Impact of Pleistocene Eustatic Fluctuations on Evolutionary Dynamics in Southeast Asian Biodiversity Hotspots.

Pleistocene climatic fluctuations (PCF) are frequently highlighted as important evolutionary engines that triggered cycles of biome expansion and contraction. Although there is ample evidence of the impact of PCF on biodiversity of continental biomes, the consequences in insular systems depend on the geology of the islands and the ecology of the taxa inhabiting them. The idiosyncratic aspects of insular systems are exemplified by the islands of the Sunda Shelf in Southeast Asia (Sundaland), where PCF-induced eustatic fluctuations had complex interactions with the geology of the region, resulting in high species diversity and endemism. Emergent land in Southeast Asia varied drastically with sea-level fluctuations during the Pleistocene. Climate-induced fluctuations in sea level caused temporary connections between insular and continental biodiversity hotspots in Southeast Asia. These exposed lands likely had freshwater drainage systems that extended between modern islands: the Paleoriver Hypothesis. Built upon the assumption that aquatic organisms are among the most suitable models to trace ancient river boundaries and fluctuations of landmass coverage, the present study aims to examine the evolutionary consequences of PCF on the dispersal of freshwater biodiversity in Southeast Asia. Time-calibrated phylogenies of DNA-delimited species were inferred for six species-rich freshwater fish genera in Southeast Asia (Clarias, Channa, Glyptothorax, Hemirhamphodon, Dermogenys, Nomorhamphus). The results highlight rampant cryptic diversity and the temporal localization of most speciation events during the Pleistocene, with 88% of speciation events occurring during this period. Diversification analyses indicate that sea-level-dependent diversification models poorly account for species proliferation patterns for all clades excepting Channa. Ancestral area estimations point to Borneo as the most likely origin for most lineages, with two waves of dispersal to Sumatra and Java during the last 5 myr. Speciation events are more frequently associated with boundaries of the paleoriver watersheds, with 60%, than islands boundaries, with 40%. In total, one-third of speciation events are inferred to have occurred within paleorivers on a single island, suggesting that habitat heterogeneity and factors other than allopatry between islands substantially affected diversification of Sundaland fishes. Our results suggest that species proliferation in Sundaland is not wholly reliant on Pleistocene sea-level fluctuations isolating populations on different islands. [Dispersal; diversification; eustatic fluctuations; freshwater fishes; insular systems; Milankovitch cycles; paleoenvironments; vicariance.].

Disentangling the taxonomy of the subfamily Rasborinae (Cypriniformes, Danionidae) in Sundaland using DNA barcodes.

Sundaland constitutes one of the largest and most threatened biodiversity hotspots; however, our understanding of its biodiversity is afflicted by knowledge gaps in taxonomy and distribution patterns. The subfamily Rasborinae is the most diversified group of freshwater fishes in Sundaland. Uncertainties in their taxonomy and systematics have constrained its use as a model in evolutionary studies. Here, we established a DNA barcode reference library of the Rasborinae in Sundaland to examine species boundaries and range distributions through DNA-based species delimitation methods. A checklist of the Rasborinae of Sundaland was compiled based on online catalogs and used to estimate the taxonomic coverage of the present study. We generated a total of 991 DNA barcodes from 189 sampling sites in Sundaland. Together with 106 previously published sequences, we subsequently assembled a reference library of 1097 sequences that covers 65 taxa, including 61 of the 79 known Rasborinae species of Sundaland. Our library indicates that Rasborinae species are defined by distinct molecular lineages that are captured by species delimitation methods. A large overlap between intraspecific and interspecific genetic distance is observed that can be explained by the large amounts of cryptic diversity as evidenced by the 166 Operational Taxonomic Units detected. Implications for the evolutionary dynamics of species diversification are discussed.