Over a number of million years, the spider (a species found only within the Canary Islands) has done something extraordinary: it has reduced its genome size by nearly half while adapting to its island environment. Despite this dramatic reduction, the genome of this species isn’t only more compact but additionally has the next level of genetic diversity than related terrestrial spiders.
Published, the research marks the primary time scientists have seen that an animal species has reduced its genome by nearly 50% throughout the means of colonizing oceanic islands.
Challenging long-held evolutionary assumptions
Traditionally, scientists believed that species that colonized islands evolved larger genomes with more repetitive DNA. This recent discovery turns that concept on its head and deepens the talk over considered one of biology's central mysteries: how and why genome size modified throughout the evolution of life.
The research was led by Julio Rojas and Sara Gerao from the Faculty of Biology and the Biodiversity Research Institute (IRBIO) on the University of Barcelona. The study's first creator, Vadim Pisrenko (UB-Irebio), worked with colleagues from the University of La Laguna, the Spanish National Research Council (CSIC), and the University of Neuchâtel in Switzerland.
Scientists have long known that genome size—the whole variety of DNA base pairs containing an organism's genetic instructions—can vary widely between species, even amongst those of comparable complexity. This recent study provides a striking example of this phenomenon and suggests that evolution can move in unexpected directions.
An evolutionary puzzle within the Canary Islands
Spiders of the genus have experienced remarkable diversity within the Canary Islands, a region often described as a natural laboratory for the study of evolution in isolation. About 50% of the 14% species within the genus, which emerged on the islands a number of million years ago, have evolved there.
Using advanced DNA sequencing tools, the researchers compared two closely related species: , which inhabit parts of Catalonia and southern France, and , native to the island of Gran Canaria.
“This species has a genome of 3.3 billion base pairs (3.3 GB, DNA letters), which is almost twice that of the species (1.7 GB). Interestingly, despite having a smaller genome, it shows more genetic diversity than the Canary Islands,” explain the members of the Evolutionary Genomics and Bioinformatics, ” Barcelona (BIB) platform.
A rare case of genome downsizing
Genomic sequencing also revealed that a haploid chromosome number is 4 autosomes plus one X chromosome, while it has six autosomes and one X chromosome.
“The genome downsizing of the spider D. tuloscens, associated with the colonization process of the Canary Islands, is one of the first documented cases of rigorous genome downsizing using high-quality reference genomes,” says Professor Julio Rojas, Director of the Evolutionary Genomics and Bioinformatics Research Group.
“This phenomenon is now being described in detail for a phylogenetically closely related animal species for the first time,” he added.
Searching for causes of decline within the genome
Among such evolutionarily similar species, which share similar habitats and diets, “differences in genome size cannot be easily attributed to environmental or behavioral factors,” says Professor Sarah Gerau. “Phylogenetic analysis—Gerau continues—combined with flow cytometry measurements, shows that the common ancestor had a very large genome (about 3 GB). This suggests that there was a drastic genome reduction during or after the arrival of the islands.”
This result’s clearly counterintuitive for 2 reasons. On the one hand, although less frequent in animals, probably the most common pattern is a rise in genome size by whole-genome duplication, “especially in plants, where the appearance of polyploid species (with multiple chromosome ends) is common. In contrast, such a rapid decrease in genome size over a relatively short period of time is more rare.”
Secondly, these results contradict theories that argue that on islands, the founder effect – the means of colonization by a small number of people – results in a decrease in selective pressure and, because of this, the genome should grow to be an increasing number of wealthy 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 Pisrenko. This pattern suggests the presence of a non-adaptive mechanism, “by which the population in the Canary Islands would have remained relatively numerous and stable for a long period of time. This would have made it possible to maintain a strong selective pressure and thereby eliminate unnecessary DNA,” says Pisrenko.
Shedding light on the genomic mysteries of evolution
It continues to be unclear why some species have collected large amounts of repetitive DNA while others develop smooth genomes. These findings may help explain this long-standing mystery in evolutionary biology.
Some scientists suggest that changes in genome size reflect direct adaptations to the environment. Others argue that these changes lead to a balance between the buildup of repetitive elements (equivalent to transposons) and their elimination by natural selection.
“This study supports the idea that, rather than direct adaptation, genome size in these species depends primarily on the balance between accumulation and removal of this repetitive DNA,” the researchers concluded.