Genetic pollution risk in newts at the far end of the speciation continuum

Invasive species can erase native biodiversity through genetic pollution: the diminishing of native genetic variation via anthropogenic hybridization. We investigated hybridization between the invasive crested newt Triturus anatolicus and the native marbled newt T. marmoratus in Montnegre i el Corredor Natural Park (Catalonia, Spain). Using genome-wide target sequence capture (NewtCap) and mitochondrial DNA barcoding, we determine hybrid viability, hybrid class composition, and hybridization directionality, and compared invasive genotypes to reference populations across the Turkish range to infer provenance. We detect numerous F1 hybrids and a relatively high frequency of backcrosses compared to the natural marbled and crested newt hybrid zone in France. All F1 hybrids carry T. anatolicus mitochondrial DNA, indicating asymmetric hybridization with T. anatolicus mothers. Later-generation hybrids demonstrate hybrid fertility. All later-generation hybrids are products of advanced crosses with T. marmoratus. Nuclear and mitochondrial data trace the invasive population to Lake Abant in northwestern Türkiye. The observed backcrossing toward the native species indicates that invasive alleles can seep into the native gene pool, meaning the prerequisites for genetic pollution are in place. Our study underscores the importance of timely genome-wide monitoring when a closely related species is introduced into the range of a native species, to inform conservation management in order to prevent the erosion of native genetic diversity.