Comparative analysis of phenotypic variability of introduced land snail Cepaea nemoralis (Gastropoda: Helicidae) in two large Eastern European cities

Variation in the shell colour and banding polymorphism of Cepaea nemoralis was recorded in 20 sites in Minsk, Belarus (a total of 3965 adults collected in 2014–2021). This variation was compared with that in 16 sites from Lviv, Western Ukraine (total 3235 adults collected in 2019–2021). Unlike in Lviv, a remarkable spatial differentiation of the phenotypic composition was found in Minsk. The samples collected in the north-eastern part of Minsk were characterized by a greater degree of phenotypic diversity and by higher frequencies of unbanded and brown shells. Samples from the southern and eastern parts of the city were generally lighter and characterised by high frequencies of shells with a single central band (mid-banded). Differences between the two parts of Minsk were greater than those between Minsk and Lviv. This may be related to the history of colonization of Minsk by C. nemoralis. The Lviv samples were, on average, darker, they contained less frequently mid-banded shells and more frequently unbanded shells, especially pink unbanded ones. The Fst values characterizing the level of phenotypic variability and calculated for Minsk and Lviv are quite high and comparable with those for other urban areas recently colonized by C. nemoralis.

Deep structure, long-distance migration and admixture in the colour polymorphic land snail Cepaea nemoralis

Although snails of the genus Cepaea have historically been important in studying colour polymorphism, an ongoing issue is that there is a lack of knowledge of the underlying genetics of the polymorphism, as well as an absence of genomic data to put findings in context. We, therefore, used phylogenomic methods to begin to investigate the post‐glacial history of Cepaea nemoralis, with a long‐term aim to understand the roles that selection and drift have in determining both European‐wide and local patterns of colour polymorphism. By combining prior and new mitochondrial DNA data from over 1500 individuals with ddRAD genomic data from representative individuals across Europe, we show that patterns of differentiation are primarily due to multiple deeply diverged populations of snails. Minimally, there is a widespread Central European population and additional diverged groups in Northern Spain, the Pyrenees, as well as likely Italy and South Eastern Europe. The genomic analysis showed that the present‐day snails in Ireland and possibly some other locations are likely descendants of admixture between snails from the Pyrenees and the Central European group, an observation that is consistent with prior inferences from mitochondrial DNA alone. The interpretation is that C. nemoralis may have arrived in Ireland via long‐distance migration from the Pyrenean region, subsequently admixing with arrivals from elsewhere. This work, therefore, provides a baseline expectation for future studies on the genetics of the colour polymorphism, as well as providing a comparator for similar species.