This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: A human, an octopus, and a coral could hardly look more different—yet deep inside their cells, their chromosomes still carry recognizable pieces of a genome inherited from an animal ancestor that lived more than 600 million years ago. Since then, their chromosomes have fused, split and rearranged countless times.
Today, thousands of animal genomes have been sequenced. For the first time, an international team led by scientists from the University of Vienna set about comparing them all at once. So far, it has been a major challenge to make sense of how their genomes changed over such vast timescales.
The study published in Science Advances maps how those pieces have been reshuffled across the world of animals and reveals that animal genomes evolve along a limited set of irreversible "evolutionary highways." The latest findings provide an important basis for the conservation of animal biodiversity. "Understanding these rules of evolution doesn't just tell us about the past," said Oleg Simakov, a professor at the University of Vienna who co-led the study. "It also lets us ask where genome evolution might go next and enables us to identify key measures for the conservation of animal biodiversity." Most sequenced genomes are "drafts" that show which genes an animal has but not how they are arranged.
Chromosome-scale assemblies instead place every gene in order along complete chromosomes—they are much harder to produce, and only recently have enough animals been sequenced this way to allow a comparison across the animal kingdom. The team analyzed more than 5,800 publicly available chromosome-scale genomes spanning 4,454 species across 19 animal phyla—the largest such comparison across the animal tree of life to date. They developed a new framework, called evolutionary genome topology, that projects this enormous diversity onto a single map.
The approach revealed that genomes do not change at random. Instead, they travel along "evolutionary highways," paths revealed by hundreds of present-day species whose genomes show evidence of traveling on or "getting off" the highway at different times and rates. "For the first time, we can see thousands of genomes on a single map and trace the unique paths along which animals' DNA evolved.
Viewing the map as a whole gives us a picture of the patterns by which animal genomes have changed over time," said Darrin Schultz, who led the work as a postdoctoral researcher at the University of Vienna and is now an assistant professor at Lehigh University and Lehigh Oceans. "And if we fold the map up in a different way, we can compare how different groups of animals took different paths from each other after splitting onto different evolutionary paths." At the heart of these patterns is a process the team named "fusion-with-mixing" in an earlier study: When two chromosomes fuse, their genes intermingle in a way that cannot be undone, leaving a permanent record of the event. Because these changes run only one way, they serve as reliable markers of shared ancestry, evidence already used to reveal the sibling group to all other animals.
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