Effects of landscape features and flooding on the genetic structure of a small wetland rodent, the harvest mouse ( Micromys minutus )

The natural patchiness of wetlands and flooding events are likely to strongly affect the genetic structure of their terrestrial species. However, these effects are not well understood yet, especially for small mammals. We investigated at different spatial scales the genetic structure of the harvest mouse (Micromys minutus), a threatened small mammal strongly tied to wetlands, and the effects on gene flow of flooding and of the different types of landscape elements composing a wetland. 309 harvest mice were sampled in eight sites in Western Europe. Their genetic structure and diversity at 15 microsatellite loci were analyzed at a regional spatial scale and at a local scale, where the resistance of land cover and flooding to gene flow was also assessed, with the optimization procedure implemented in ResistanceGA. At a regional scale, our study revealed a strong genetic differentiation between populations from Northern Europe to the Mediterranean Sea. At the local scale, in a flooded wetland in France, the species exhibited a large genetic cluster over at least 45 km2, in spite of a large river crossing it. Winter floods explained genetic structure better than landscape features alone, with a stronger resistance to gene flow in reed beds where vegetation level above water was high: contrary to meadows, from which individuals are forced to escape, reed beds can be “golden prisons”, i.e. refuges during floods but with very low possibility of movements. Several parameters influencing the functioning of a flooded population of harvest mouse are here highlighted that can also be useful for the development of plans to safeguard wetland ecosystems.