Reproductive isolation is crucial for the process of speciation to progress. Sex chromosomes have been assigned a key role in driving reproductive isolation but empirical evidence from natural population processes has been restricted to organisms with degenerated sex chromosomes such as mammals and birds. Here we report restricted introgression at sex-linked compared to autosomal markers in a hybrid zone between two incipient species of European tree frog, Hyla arborea and H. orientalis, whose homologous X and Y sex chromosomes are undifferentiated. This large X-effect cannot result from the dominance or faster-X aspects of Haldane's rule, which are specific to degenerated sex chromosomes, but rather supports a role for faster-heterogametic-sex or faster-male evolutionary processes. Our data suggest a prominent contribution of undifferentiated sex chromosomes to speciation.

Department of Ecology and Evolution University of Lausanne Biophore CH 1015 Lausanne Switzerland

Department of Evolutionary Biology and Conservation of Vertebrates Wrocław University Sienkiewicza 21 50 335 Wrocław Poland

Department of Population Biology Institute of Vertebrate Biology Academy of Sciences of the Czech Republic External research facility Studenec 675 02 Koněšín Czech Republic

Laboratory of Behavioral Ecology and Evolution School of Biological Sciences Seoul National University 151 747 Seoul Republic of Korea

Leibniz Institute of Freshwater Ecology and Inland Fisheries Müggelseedamm 301 D 12587 Berlin Germany

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