professor of evolution and ecology at the University of California, Davis, and senior author of the study, “Our results have important implications for understanding the evolution of mammals and for conservation efforts.”
The researchers used 32 living species, representing 23 of the 26 known orders of mammals, and their high-quality genome sequences. They included chimpanzees, humans, wombats, domestic cattle, rhinos, bats, pangolins, and manatees. The genomes of the chicken and Chinese alligator were also analyzed as comparison samples. The Earth BioGenome Project and other extensive biodiversity genome sequencing initiatives are producing some of these genomes. The Working Group for the Earth BioGenome Project is presided over by Lewin.
According to Joana Damas, first author of the study and a postdoctoral scientist at the UC Davis Genome Center, the reconstruction reveals that the mammalian ancestor had two sex chromosomes in addition to 19 autosomal chromosomes, which govern the inheritance of an organism’s traits aside from those controlled by sex-linked chromosomes. In all 32 genomes, the team found 1,215 clusters of genes that always appear in the same order and on the same chromosome. These basic parts of the genomes of all mammals hold the genes that an embryo needs to grow up healthy.
Over 300 million years, chromosome stability
The researchers found nine complete chromosomes or pieces of chromosomes in the mammal ancestor. The order of the genes in these chromosomes is the same as in modern birds.
According to this remarkable discovery, the order and orientation of genes on chromosomes have been stable over an extended evolutionary timeframe of more than 320 million years. Most breaks, rearrangements, and duplications of sequences, which are the main causes of genome evolution, were more likely to happen in the spaces between these conserved blocks, which had more repetitive sequences.
“Understanding where and why selective pressures vary across genomes depends on reconstructions of ancestral genomes. “The relationship between chromatin architecture, gene regulation, and linkage conservation is clearly established by this study,” Professor William Murphy of Texas A&M University, who did not contribute to the writing of the paper, says. This sets the stage for figuring out how natural selection has affected the evolution of chromosomes all along the mammalian tree of life.
The researchers were able to trace the ancestry of the chromosomes back to their common ancestor. They discovered that there were variations in chromosome rearrangement rates among mammalian lineages. 66 million years ago, when an asteroid impact wiped out the dinosaurs and gave rise to mammals, there was an acceleration in rearrangement in the ruminant lineage, which gave rise to modern cattle, sheep, and deer.
According to co-author Dr. Camilla Mazzoni, head of “Evolutionary and conservation genetics” at the Berlin Center for Genomics in Biodiversity Research and Research Group Leader in Evolutionary and Conservation Genomics at the Department of Evolutionary Genetics at Leibniz-IZW, “the results will help understand the genetics behind adaptations that have allowed mammals to flourish on a changing planet over the last 180 million years.”
