The invasive marsh frogs advancing into The Netherlands carry a diverse mix of mtDNA

Abstract The marsh frog ( Pelophylax ridibundus complex) has been dubbed the number one amphibian invader in Western Europe. In The Netherlands, marsh frogs are native to the north, centre and west of the country, whereas pool frogs ( P. lessonae ) are mainly found in the south and east; their hybridogenetic hybrid, the edible frog ( P . klepton esculentus ), is distributed throughout The Netherlands. Over the past decades, the southeastern province of Limburg has been overrun by marsh frogs. To expose the invasion history of marsh frogs in The Netherlands, we mtDNA barcoded hundreds of water frogs from across the country. Throughout The Netherlands, we primarily detect pool frog mtDNA: the only mtDNA type that is expected to occur here in any native water frog, regardless of taxon, due to asymmetric introgression. By contrast, we identify four distinct marsh frog mtDNA haplotypes in Limburg, originating from four regions of the native marsh frog range: (1) Central, Eastern and Southeastern Europe, (2) western Balkan Peninsula, (3) western and northern Turkey, Iran and eastern Ukraine into Russia, up to the Ural Mountains and (4) central southern Turkey. Additionally, two presumably introduced Central, Eastern and Southeastern European marsh frog haplotypes are detected locally elsewhere in The Netherlands, within the native marsh frog range. Our results align with the broader pattern that invasive marsh frogs constitute a ‘cocktail’ of genetically diverse lineages. As the invasion front in Limburg approaches native water frog populations, we urge that conservation measures be taken promptly.