Inferring the sources of postglacial range expansion in two large European land snails

Abstract

Exact locations of glacial refugia are relevant for the study of contemporary biodiversity, not only as places less disturbed during the climatic changes but also as sources of rapid expansion of the biota after the Last Glacial cycle. If continuously inhabited over several of the Quaternary glacial cycles, the refugia are readily identifiable by the accumulated genetic diversity. However, the sources of the Holocene range expansion, particularly important for the emergence of present‐day bio‐ and phylogeographic patterns and for realistic estimation of species’ expansion rates, might have been located at the fringes of the glacial species ranges and lack unique lineages. This problem is pertinent when the variation is explored at slowly evolving genetic markers. We suggest that the location of such source refugia may be approximated by reconstructing the geographic location as a continuous trait evolving along the branches of a phylogenetic tree. We applied this approach, using the BEAST software, on two large southeast European land snail species: Caucasotachea vindobonensis and Helix thessalica. We found evidence for C. vindobonensis refugia in the western Balkans; notable is an apparently old refugium in Bosnia and Herzegovina. The plausible sources of the species’ Holocene range expansion, however, were located around the south‐western end of the Carpathians. Although the source areas were likely similar in H. thessalica, some expansion sources suggested by the analyses (e.g., Podolia, Ukraine) appeared implausible and driven by sampling clustered in that area. The applied approach allows for additional exploitation of the mitochondrial data gathered during the past two decades of animal phylogeography studies.

In both directions: Expansions of European land snails to the north and south from glacial refugia

“…In contrast to the unlikely northern refugium, we have confidence in the southward expansion inferred for M. incarnatus (Figure 4). Southward expansion has also been previously inferred in C. vindobonensis (Korábek et al, 2020), and our present analysis confirms that this result is robust to latitudinal sampling bias. Although only 12 samples from Bulgaria were included in the earlier analysis compared to 82 used here, the estimated geographic source of the Bulgarian haplotypes remains similar.…”

“…For C. vindobonensis and H. thessalica , we base our discussion on the same clades as earlier (Korábek et al, 2020), marked with filled circles in Figure 1. In H. pomatia , haplotypes from clade p‐blue (Figure 1) dominate in the postglacially colonized areas (Figure 2).…”

The introduction of the European Caucasotachea vindobonensis (Gastropoda: Helicidae) in North America, its origin and its potential range

“…The species was actually included in Cepaea until recently, when molecular phylogenetics demonstrated that it is not related to western and central European Cepaea, but to Caucasian Caucasotachea (Korábek et al 2015;Neiber and Hausdorf 2015;Neiber et al 2016). Its natural range extends from the eastern rim of Germany, Poland, and an isolated occurrence in Latvia in the northwest, southwards through the Czech Republic, Austria, and northeastern Italy, across the Balkan Peninsula and southeastern Europe, with isolated occurrences in southern Russia eastwards to the Ural Mountains and from Crimea along the Caucasus to Dagestan (Electronic supplementary material 1; Neiber and Hausdorf 2015;Kajtoch et al 2017;Korábek et al 2020). Caucasotachea vindobonensis becomes adult within two years and copulates in spring and lays 29-67 eggs (mean of 49.3 ± 12.5) in their third year (Staikou 1998).…”

“…We constructed a median-joining network (Bandelt et al 1999) based on 292 partial cox1 sequences from the studies of Kajtoch et al (2017) and Korábek et al (2020) using the program PopART (Leigh and Bryant 2015) with e = 0.…”

“…We compiled European records of C. vindobonensis from Kajtoch et al (2017), Korábek et al (2020), iNaturalist (https://www.inaturalist.org/ observations?place_id=any&taxon_id=922180, accessed on 11.11.2020), and the material in the Zoological Museum Hamburg for environmental niche modelling (Electronic supplementary material 1). We used Maxent version 3.4.1 (Philips et al 2020) for ecological niche modelling as implemented in the 'kuenm' workflow (Cobos et al 2019).…”