Pre-Pleistocene geological events shaping diversification and distribution of primitively segmented spiders on East Asian margins

Aim Phylogeographical research in the East Asian continent and islands (East Asian Margins, or EAM) suggests predominant Pleistocene over-water dispersal events from continent to islands, but more ancient biogeographical patterns in EAM remain obscure. We explored biogeographical histories and population genetic structures of the primitively segmented spiders, Heptathela and Ryuthela (Liphistiidae), broadly codistributed across EAM islands. To test competing biogeographical hypotheses, we looked for geohistoric events that may have shaped their distributional patterns. Location Kyushu and Ryukyus. Methods We sampled 17 Heptathela and Ryuthela species on 10 out of 12 EAM islands and sequenced fragments of two mitochondrial and three nuclear genes. We performed phylogenetic, network, time-calibrated and biogeographical analyses to identify lineages, estimate their divergence times, and reconstruct ancestral ranges. We also assessed genetic diversity and historic demography of each lineage. Results Phylogenetic origin of Heptathela + Ryuthela is estimated at 26.6-18.4 Ma (95% HPD) when EAM islands became separated from the continent. The crown ages of Heptathela (18.0-10.7 Ma) and Ryuthela (17.8-8.7 Ma) coincide with the formation of Japan Sea and Okinawa Trough respectively. A split within Ryuthela (13.9-6.0 Ma) coincides with the opening of the Kerama Gap, but a split within Heptathela (18.0-10.7 Ma) pre-dates the formation of the Tokara Gap. Speciation within Heptathela and Ryuthela is relatively recent (6.0-0.3 Ma). Population genetic and demographic results suggest interrupted gene flow within and among islands, with most species exhibiting stable past populations. Main conclusions Historic biogeographical reconstructions strongly suggest predominant vicariant origins of EAM island liphistiids. Restricted gene flow following barrier formation is the dominant speciation force in these spiders. As they never seem to cross bodies of water, their past range expansion, successive fragmentation of EAM, and other vicariant events, must have shaped the detected genetic boundaries among and within islands.