The position, shape and dynamics of the hybrid zone between the Danube and Italian crested newt based on genome-wide data, with a cautionary tale on the discovery of a ‘distinct’ mtDNA lineage

When species that are reproductively incompletely isolated meet in parapatry and reproduce, a hybrid zone is established. The strength of reproductive isolation between these species and the dynamics of their ranges can be inferred from the extent and direction of gene flow across the hybrid zone. We determine the position, shape and dynamics of the relatively poorly studied hybrid zone between the Danube (Triturus dobrogicus) and Italian (T. carnifex) crested newts, using genome-wide nuclear DNA data (24 species-diagnostic SNPs) and an mtDNA marker (ND4). Overall, the hybrid zone is narrow, signifying strong selection against hybrids. In the north of the hybrid zone, asymmetric introgression of T. dobrogicus alleles into T. carnifex indicates postglacial expansion by T. carnifex. We also report a highly distinct haplotype lineage that presumably reflects ‘cryptic pseudogenes’; while the relevant sequences do not bear the hallmarks of a pseudogene, their apparent distinctiveness is not supported by an independent mtDNA marker (ND2).