Riverine barriers to gene flow in a salamander with both aquatic and terrestrial reproduction

The riverine barrier hypothesis (RBH) posits that rivers comprise geographical barriers to gene flow for terrestrial organisms, thus promoting genetic differentiation between populations. Here, we explored the RBH on larviparous and pueriparous populations of the live-bearing fire salamander (Salamandra salamandra). While larviparous fire salamanders exhibit a semi-aquatic life cycle (females deposit pre-metamorphic larvae on water), pueriparous salamanders present a fully terrestrial life cycle (females deliver terrestrial juveniles) and, therefore, a greater independence from water for survival and reproduction. We performed a fine-scale sampling of opposite transects in 11 rivers (six and five for larviparous and pueriparous populations, respectively) to test the hypothesis that rivers are more effective barriers for pueriparous salamanders due to their terrestrial life cycle. We carried out individual- and population-based genetic analyses using 14 microsatellites and a mitochondrial marker to examine the extent to which rivers hinder short- and long-term gene flow. We found that rivers are semi-permeable obstacles for both larviparous and pueriparous salamanders, although they appear to be more effective barriers for the latter when rivers with similar attributes are compared. We also found that river width and possibly the presence of crossing structures may influence the genetic barrier effects of rivers in fire salamanders. This is one of the very few studies in amphibians showing how different reproductive strategies influence the barrier effects imposed by rivers.