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Unraveling the mysteries of the first mammals

According to some scientists, mammals are currently the most dominant species on planet earth. They bear fur, give birth to live young, and produce milk for their offspring, and scientists are beginning to better understand how mammals like us became so powerful. Not much is known about our extinct common ancestor either. All mammals, from the little mouse to the loyal dogs to the great blue whale, are genetically related. But what did this important keystone species eat or look like? A study today Proceedings of the National Academy of Sciences is revealing some new clues about our common ancestor locked up within the animal’s genome.

The international team of researchers analyzed high-quality genome sequences from 32 living species, including humans, chimpanzees, wombats, rabbits, manatees, domestic cattle, rhinos, bats and pangolins, to create a computer model of what the genome of the first mammal. been like. These sequences represented 23 of the 26 known orders of mammals. In Linnaean taxonomythe way of classifying living beings developed by Swedish botanist Carlous Linnaeus, all living things are divided into three kingdoms (plants, animals, and minerals), and then subdivided into classes, orders, genera, and species. As a comparison group, the analysis included the genome sequences of chickens (birds) and Chinese alligators (reptiles).

[Related: Mammals’ ears may hold the secret to warm-bloodedness.]

The ancestor possibly resembled the rat. morganucodonone of the earliest mammals that weighed only a few ounces and lived about 200 million years ago.

According to Joana Damas, first author of the study and a postdoctoral researcher at the UC Davis Genome Center, the team looked back in time and reconstructed the genome based on similarities found in modern mammalian genomes. His reconstruction showed that the mammalian ancestor had 19 autosomal chromosomes plus two sex chromosomes. Autosomal chromosomes control the inheritance of characteristics of an organism except those controlled by sex-linked chromosomes. In most cells, the sex-linked chromosomes are paired (an X and a Y for males and XX for females). Based on this new genome construction, the first mammal had 38 total chromosomes, compared to humans with 46 pairs.

“We can confirm that it had an X chromosome, but we are not sure about the Y because we could not reconstruct it due to data limitations. There are other possible configurations for the sex chromosomes,” explains Harris Lewin, Distinguished Professor of Evolution and Ecology at the University of California, Davis, and lead author of the paper. popular science. “It is probably based on marsupial other montrema karyotypes [an individual’s complete set of chromosomes]that the ancestor also had a Y chromosome.”

The team also identified 1,215 gene blocks that are consistently found on the same chromosome and in the same order across the 32 genomes used in the study.

“Our results have important implications for understanding mammalian evolution and for conservation efforts,” Lewin said. in a sentence.

Furthermore, the team found nine complete chromosomes within the mammalian ancestor whose genes were arranged in the same order as the chromosomes of modern birds. “This remarkable finding shows the evolutionary stability of the order and orientation of genes on chromosomes over a prolonged evolutionary period of more than 320 million years.” Lewin added in a press release.

[Related: The fight to save America’s most endangered mammal.]

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

“Ancestral genome reconstructions are critical for interpreting where and why selective pressures vary between genomes. This study establishes a clear relationship between chromatin architecture, gene regulation, and linkage conservation,” said William Murphy, professor of evolutionary biology at Texas A&M University, in a press release. “This provides the basis for evaluating the role of natural selection in the evolution of chromosomes in the mammalian tree of life.” Murphy was not an author on the article.

The researchers followed ancestral chromosomes forward in time from the common ancestor to the present day and found that the rate of chromosome rearrangement differed between mammalian lineages. For example, in the lineage that leads to our modern cattle, sheep, and deer (called ruminants) there was an acceleration in chromosome rearrangements 66 million years ago, when an asteroid impact wiped out the dinosaurs. This mass extinction event led to the rise of mammals as the dominant species on Earth.

Going forward, the results of this study will help scientists better understand the genetics behind the adaptations that have allowed mammals to thrive on an ever-changing planet for the past 180 million years.


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