The study focused on intraspecific genetic diversity in G. truncatula sensu lato across its broad range. Earlier phylogenetic and DNA-barcode studies had concentrated largely on Western Europe or populations invasive in South America, leaving Eastern Europe, Asia and the Caucasus comparatively understudied. The authors generated sequences from specimens collected in Eastern Europe, the Caucasus and Central Asia and combined them with all available G. truncatula COI sequences identified in the cited studies or GenBank.

For the phylogenetic analysis, the researchers used a maximum of 78 sequences and a 574-base-pair alignment, with representatives of several other lymnaeid genera included as outgroups. Species identification of newly collected material was based on shell morphology and features of the copulatory apparatus. Alignments were prepared with MAFFT and filtered with Gblocks under the least stringent settings. Maximum-likelihood tree support was assessed with 100,000 bootstrap replicates, while Bayesian inference used two independent RevBayes runs of 20 million generations each, with the first 2 million generations discarded as burn-in.

The resulting trees had largely congruent topologies for sequences identified as G. truncatula. Three sequences from northern Western Europe formed a distinct group, represented by two haplotypes and separated from the rest of the haplotype network by 36 mutational steps. The remaining sequences formed a major clade divided into an eastern group containing populations from Asia, the Caucasus and Eastern Europe and a western group containing populations from Western Europe and, in part, North Africa. A Templeton, Crandall and Sing network contained 36 haplotypes overall and separated the eastern and western subgroups within the main haplogroup by 18 mutational steps.

Analysis of molecular variance showed pronounced geographic differentiation between the eastern and western clades: 70.6% of total genetic variance was attributed to differences between them, compared with 29.4% within them. The Phi statistic was 0.707 with p < 0.001. Nucleotide diversity, calculated excluding invasive populations, was higher in the western clade at 2.0% than in the eastern clade at 0.97%. The diversity remained elevated within Western Europe alone, excluding North African populations, at 1.9%.

Regional patterns were also apparent within the two major clades. The eastern clade contained 18 haplotypes, including haplotypes unique to the Caucasus and the Himalayas, although these regional groups differed by only a few mutational steps. Populations from the Tien Shan shared haplotypes with Eastern Europe, which had the highest haplotype diversity within the eastern group. The western clade contained 12 haplotypes, including three from North Africa and three from South America, with the remainder restricted to Western Europe. North African populations shared no haplotypes with European populations.

Both the eastern and western subgroups occurred in the invasive South American range, but invasive populations shared no haplotypes with populations in the native range. Eurasian populations also had substantially more haplotype diversity than South American populations, a pattern consistent with a founder effect and a bottleneck during colonization. The authors considered this evidence inconsistent with a South American origin for the species and instead proposed that G. truncatula colonized South America independently on at least two occasions, involving both major lineages.

The authors associated the east–west division with Pleistocene glacial cycles, hypothesizing that the initial divergence may have begun in the early Pleistocene. During the Last Glacial Maximum, approximately 25,000 to 17,000 years ago, the southward advance of the Scandinavian ice sheet and Alpine glaciation formed a barrier between partially glaciated Western Europe and Eastern Europe and Siberia. They further proposed that the Last Glacial Maximum reinforced lineage separation by restricting gene flow. This remains a historical hypothesis rather than a directly tested process in the study.

Several limitations prevent firm taxonomic conclusions. The deeply divergent northern European group is represented by only three GenBank sequences without corresponding morphological data, and limited sampling could make the apparent lack of admixture between geographic lineages an artifact. Although the two major clades show deep intraspecific divergence, their COI p-distance remained below the commonly proposed species-level threshold of 0.05 for Lymnaeidae, which the authors said was insufficient evidence for delimitation. Sequence data from topotypic Galba almaatina did not support its species-level distinctiveness, leading the authors to treat it as a synonym of G. truncatula, while they concluded that a broader revision would require topotypic material and integrated molecular, morphological and ecological evidence.

Subject of Research: Phylogeographic and genetic structure of the dwarf pond snail Galba truncatula across Eurasia, North Africa and South America

Article Title: A relic of glacial isolation: deep phylogeographic splits in dwarf pond snail Galba truncatula ( Gastropoda , Lymnaeidae )

Article References: Nekhaev, I. O., Ishayeva, A. A., Omarova, A. M., Khrebtova, I., Kondakov, A., Kim, L. V., & Aksenova, O. V. (2025). A relic of glacial isolation: deep phylogeographic splits in dwarf pond snail Galba truncatula (Gastropoda, Lymnaeidae). Frontiers of Biogeography, 18, Article e158818. https://doi.org/10.21425/fob.18.158818

Image Credits: AI Generated

DOI: 10.21425/fob.18.158818

Keywords: Galba truncatula, dwarf pond snail, phylogeography, COI sequencing, mitochondrial lineages, haplotype network, Pleistocene glaciation, population genetics, South America, taxonomy, relic, glacial