Low genetic diversity and strong but shallow population differentiation suggests genetic homogenization by metapopulation dynamics in a social spider.

Mating systems and population dynamics influence genetic diversity and structure. Species that experience inbreeding and limited gene flow are expected to evolve isolated, divergent genetic lineages. Metapopulation dynamics with frequent extinctions and colonizations may, on the other hand, deplete and homogenize genetic variation, if extinction rate is sufficiently high compared to the effect of drift in local demes. We investigated these theoretical predictions empirically in social spiders that are highly inbred. Social spiders show intranest mating, female-biased sex ratio, and frequent extinction and colonization events, factors that deplete genetic diversity within nests and populations and limit gene flow. We characterized population genetic structure in Stegodyphus sarasinorum, a social spider distributed across the Indian subcontinent. Species-wide genetic diversity was estimated over approximately 2800 km from Sri Lanka to Himalayas, by sequencing 16 protein-coding nuclear loci. We found 13 SNPs in 6592 bp (π = 0.00045) indicating low species-wide nucleotide diversity. Three genetic lineages were strongly differentiated; however, only one fixed difference among them suggests recent divergence. This is consistent with a scenario of metapopulation dynamics that homogenizes genetic diversity across the species' range. Ultimately, low standing genetic variation may hamper a species' ability to track environmental change and render social inbreeding spiders 'evolutionary dead-ends'.