Tree frogs with foreign sex chromosomes are less fit

During the last glaciation, a huge ice shield reached up to the region of today's Berlin. By the time it started to melt about 20.000 years ago, it enabled a gradual re-colonization of the northern latitudes by many plant and animal species. Often, they took different colonization routes around the mountain ranges, for example the Carpathians - with astonishing outcomes for a special kind of re-unification that, for instance, happens in Poland: In the region of the Vistula River, two evolutionar