The five Natrix species are the most abundant snakes across much of their range. Three of them form what researchers call the grass snake complex: Natrix astreptophora from the Iberian Peninsula and North Africa, N. helvetica from western and central Europe, and N. natrix from eastern and northern Europe and Asia. Two further species complete the picture: the viperine snake N. maura of southwestern Europe and North Africa, and the dice snake N. tessellata, which stretches across eastern Europe and Asia. Over the past decade, a series of phylogeographic and systematic studies on grass snakes relied on a constantly growing private database of microsatellite genotypes and mitochondrial DNA sequences. The new publication makes the current version of that underlying database openly available, supplemented with mtDNA sequences from all other available sources for the genus.

The technical scope of the dataset is considerable. It contains georeferenced information for 3,061 grass snakes belonging to the grass snake complex, 69 genetically verified hybrids between N. natrix and N. tessellata, and mtDNA sequences for a single F1 hybrid between N. astreptophora and N. maura. In addition, the resource includes mitochondrial sequences for 130 viperine snakes and 433 dice snakes, plus 59 microsatellite genotypes for dice snakes. For each sample, and whenever known, the dataset reports its mitochondrial identity, meaning the mtDNA clade and individual cyt b and ND4 haplotypes, its microsatellite genotype, and its collection site in WGS 84 coordinates. For the grass snake complex, some dice snakes and the N. natrix x N. tessellata hybrids, genotypes at 13 microsatellite loci are also provided where available.

The mitochondrial markers used are the cyt b gene and the ND4 gene, the latter often together with adjacent DNA coding for transfer RNAs. These short sequences allow researchers to assign individual snakes to mitochondrial lineages and, in many cases, to determine specific haplotypes. Microsatellite genotypes are presented as rounded and standardised allele lengths at each locus, using the values of the original sources. With the exception of approximately 200 Swiss samples, all microsatellite genotypes were generated in the same laboratory, Senckenberg Dresden, ensuring consistency across the dataset. Fragment lengths from the Swiss samples were calibrated as described in the original study, allowing them to be integrated reliably with the rest of the data.

Haplotype nomenclature follows established conventions from previous studies: for grass snakes, the system introduced by Kindler and colleagues in 2017 and subsequent work; for dice snakes, the nomenclature of Guicking and colleagues from 2009 and Asztalos and colleagues from 2021; and for cyt b haplotypes of viperine snakes, that of Guicking and colleagues from 2008. An equivalent nomenclature is applied to ND4 haplotypes of N. maura, and a new acronym, ALG, is introduced for an additional N. maura clade from eastern Morocco and Algeria described by Barata and colleagues in 2008. Sampling sites were individually verified using Google Maps, and incorrect spellings and coordinates were corrected wherever possible. The dataset also contains several previously unpublished haplotypes for grass snakes, including new cyt b variants such as c14, c15 and c16, gn42 through gn46, h51, and r87 through r89, as well as new ND4 plus tRNA haplotypes c6, c7 and r47. These sequences were generated following established laboratory procedures, and haplotypes of new sequences were identified using TCS parsimony networks as implemented in the software Hapsolutely. GenBank and ENA accession numbers and sequences for all haplotypes are provided in the accompanying supplementary tables.

One of the most valuable aspects of the resource is its treatment of hybridisation. Hybridisation between N. astreptophora and N. maura, and between N. natrix and N. tessellata, is rare. A single F1 hybrid between the Iberian grass snake and the viperine snake has been genetically verified, whereas several F1 hybrids as well as backcrosses or later-generation hybrids are known for the grass snake x dice snake pairing. This allows hybrids to be flagged in the dataset, although hybridisation amongst the mentioned species is not restricted to clearly circumscribed geographic regions. In contrast, hybridisation and limited gene flow between N. astreptophora and N. helvetica, and between N. helvetica and N. natrix, are more frequent along narrow geographic contact zones. In these latter cases, identification of pure individuals and hybrids can vary amongst analyses depending on the samples included.

Because of this analytical complexity and the prolonged computation time required, the researchers did not attempt to classify all samples of the three grass snake species using STRUCTURE analyses across the entire database. Instead, they treat all samples as representing the grass snake complex and show in the accompanying maps the extent of the hybrid zones inferred from previous studies. Any sample within these regions may represent either one of the parental species or a hybrid. Beyond the hybrid zones, pure individuals are expected, although translocations may create exceptions. This pragmatic approach allows the database to remain useful even where species boundaries are genetically blurred, while the accompanying maps provide clear visual guidance on where hybrid zones lie.

The database also serves as a monitoring tool for anthropogenic hybridisation. Long-distance translocation of grass snakes and their hybridisation with native individuals are well documented. This is evident in grass snakes from Newdigate in Surrey and the West Midlands, both in Great Britain, and at several sites in France, Germany, Italy, the Netherlands and Switzerland, where non-native mitochondrial lineages have been detected. At some of these sites, non-native or admixed populations have become established, threatening the genetic integrity of local grass snakes. The new resource allows researchers to track population dynamics in natural hybrid zones and to detect future anthropogenic admixture beyond these zones, providing a baseline against which any emerging admixture can be measured.

The geographic coverage of the dataset is Palaearctic-wide, reflecting the full distribution of the genus. The accompanying maps show the distribution of the mitochondrial lineages of grass snakes, viperine snakes and dice snakes, with individual haplotypes listed in the supplementary tables. For the grass snake complex, one of the maps also depicts the sites of the genotyped samples, giving researchers a direct visual link between mitochondrial lineages and microsatellite genotypes across the range. The data are provided in two Excel tables: the first containing mtDNA haplotypes and microsatellite genotypes of Natrix species, with short mtDNA sequences that allow only mitochondrial lineage assignment marked with asterisks, and the second listing individual haplotypes for the cyt b gene and the ND4 gene plus adjacent tRNA coding DNA. Both tables carry a release date of 1 August 2026.

The resource is published under the Open Data Commons Attribution License, making it freely available for reuse in future population genetic, phylogeographic and taxonomic studies. For a group of snakes that dominate the reptile fauna across much of Europe, North Africa and Central Asia, this consolidated repository represents a shift from fragmented single-study datasets to a shared, range-wide evidence base. It can serve researchers studying the evolutionary history of the genus, conservationists monitoring the genetic integrity of native populations, and wildlife managers assessing the consequences of translocated non-native snakes. As anthropogenic pressures on reptile populations continue to grow, such a comprehensive genetic baseline will be essential for understanding how these abundant snakes respond to changing environments and shifting species boundaries.

Subject of Research: Genetics

Article Title: Mitochondrial haplotypes and microsatellite genotypes for grass and water snakes of the genus Natrix : A range-wide resource ( Squamata , Natricidae )

Article References: Criado-Flórez, A., Asztalos, M., Kindler, C., Neumann, A., Schultze, N., & Fritz, U. (2026). Mitochondrial haplotypes and microsatellite genotypes for grass and water snakes of the genus Natrix: A range-wide resource (Squamata, Natricidae). Biodiversity Data Journal, 14, Article e208590. https://doi.org/10.3897/bdj.14.e208590

Image Credits: AI Generated

DOI: 10.3897/BDJ.14.e208590

Keywords: Natrix, grass snakes, water snakes, mitochondrial DNA, microsatellites, phylogeography, hybridisation, Palaearctic, Mitochondrial, haplotypes, microsatellite, genotypes