A spider species exclusive to the Canary Islands, Dysdera tilosensis, has exhibited a reduction in its DNA content by half, a finding that contradicts conventional evolutionary assumptions for island species. This case represents an unprecedented instance of genome downsizing in an animal.
The findings, published in Molecular Biology and Evolution, detail how D. tilosensis maintains high genetic diversity despite possessing a genome significantly smaller than that of its mainland relatives.
Research led by the University of Barcelona, in collaboration with the Spanish National Research Council (CSIC) and the University of Neuchâtel, involved comparing D. tilosensis, found solely on Gran Canaria, with Dysdera catalonica, a continental species.
The study revealed D. catalonica has a genome of approximately 3.3 billion base pairs, while D. tilosensis has 1.7 billion, nearly half the size. Despite the smaller genome, the Canary Islands species displayed greater genetic diversity, which counters the assumption that small, isolated populations experience reduced diversity due to genetic bottlenecks.
Chromosome counts also varied between the species: D. catalonica has four autosomes and one X sex chromosome, while D. tilosensis has six autosomes plus the X.
These findings challenge established evolutionary patterns. Traditional understanding suggests island species often expand their genomes by accumulating non-functional DNA due to decreased selection pressure. However, D. tilosensis demonstrates the opposite trend.
Genome analysis indicates the common ancestor of both spiders had a genome close to 3 billion base pairs, confirming D. tilosensis's smaller genome as a derived trait resulting from downsizing after its arrival on the Canary Islands. Genome shrinkage is less common than expansion, particularly in animals over a relatively short evolutionary period. This discovery also challenges assumptions regarding the founder effect.
Researchers suggest that strong natural selection may have persisted in the island population. Instead of adapting by adding new DNA, the spider may have undergone genomic streamlining, eliminating repetitive, non-functional sequences. This process points to non-adaptive mechanisms, where purifying selection efficiently removed redundant DNA. The team proposes that the D. tilosensis population likely remained large and stable for an extended period after colonizing Gran Canaria.