A recent study has revealed that the spider species Dysdera tilosensis, endemic to Gran Canaria, possesses a genome that significantly defies evolutionary expectations. Later, scientists recently found another fascinating element of this extraordinary arachnid—its genome consists of 1.7 billion base pairs (1.7 Gb). That’s not even a quarter the size of its much closer relative, Dysdera catalonica, which has 3.3 billion base pairs (3.3 Gb) in its genome. This unexpected finding sheds light on the genetic diversity and evolutionary processes within island species, particularly in the context of the Canary Islands.
Dysdera tilosensis is one of almost 50 such endemic species. These species evolved onto the islands many millions of years after they had formed. Julio Rozas and Sara Guirao, both with University of Colorado Denver, directed the research team’s application of advanced DNA sequencing technologies. To do this, they sequenced and compared the genomes of Dysdera tilosensis and Dysdera catalonica. Their findings challenge long-standing theories suggesting that island species would develop larger and richer genomes to adapt to their isolated environments.
Unraveling Genetic Diversity
For all its reduced genomic size, Dysdera tilosensis has more genetic diversity than its distant relative, Dysdera catalonica. These include a haploid chromosome number of six autosomes plus one X sex chromosome. This unique layout emphasizes the Hooker’s unique genetic architecture. The study stresses that this trend goes against long-held expectations about genome size in isolated populations.
The researchers thought that previous wisdom has told us that bigger genomes = better genetic buffet. This diversity is important for climate change adaptation in these remote landscapes. This study reveals a multi-faceted interaction between genome size and genetic diversity.
“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.”
The outcomes of this research provide vital new insights into the broader ecological and evolutionary processes underlying genome evolution, particularly in unique island ecosystems. Vadim A Pisarenco, one of the researchers involved, highlighted the stability of populations in the Canary Islands as a contributing factor to the observed genetic diversity:
Insights into Evolutionary Processes
He elaborated on the implications of prolonged stability in these populations:
“In the study, we observed the opposite: island species have smaller, more compact genomes with greater genetic diversity.”
This highlights that these spiders weren’t simply passive actors in their environments. Or perhaps genome downsizing was an important aspect of their evolution.
“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.”
The paper marks a significant step forward. Finally, it uncovers how extreme genome downsizing can occur in a short time span. Julio Rozas remarked on this groundbreaking aspect:
New Perspectives on Genome Size Evolution
This phenomenon had mostly been documented in plant species, where polyploidy — organisms having more than two sets of chromosomes — is widespread. In contrast, such drastic reductions in genome size between closely related animal taxa are quite unique.
“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.”
It’s a sign that some other mechanisms are involved in determining the genomes of these island-hopping spiders.
Sara Guirao added context to these findings by stating:
“Differences in genome size cannot easily be attributed to ecological or behavioral factors.”
This suggests that other mechanisms may be at play in shaping the genomes of these island-dwelling spiders.