Mitochondrial tracing of amphibian introductions amid cyto-nuclear discordance: a case study of two allochthonous populations of green toads (Bufotes viridis) in France

Abstract Taxonomic identification of introduced amphibian populations – a crucial first step for conservation management – often relies on mitochondrial DNA (mtDNA) barcoding, which can be intricate with recent histories of introgressive hybridization. The European green toad ( Bufotes viridis ) is characterized by cryptic phylogeographic subspecies and extensive cyto-nuclear discordance, particularly between European B. v. viridis and Asian B. v. sitibundus . Using an extensive reference dataset (2440 individuals from 730 localities) of two mtDNA genes, and haplotype-level analysis, we identified two well-established allochthonous green-toad populations in eastern and southern France. Both are assigned to B. v. viridis , despite distinct mitochondrial lineages: a typical viridis haplotype and a sitibundus haplotype largely associated with B. v. viridis ; their native distributions suggest independent origins. This case shows that mtDNA barcoding can remain informative when native diversity is well characterized and highlights green toads as a pioneer species highly adaptable to human-modified landscapes that warrants attention in bioinvasion monitoring.