An international team has reconstructed the genome organization of the oldest common ancestor of all mammals. The reconstructed ancestral genome could help understand the evolution of mammals and the preservation of modern animals. The earliest mammalian ancestor likely resembled the fossil animal ‘Morganucodon’ that lived about 200 million years ago. The work is published in the scientific journal Proceedings of the National Academy of Sciences.
Every modern mammal, from the platypus to the blue whale, descended from a common ancestor that lived about 180 million years ago. We don’t know much about this animal, but the organization of its genome has now been computationally reconstructed by an international team of scientists.
“Our findings have important implications for understanding mammalian evolution and conservation efforts,” says Harris Lewin, professor of evolution and ecology at the University of California, Davis, and senior author of the research paper.
The scientists relied on high-quality genome sequences from 32 living species representing 23 of the 26 known orders of mammals. Among them were humans, chimpanzees, wombats, rabbits, manatees, cattle, rhinos, bats, and pangolins. The analysis also included chicken and Chinese crocodile genomes as comparison groups. Some of these genomes are produced as part of the Earth BioGenome Project and other large-scale efforts to sequence the genome of biodiversity. Lewin leads the working group for the Earth BioGenome Project.
Reconstructions show that the mammalian ancestor had 19 autosomal chromosomes, which control the inheritance of characteristics of the organism outside of those controlled by sex-linked chromosomes, (they are paired in most cells, making them 38 in total) as well as two sex chromosomes, she said. Joanna. Damas, first author of the study and a postdoctoral scientist at the UC Davis Genome Center. The team identified 1,215 blocks of genes that occur consistently on the same chromosome in the same order across all 32 genomes. The building blocks of all mammalian genomes contain genes necessary for the development of a normal embryo.
Chromosomes are stable over 300 million years
Scientists have found nine complete chromosomes, or chromosome fragments, in a mammalian ancestor whose gene arrangement is the same as in the chromosomes of modern birds.
“This remarkable discovery demonstrates the evolutionary stability of the ordering and orientation of genes on chromosomes over an evolutionary time frame spanning more than 320 million years,” says Lewin. In contrast, regions between these conserved blocks contained more repeating sequences and were more susceptible to breakage, rearrangement, and sequence repeats, which are key factors for genome evolution.
Says Professor William Murphy, of Texas A&M University, who was not an author on the paper. “This provides the basis for assessing the role of natural selection in chromosome evolution across the mammalian tree of life.”
Scientists were able to trace back in time the ancestral chromosomes from the common ancestor. They found that the rate of chromosome rearrangements differed between mammalian strains. For example, in the lineage of ruminants (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 gave rise to mammals.
“The results will help understand the genes underlying the adaptations that have allowed mammals to thrive on a changing planet over the past 180 million years,” explains co-author Dr Camila Mazzoni, Head of the Department of “Evolutionary Genetics and Conservation” at the Berlin Center for Genomics and Head of Research and Research Group Biodiversity in evolutionary genomes and conservation in the Leibniz-IZW Department of Evolutionary Genetics.
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