Natracine Snakes Came Out of Asia to Colonize Africa and the Western Hemisphere

Natracines, often called water snakes, have a Holarctic distribution with about 29 genera and 210 species found across Asia, Europe, North Africa, sub-Saharan West Africa, as well as North and Central America. Many of these snakes are associated with aquatic habitats and feed on fish and frogs, some have adapted to xeric conditions and may feed on invertebrates or mammals. Molecular studies suggest the natricines evolved from a colubroid ancestor in the Eocene/Oligocene, suggesting they most likely encountered the same environmental conditions as the ratsnakes, skinks and crotalinae snakes as they dispersed across the globe. Molecular studies suggest the New World natricines form a monophyletic group (the Thamnophiini) implying they originated from a single dispersal event from an Old World ancestor. Edmund Malnate (1960) had previously hypothesized natricines originated in Asia and dispersed from there to Europe, Africa, Australia and North America, and that Natrix (sensu lato) dispersed to North America over the Bering Strait. Given the timing of origin of the global natricines in Eocene, it is also possible that they could have colonized the New World via the Atlantic through the Thulean Land Bridge or though the Pacific via Beringia. However, dispersals after the first half of the Cenozoic are unlikely to have occurred via the Thulean Land Bridge, due to declining environmental conditions suitable for ectotherms. Crucially, relationships among Old World genera and Thamnophiini are unclear, and divergence dating and ancestral area reconstructions are needed to understand basic biogeographic patterns relating to areas of origin, dispersal, and the rise of the several regional assemblages.

Pen Guo of the College of Life Science at Yibin University in China and a researh team of other Chinese workers, as well as two Americans (R. Alexaner Pyron and Frank Burbrink) examine the temporal and geographic origins of natricine snakes in order to provide a comparison to the patterns and timing of origin of the ratsnakes, skinks, and crotalines, which all demonstrate similar timing and routes of dispersal through Beringia to the New World. In addition to sequencing several new species, they test hypotheses about the timing and area of origin of natricines in the Old W.orld . They also attempt to understand the causes of the Holartic distribution, which for the other groups of squamates have suggested a single unidirectional dispersal from Asia through Beringia to the New World during the Oligocene and Miocene (the Cenozoic Beringial Dispersal Hypothesis).

Using a combination of six mitochondrial gene fragments (12S RNA, cyt b, ND1, ND2, ND4 and CO1) and one nuclear gene (c-mos) from 22 genera the authors infered phylogenetic relationships among natricine snakes and examine the date and area of origin of these snakes. The phylogenetic results indicate the subfamily Natricinae is strongly supported as monophyletic including a majority of extant genera, and a poorly known and previously unassigned species Trachischium monticola; two main clades are inferred within Natricinae, one containing solely taxa from the Old World and the other comprising taxa from a monophyletic New World group with a small number of Old World relatives. They found that within the Old World clade, the genera Xenochrophis and Amphiesma are apparently not monophyletic. Divergence dating and ancestral area estimation indicate that the Old World natricines originated in tropical Asia during the later Eocene or the Oligocene. Additionally they recover two major dispersals events out of Asia, the first to Africa in the Oligocene, 28 million years ago and the second to the Western Palearctic and the New World at 27 million years ago. This date is consistent with the dispersal of numerous other Old World groups into the New World.

Trachischium monticola, a fossorial, worm-eating natracine
snake from South Asia

Citation

Peng Guo, Qin Liu, Yan Xu, Ke Jiang, Mian Hou, Li Ding, R. Alexander Pyron, Frank T. Burbrink. 2012. Out of Asia: Natricine snakes support the Cenozoic Beringian Dispersal Hypothesis. Molecular Phylogenetics and Evolution, 63:825-833.