The limits of mtDNA analysis for determining the provenance of invasive species: a midwife toad example

Abstract

For most if not all European herpetofauna, range-wide mtDNA phylogeographies have been published. This facilitates establishing the provenance of introduced populations. However, precision is contingent on the spatial genetic structure across the range of the taxon under study and, in particular, from where within that range the introduction was sourced. In the Netherlands, the common midwife toad, Alytes obstetricans, only naturally occurs in the extreme southeast and is on the decline there. Yet, introduced populations thrive elsewhere in the country. We use mtDNA analysis to try to determine the origin of two introduced populations along the Dutch coast, in the city of The Hague and the dune area Meijendel. We compiled a database of hundreds of individuals from throughout the distribution range and added over 130 individuals from both native and introduced populations from the Netherlands, Belgium and Germany. The mtDNA haplotypes found in the introduced populations are associated with postglacial expansion. The main haplotype predominates in the natural range in the Netherlands, but also occurs much more widely across western Europe, north of the Pyrenees. A closely related haplotype, newly identified from The Hague, was not found in the native Netherlands range, suggesting an origin from abroad. The combination of low phylogeographic resolution and low sampling density in the postglacially colonized part of the range hampers our ability to determine the provenance of the introduced A. obstetricans populations.

An isolated crested newt population in Dutch coastal dunes: distribution relict or introduction?

“…It is unclear if the isolated population in the coastal dunes is the remnant of a formerly larger natural distribution (Creemers and van Delft, 2009). Human-mediated establishment of the coastal dune population is a realistic scenario as well, considering that introductions of the common midwife toad Alytes obstetricans, naturally occurring only in the extreme southeast of the Netherlands (Vliegenthart et al, 2023), and of two non-native tree frog Hyla species (Kuijt et al, 2023), have been well documented in the same area (albeit these only took place in the 21st century).…”

Extreme genetic depletion upon postglacial colonization hampers determining the provenance of introduced palmate newt populations

“…Invasive species -defined here following Simberloff (2013) as species that have established a reproductive population outside of their native range -are one of the main drivers of the current sixth mass extinction (Bellard et al, 2016;Nunes et al, 2019;Pyšek et al, 2020;Bellard et al, 2021;Dueñas et al, 2021). Documenting introduced populations and tracing their origins are key steps when considering conservation actions (Allendorf et al, 2010;Vliegenthart et al, 2023). For this purpose, mtDNA barcoding is commonly employed, given its cost effectiveness (Hebert et al, 2003;Mir et al, 2021;Gostel and Kress, 2022).…”

“…Typically, genetic variation is concentrated in glacial refugia, whereas it is generally low or absent in postglacially colonized terrain (Hewitt, 2000). Furthermore, phylogeographic sampling is typically biased towards glacial refugia, thereby reducing the likelihood to capture genetic variation present in postglacially colonized areas (Vliegenthart et al, 2023). Therefore, the accuracy with which mtDNA barcoding can pinpoint the provenance of invasive species strongly depends on the source locality of the introduced population.…”

“…Therefore, the accuracy with which mtDNA barcoding can pinpoint the provenance of invasive species strongly depends on the source locality of the introduced population. As a consequence, their origin can be determined with high spatial resolution in some cases (Dubey et al, 2019;Dufresnes et al, 2019a;Koster et al, 2023;Kuijt et al, 2023;Robbemont et al, 2023), but remains imprecise in others (de Brouwer et al, 2023;Vliegenthart et al, 2023).…”

“…This means that the provenance of these introduced localities cannot be determined with a high degree of accuracy. A similar lack of phylogeographic resolution has hampered the ability to determine the provenance of other amphibian populations introduced in the Netherlands, namely the alpine newt Ichthyosaura alpestris, the great crested newt Triturus cristatus, the common tree frog Hyla arborea and the common midwife toad Alytes obstetricans (de Brouwer et al, 2023;Kuijt et al, 2023;Robbemont et al, 2023;Vliegenthart et al, 2023).…”