Genetic relationships and diversity patterns within the invasive range of the Mediterranean Painted Frog.
Range expansions are an intrinsic part of the biogeography of most species. Among them, some invasive species represent a paradigmatic case in which local introductions are followed by rapid expansions in several directions. Theoretically, this creates a recursive founder effect, leading to sustained losses of genetic diversity and - through stochastic changes in allele frequency - to a population structure that mirrors invasion history. Here, we use restriction site-associated DNA (RAD) markers to detect these patterns in the invasive range of the Mediterranean Painted Frog (Discoglossus pictus) in Europe. We identify and genotype a large panel of loci using genotyping-by-sequencing in several sites along its two main directions of invasion - northwards and southwards. Although the frog has a few translocated isolated populations, most of the invasive range is contiguous, allegedly corresponding to a self-sustained expansion from a single introduction point in Banyuls de la Marenda (Southern France) more than a century ago. As expected, the highest genetic diversity was found in this location, with progressive losses of genetic diversity north- and southwards. Genetic similarities among sampling sites also coincided with predictions according to the documented process of expansion, showing a latitudinal population structure matching predictions according to geographic distance among locations. This poses the two opposite expansion directions as independent processes of loss of genetic diversity and creation of population structure, that is, two different cases of range expansion which are ready for further examinations.