Creature that has chomped down on its DNA by half is evolving in reverse

Scientists have discovered a spider species with a smaller genome while maintaining high genetic diversity, in a first-of-its-kind case. The creature on the Canary Islands is changing the way evolution is believed to work. Throughout its time on Earth, Dysdera tilosensis has reduced its DNA content by half, a genome downsizing seen for the first time in an animal. The island spider was compared to its mainland counterpart, and the two closely related spiders were found to be extremely different. The study was published in Molecular Biology and Evolution by a team of researchers from the University of Barcelona. The scientists collaborated with the Spanish National Research Council (CSIC) and the University of Neuchâtel to study this unique spider that has managed to do the unthinkable. They used high-quality sequencing methods to compare Dysdera catalonica, a continental species found in Catalonia and southern France, and D. tilosensis, which lives only on Gran Canaria.

Smaller genome, greater genetic diversity

The researchers were stunned to see that D. catalonica has a genome of about 3.3 billion base pairs, while D. tilosensis carries just 1.7 billion. Despite carrying almost half the genome, the latter does not display reduced genetic diversity. Professor Julio Rozas wrote, "Despite having a smaller genome, the species from the Canary Islands shows greater genetic diversity." The findings contradict the belief that creatures with small, isolated populations experience hurdles in genetic growth and a drop in diversity. The number of chromosomes was also found to differ, with D. catalonica recording four autosomes and one X sex chromosome, while D. tilosensis has six autosomes plus the X.

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D. tilosensis defies the basic rule of evolution

Evolutionary biology has long believed that species living in isolated spots, such as islands, accumulate non-functional DNA and expand their genomes over several years. However, the reverse is in fact true for D. tilosensis. Professor Sara Guirao says that the two spiders had a common ancestor, and genome analysis and flow cytometry measurements indicate that it had a genome close to 3 billion base pairs. Since D. tilosensis carries just 1.7 billion of these, it proves that the reduction is acquired and has happened over time. The finding shows that instead of adapting by adding new DNA, the spider likely underwent a process of genomic streamlining, which caused repetitive sequences that had no function to be eliminated.