Genome Reduction in Island Colonizing Species

In a study published in the journal Molecular Biology and Evolution, scientists have, for the first time, observed a genome reduction of nearly 50% in a species of animal during its colonization of oceanic islands. This discovery challenges traditional assumptions about genome evolution and contributes to the ongoing discussion of a central mystery in biology: how and why genome size changes during the evolution of life.

Challenging Traditional Evolutionary Assumptions

Scientists have long believed that species colonizing islands tend to develop larger genomes with more repetitive DNA. However, this new discovery calls this assumption into question and raises inquiries about how genome size changes during evolution.

This research was led by Julio Rosas and Sara Girau from the School of Biology and the Institute of Biological Research at the University of Barcelona, in collaboration with researchers from the University of La Laguna, the Spanish National Research Council, and the University of Neuchâtel in Switzerland.

An Evolutionary Puzzle in the Canary Islands

Spiders of the genus Dysdera have shown remarkable diversity in the Canary Islands, a region considered a natural laboratory for studying evolution in isolation. Approximately 50 endemic species of Dysdera have evolved there since the islands emerged a few million years ago.

Using advanced DNA sequencing tools, researchers compared two closely related species: Dysdera catalonica, found in parts of Catalonia and southern France, and D. tilosensis, found on the island of Gran Canaria.

A Rare Case of Genome Reduction

Genomic sequencing revealed that D. catalonica has four haploid chromosomes plus one X chromosome, while D. tilosensis has six chromosomes and one X chromosome.

Professor Julio Rosas explains that the genome reduction in the D. tilosensis spider associated with its colonization of the Canary Islands is one of the first documented cases of radical genome reduction using high-quality reference genomes.

Investigating the Causes of Genome Reduction

In evolutionarily similar species that share habitats and diets, differences in genome size cannot easily be attributed to environmental or behavioral factors. Evolutionary analyses paired with flow cytometry measurements suggest that the common ancestor had a large genome, indicating that genome reduction occurred during or after reaching the islands.

This result appears contradictory for two reasons: on one hand, the more common pattern is an increase in genome size through whole-genome duplication, and on the other hand, the findings contradict theories suggesting that colonization reduces selective pressure.

Conclusion

It remains unclear why some species accumulate large amounts of repetitive DNA while others develop simpler genomes. These discoveries may help clarify this longstanding puzzle in evolutionary biology. Scientists propose that changes in genome size reflect a balance between the accumulation and removal of repetitive elements through natural selection.