Hybrid zones of Natrix helvetica and N. natrix: Phenotype data from iNaturalist and genetics reveal concordant clines and the value of species-diagnostic morphological traits

Abstract

Using georeferenced photographic records of 2944 grass snakes from Germany, Austria, and northern Italy as well as previously published mtDNA sequences (n = 1062) and microsatellite data (n = 952) for grass snakes from the same regions, we examined whether or not coloration and pattern reliably allow to differentiate between Natrix natrix and N. helvetica and if so, whether the distribution patterns revealed by phenotypes and genetics are congruent. Furthermore, we used cline analyses across hybrid zones to test whether the phenotypic transition from one species to the other parallels the steep clines unveiled by genetics. Our results suggest that the two species can be reliably differentiated using coloration and pattern. The most powerful diagnostic traits are the presence/absence of side bars on the body flanks, the number of occipital spots, and the shape of the posterior dark occipital spot. The distributions of morphologically identified N. natrix and N. helvetica match their genetically confirmed ranges. Single conflicting individuals morphologically identified as N. natrix or hybrids within the distribution range of N. helvetica either represent misidentifications or translocated snakes. For the genetic markers and phenotypes, our cline analyses revealed concordant steep clines across hybrid zones. However, the southern part of the hybrid zone in Italy, for which no sufficient genetic data are available, should be studied in more detail because the phenotypic data suggest a smooth cline in this region. The unexpected high percentage of putative hybrids with dorsal stripes in this region also calls for further research. For northwestern Germany, another region for which no genetically verified records are available, iNaturalist data suggest that the contact zone of N. natrix and N. helvetica is near the Ems River and extends from there southeastwards to the region of Höxter, North Rhine-Westphalia.

Natrix natrix after dark: citizen science sheds light on the common grass snake’s nightlife

“…This is not the first instance of citizen science platforms, such as iNaturalist, being used to gain knowledge about grass snakes. In recent years, georeferenced images from these platforms have proven to be extremely valuable in providing important morphological and distributional data related to grass snakes ( Fritz & Ihlow, 2022 ; Fritz, Grismer & Asztalos, 2023 ; Jablonski et al, 2023 ).…”

Genetic identification, morphology and distribution of Natrix helvetica subspecies in southern and western Switzerland (Reptilia, Squamata, Serpentes)

“…This does not contradict the validity of the involved two subspecies because morphological traits that can be distinguished by humans are neither necessarily biologically relevant nor a prerequisite for taxonomic distinctness (compare, for instance, Kindler and Fritz 2018 ; Dufresnes et al 2023 , 2024 ). Also, we could have missed some relevant traits in coloration and pattern that were only recently highlighted ( Fritz et al 2023 ). According to these authors, some individuals of N. h. sicula show a “spotted” colour pattern that never occurs in the nominotypical subspecies.…”

Figure 2 from: Schild A, Baur H, Hertwig ST, Fritz U, Ursenbacher S (2024) Genetic identification, morphology and distribution of Natrix helvetica subspecies in southern and western Switzerland (Reptilia, Squamata, Serpentes). ZooKeys 1205: 223-238. https://doi.org/10.3897/zookeys.1205.123911

“…This does not contradict the validity of the involved two subspecies because morphological traits that can be distinguished by humans are neither necessarily biologically relevant nor a prerequisite for taxonomic distinctness (compare, for instance, Kindler and Fritz 2018;Dufresnes et al 2023Dufresnes et al , 2024. Also, we could have missed some relevant traits in coloration and pattern that were only recently highlighted (Fritz et al 2023).…”