A common toad hybrid zone that runs from the Atlantic to the Mediterranean

We document the distribution of the common toad Bufo bufo and the spined toad B. spinosus at their contact zone across France with data from a mitochondrial DNA RFLP assay, complementing similar work including nuclear markers in the northwest and southeast of France and in Italy. We also reconstruct geographical clines across the species' contact zone in central France. Bufo bufo is found in the north-eastern half of France. Bufo spinosus is found in the south-western complement. The contact zone they form runs from the Atlantic coast near Caen, France, to the Mediterranean coast near Savona, Italy, and has a length of over 900 km. In central France B. bufo and B. spinosus engage in a hybrid zone with a unimodal genetic signature. Hybrid zone width is ca. 10 km at mitochondrial DNA and averages at 61 km for four nuclear loci. The hybrid zone is distinctly asymmetric with a signature of B. spinosus in B. bufo and not the other way round. We attribute this observation to B. bufo moving southwards at the expense of B. spinosus, with introgression in the direction of the advancing species. We noted substantial geographic variation in characters for species identification. Morphological species identification performs well in France, but breaks down in Italy. Mitochondrial DNA is inconclusive in south-eastern France and Italy. The nuclear genetic markers perform consistently well but have not yet been applied to the zone in full. Possible, but surely heterogeneous ecological correlates for the position of the hybrid zone are mountains and rivers.