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| The sequencing of the genomes of a spider from the mainland (Dysdera catalonica, left) and one from the Canary Islands (Dysdera tilosensis, left) opens a new perspective for understanding how genome size evolves in similar species, an enigma that has baffled the scientific community for years. Photo Credit: Courtesy of University of Barcelona |
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.
Spiders of the genus Dysdera have diversified spectacularly in the habitats of the Canary Islands.
Image Credit: Courtesy of University of Barcelona
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 analyze 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 analyzed 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.
“The genome downsizing of the spider D. tilosensis, associated with the colonization process of the Canary Island, is one of the first documented cases of drastic genome downsizing using high-quality reference genomes,” says Professor Julio Rozas, director of the Evolutionary Genomics and Bioinformatics research group.
“This phenomenon is now being described for the first time in detail for phylogenetically closely related animal species,” he continues.
How can genome reduction be explained?
In such evolutionarily similar species, which share similar habitats and diet, “differences in genome size cannot easily be attributed to ecological or behavioral factors,” says Professor Sara Guirao. “Phylogenetic analysis — Guirao continues — combined with flow cytometry measurements, reveals that the common ancestor had a large genome (about 3 Gb). This indicates that the drastic genome reduction occurred during or after the arrival on the islands.”
This result is clearly paradoxical for two reasons. On the one hand, although less frequent in animals, the most common pattern is the increase in genome size via whole-genome duplications, “especially in plants, where the appearance of polyploid species (with multiple chromosome endowments) is common. In contrast, such sharp reductions in genome size over a relatively short period of time are much rarer,” says Guirao.
Secondly, the findings contradict theories that argue that, on islands, the founder effect — the process of colonization by a small number of individuals — leads to a reduction in selective pressure and, as a result, genomes should be larger and richer in repetitive elements.
“In the study, we observed the opposite: island species have smaller, more compact genomes with greater genetic diversity,” says doctoral student Vadim Pisarenco. This pattern suggests the presence of non-adaptive mechanisms, “whereby populations in the Canary Islands would have remained relatively numerous and stable for a long time. This would have made it possible to maintain a strong selective pressure and, as a consequence, eliminate unnecessary DNA,” says Pisarenco.
bove, from left to right, Carlos Tinedo, Alejandro Sánchez-Gracia, Vadim A. Pisarenco Julio Rozas and Sara Guirao-Rico. Below, from left to right, Andrea Barrio Silvia García-Juan, Marta Olivé-Muñiz and Carlos Eduardo Arboleda.
Photo Credit: Courtesy of University of Barcelona
Deciphering one of the great enigmas in evolutionary biology
It is still a mystery why, in similar species, some genomes accumulate numerous repetitive DNA sequences while others are more compact. The study could provide a fresh perspective on solving this open question in evolutionary biology.
According to some hypotheses, these changes in the genome are of direct adaptive value. Other explanations propose non-adaptive mechanisms, in which genome size is the result of a balance between the accumulation of repetitive elements (such as transposons) and their removal by purifying selection.
“This study supports the idea that, rather than direct adaptation, genome size in these species depends primarily on a balance between the accumulation and removal of this repetitive DNA,” the researchers conclude.
Title: How Did Evolution Halve Genome Size During an Oceanic Island Colonization?
Authors: Vadim A Pisarenco, Adrià Boada-Figueras, Marta Olivé-Muñiz, Paula Escuer, Nuria Macías-Hernández, Miquel A Arnedo, Pablo Librado, Alejandro Sánchez-Gracia, Sara Guirao-Rico, and Julio Rozas
Source/Credit: University of Barcelona
Reference Number: gen101325_01