Genomic signatures of positive selection and relaxed selective constraints associated with convergent dietary specialisation

Island radiations provide an ideal setting for studying the molecular foundations of convergent evolution. The rapid diversification of Dysdera spiders in the Canary Islands is a prime example, with these arachnids repeatedly evolving a specialization on woodlice. This shift brings about morphological, metabolic, and behavioral adaptations that enable the exploitation of heavy-metal-rich prey.

In this study, researchers examined two independent instances of dietary specialization on different Canary Islands. By analyzing six high-quality genome assemblies, including three newly generated for this research, they sought to uncover genomic signals of positive selection and relaxed selection in protein-coding genes, upstream regulatory regions, and gene family turnover rates.

Signatures of positive selection were detected in both specialist spiders, while there was a twofold increase in gene loss rates compared to their generalist ancestors. Crucially, positive selection focused on a smaller, functionally cohesive set of genes, potentially involved in detoxification through heavy metal excretion. Conversely, relaxation of selection impacted a wider array of genes linked to functions that may have become superfluous under the new dietary regime.

Moreover, the researchers observed contractions in gene families related to vesicular transport, likely indicating a diminished reliance on vesicle-mediated metal accumulation. Overall, these findings indicate that similar ecological pressures can prompt parallel, yet distinct, adaptive and non-adaptive responses. This suggests partially predictable evolutionary trajectories in island radiations.