In a few million years, the spider Dysdera tilosensis - a species endemic to the Canary Islands - has reduced the size of its genome by half during the process of colonization and adaptation to its natural habitat. In addition to being smaller, this genome is more compact and contains more genetic diversity than that of other similar continental spiders. The discovery, published in the journal Molecular Biology and Evolution , reveals for the first time how an animal species managed to reduce its genome by almost half during oceanic island colonization.
This study contradicts the more traditional evolutionary view - on island-colonizing species, whose genomes are larger and often have more repetitive elements - and expands the scientific debate on a major puzzle in evolutionary biology: how and why does genome size change during the evolution of living beings?
The study is led by Julio Rozas and Sara Guirao, experts from the Faculty of Biology and the Biodiversity Research Institute (IRBio) of the University of Barcelona. The paper, whose first author is Vadim Pisarenco (UB-IRBio), also involves teams from the University of La Laguna, the Spanish National Research Council (CSIC) and the University of Neuchâtel (Switzerland).
This research offers a surprising perspective to explain a phenomenon that has puzzled scientists for decades: the size of the genome - the total number of DNA base pairs encoding an organism's genetic information - varies enormously between species, even those with similar biological complexity.
A smaller genome in an island species: an evolutionary paradox?
Spiders of the genus Dysdera have diversified spectacularly in the habitats of the Canary Islands. This archipelago is considered a veritable natural laboratory in which to analyse how species and their genomes evolve in a context of geographical isolation. Nearly 50 endemic species - 14% of all the species of this genus described in the world - have emerged since the islands were formed a few million years ago.
Applying advanced DNA sequencing technologies, the team has analysed the genomes of two evolutionarily close spider species: Dysdera catalonica, a continental species, present in the northern half of Catalonia and southern France, and D. tilosensis, which is endemic to the island of Gran Canaria.
"The species D. catalonica has a genome of 3.3 billion base pairs (3.3 Gb, the letters of DNA), 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," says Julio Rozas, professor at the UB's Department of Genetics, Microbiology and Statistics, director of the Evolutionary Genomics and Bioinformatics research group and member of the board of directors of the Bioinformatics Barcelona (BIB) platform.
Genomic sequencing also reveals that D. catalonica has a haploid chromosome number of four autosomes and one X sex chromosome, while D. tilosensis has six autosomes plus the X chromosome.
