Scientists have discovered that the spider Dysdera tilosensis, found only on the Canary Islands, has reduced its genome size by nearly half in just a few million years. This downsizing defies traditional theories predicting larger genomes in island species. Despite the smaller genome, the spider shows greater genetic diversity than its mainland relatives.
The spider Dysdera tilosensis, endemic to Gran Canaria in the Canary Islands, has undergone a remarkable genomic reduction since colonizing the islands a few million years ago. Researchers compared its genome to that of the mainland species Dysdera catalonica, revealing that D. tilosensis has 1.7 billion base pairs (1.7 Gb), almost half the 3.3 Gb of D. catalonica. This is the first documented case of an animal species halving its genome during oceanic island colonization, as detailed in a study published in Molecular Biology and Evolution.
The Canary Islands serve as a natural laboratory for evolution, hosting nearly 50 endemic Dysdera species, which represent about 14% of the genus. The research team, led by Julio Rozas and Sara Guirao from the University of Barcelona's Faculty of Biology and Biodiversity Research Institute (IRBio), included first author Vadim Pisarenco and collaborators from the University of La Laguna, the Spanish National Research Council (CSIC), and the University of Neuchâtel in Switzerland. Using advanced DNA sequencing, they found that D. tilosensis has a haploid chromosome number of six autosomes plus one X chromosome, compared to four autosomes plus one X in D. catalonica.
"The species D. catalonica has a genome of 3.3 billion base pairs (3.3 Gb), which is almost double that of the species D. tilosensis (1.7 Gb). Interestingly, despite having a smaller genome, the species from the Canary Islands shows greater genetic diversity," explains Professor Julio Rozas, director of the Evolutionary Genomics and Bioinformatics research group at the University of Barcelona.
Phylogenetic analysis and flow cytometry indicate the common ancestor had a large genome of about 3 Gb, with the reduction occurring during or after island arrival. This challenges long-held assumptions that island colonization leads to larger genomes with more repetitive DNA due to reduced selective pressure. "In the study, we observed the opposite: island species have smaller, more compact genomes with greater genetic diversity," says doctoral student Vadim Pisarenco. The findings suggest non-adaptive mechanisms, such as strong selective pressure maintaining stable populations, eliminated unnecessary DNA.
Professor Sara Guirao notes that differences in genome size between these similar species cannot be easily attributed to ecological or behavioral factors. The study supports the idea that genome size depends on a balance between accumulation and removal of repetitive elements like transposons, rather than direct environmental adaptation. This discovery deepens the debate on why some species evolve streamlined genomes while others accumulate excess DNA.