A reassessment of the biogeographic range of northern clade pool frogs (Pelophylax lessonae)

Distinguishing between native and introduced species can be difficult, particularly at range borders, where patchily distributed populations may occur away from a species' natural core range. The case of native pool frog (Pelophylax lessonae) populations at their northern range limit in Europe is particularly interesting. These are morphologically and genetically distinct populations which are patchily distributed and have been reported from the UK, Sweden and Norway, but up until 2013 were thought to be absent from Finland. When pool frog populations were discovered in southwestern Finland they were morphologically classified as belonging to this northern clade. However, the origin of these populations has been unclear and it is possible that the Finnish populations originated through human aided introductions, established themselves recently through natural migration, or are indeed previously undiscovered relic populations. To establish the origin and relationship of these frogs to other populations across Europe we used phylogeographical analysis based on microsatellite and mitochondrial DNA markers. Our results indicate that the Finnish, Norwegian, Swedish, UK, as well as Estonian populations belong to the northern clade. The Finnish frogs are most closely related to Swedish northern pool frogs, but are genetically more diverse. This suggests that the Finnish pool frogs are most likely a relic from postglacial migration, though we could not entirely rule out the possibility of a recent natural or human aided colonization from Sweden. This has implications for the conservation status of the pool frog in Finland, where it thus far has been considered an invasive alien species

Two clades of north European pool frogs Rana lessonae identified by cytochrome b sequence analysis

A 410 base-pair region of the mitochondrial cytochrome b (cytb) gene, including the coding region for amino acids 44-179, was amplified and sequenced from a total of 53 pool frogs (Rana lessonae) sampled in nine European countries across the species' biogeographical range. Just two haplotypes were found, differing by single base pair (G-A transition) at a codon second position, corresponding to a conservative (serine-asparagine) change at amino acid 75. Only haplotype A (serine 75) was found in the samples from Italy, Hungary, Switzerland, Poland, Sweden, Norway and Britain (including museum specimens). Only haplotype B (asparagine 75) was found in France, and there were equal numbers of A and B haplotypes in a small sample from the Netherlands. Comparisons with other ranid cytb sequences together with an absence of stop codons indicated that the sequences were mitochondrial rather than nuclear copies. Using these north European cytb sequences and three more from other work based on individuals from Italy, Ukraine and Luxembourg, we discuss haplotype distribution with respect to the phylogeography of R. lessonae

MHC and microsatellite diversity comparisons.

<p>A: Correlation of MHC and microsatellite allelic richness (R). B: Microsatellite (□) and MHC (•) allelic richness (R) and distance from SW France. C: Correlation of MHC and microsatellite expected heterozygosity (H_E).</p

Relationships of pairwise F_ST and distance estimates.

<p>Microsatellite data are represented by open circles and MHC data by filled circles.</p