How Did Evolution Halve Genome Size During an Oceanic Island Colonization?

Area of Science:

• Genomics
• Evolutionary Biology
• Island Biogeography

Background:

• The genus Dysdera, or red devil spiders, exhibit remarkable diversification on the Canary Islands.
• Insular species possess genomes approximately half the size of their mainland relatives, presenting a unique model for genome size evolution studies.

Purpose of the Study:

• To investigate the forces driving genome size reduction in island-dwelling Dysdera spiders.
• To resolve debates on the roles of adaptive and nonadaptive mechanisms in shaping genome size.

Main Methods:

• Comprehensive genomic analyses using three high-quality chromosome-level assemblies.
• Comparative analysis of genomic elements, including intronic, intergenic regions, and transposable elements (TEs).

Main Results:

• Significant genome size reduction observed in insular Dysdera species, affecting all genomic components.
• Transposable element (TE) loss is the primary driver of genome contraction, with autosomes showing disproportionate reduction compared to the X chromosome.
• Island species display paradoxically higher nucleotide diversity, recombination rates, and evidence of intensified natural selection, suggesting larger long-term effective population sizes.

Conclusions:

• Genome size reduction in Canary Island Dysdera is largely driven by nonadaptive processes, specifically purifying selection against deleterious DNA and TE insertions.
• Findings support the nonadaptive mutational hazard hypothesis as the main mechanism for genome size reduction in this model system.