The taxon lantzi is widely distributed in the Caucasus region (Schmidtler and Franzen 2004, Skorinov et al. 2014). A species distribution modelling exercise (Skorinov et al. 2014) revealed that suitable environmental conditions protrude into the extreme northeast of Turkey, near the border with Georgia. However, the continued occurrence of lantzi in Turkey needs confirmation; there is only a single record, dating from the beginning of the twentieth century (Schmidtler and Franzen 2004, Skorinov et al. 2014). Intriguingly, there are also old reports of the Crested newt Triturus karelinii (Strauch 1870) from this part of Turkey (Wielstra et al. 2010). Just as lantzi, T. karelinii is widely distributed in the Caucasus and, although its occurrence in Turkey is suggested by species distribution modelling (Wielstra et al. 2013c), its actual presence requires further scrutiny. In any case, lantzi is allopatric from the other Turkish taxa: Lissotriton newts are absent from north-east Anatolia (Schmidtler and Franzen 2004; Fig. 1).

The distribution of the Turkey endemic kosswigi is restricted to north-western Anatolia (our exhaustive survey revealed 49 localities; Fig. 1). The species distribution model suggests that suitable environmental conditions extend further to the east along most of the Turkish Black Sea coast (Fig. 3). However, this area appears to be devoid of Lissotriton newts (Fig. 1). Over-prediction is a well-known problem in species distribution modelling (Elith et al. 2011). This could suggest that the climate layers used to create the species distribution model do not properly reflect the factors limiting the distribution of the species, but it could also suggest that not all suitable area could be colonized due to dispersal constraints.

The taxon schmidtleri occurs in the west of Asiatic Turkey and is now known to extend into Europe, across the marine corridor connecting the Aegean and Black Seas (Nadachowska and Babik 2009, Pabijan et al. 2014). The permeability of this apparent barrier can be ascribed to sea level fluctuations related to glacial cycles and the disjunct distribution pattern of schmidtleri is mirrored by the co-distributed crested newt species T. ivanbureschi Arntzen & Wielstra, 2013 in Wielstra et al. (2013a) (Wielstra and Arntzen 2012). Although the Balkan range of schmidtleri outside of Turkey is poorly understood, the taxon appears to occur well into Bulgaria (Pabijan et al. 2014). This is in conflict with the species distribution model (Fig. 3). It could be that mitochondrial DNA does not properly reflect the range of schmidtleri and overestimates its occurrence in Bulgaria. However, we consider it more likely that, due to the lack of confirmed schmidtleri localities from Bulgaria (whereas the taxon might well be abundant there), the species distribution model underestimates the environmental space inhabited by schmidtleri.

The taxa kosswigi and schmidtleri currently appear allopatric. We have particularly surveyed the area for Lissotriton (pers. obs.) and no localities are known between the schmidtleri locality Gemlik (Olgun et al. 1999; locality 62 in Suppl. material 1) and kosswigi locality Yalova (Demirsoy 1996; locality 18 in Suppl. material 1). The Yalova locality lacks documentation and needs confirmation (note that the locality was not included in Schmidtler and Franzen 2004) and it is suggested that there is probably a larger distribution gap, with the next closest kosswigi locality from the perspective of schmidtleri being Kocaeli (museum record; locality 3 in Suppl. material 1). This apparent distribution gap disagrees with the species distribution models, which suggest suitable environmental conditions for both kosswigi and schmidtleri occur south of the Marmara Sea (Fig. 3).

Based on introgression of schmidtleri mitochondrial DNA into kosswigi (very similar to mitochondrial DNA found in schmidtleri today) it has been hypothesized that kosswigi displaced schmidtleri on the Istanbul Peninsula as the waterway between the Black and Marmara Seas rerouted within the last 10,000 years (Nadachowska and Babik 2009, Wielstra et al. 2013b). Similarly, an as yet undescribed Triturus species was proposed to have displaced T. ivanbureschi in this region (Wielstra et al. 2013a, 2013b). The species distribution models suggest suitable environmental conditions here for both kosswigi and schmidtleri and hence do not provide further insight into how kosswigi was able to locally outcompete schmidtleri (Fig. 3).

In light of the current allopatric distribution pattern of kosswigi and schmidtleri, previous reports of transitional forms are curious. Following up on a possible intermediate specimen from Sapanca, Eiselt (1966) could only confirm the presence of pure kosswigi there. Freytag (1955) indicated that in a Lissotriton population from Kanlıca (locality 37 in Suppl. material 1), on the eastern side of the Bosphorus, some males showed characteristics of schmidtleri, namely the dorsal fin being ragged and starting at the back of the head and the lack of a tail filament (cf. Fig. 2). Tabrizi (1980) studied a larger sample of newts from populations throughout the range of kosswigi. He found that four out of 70 newts in Kanlıca showed a schmidtleri-like, relatively anterior starting position of the dorsal fins; all other newts were classified as typical kosswigi. Considering the biogeographical scenario outlined above, a relict schmidtleri population in the process of being replaced by kosswigi via genetic swamping is a possibility. A study on historical gene flow between the two taxa unfortunately did not include samples from the potentially admixed populations, but did suggest ancient gene flow from schmidtleri into kosswigi (Nadachowska and Babik 2009).

Furthermore, Freytag (1957) mentioned that in a Lissotriton population from Baltalimanı (locality 104 in Suppl. material 1), on the western side of the Bosphorus, some males shared similarities with kosswigi, in terms of possessing tail filaments and smooth dorsal fins that started relatively posteriorly (Fig. 2). Yılmaz (1983) studied a larger sample encompassing more populations from European Turkey. He noted newts with kosswigi characteristics at Habibler and Küçükçekmece (localities 121 and 125 in Suppl. material 1). Out of 80 studied newts, 20 had dorsal fins that began at the forelimbs rather than at the back of the head, 41 had smooth dorsal fins and 37 had tail filaments to varying degree (17 with 0–2 mm, 10 with 2–4 mm, 5 with 4–6 mm, and 5 with over 6 mm). Schmidtler and Franzen (2004) state that in schmidtleri males can show kosswigi-like characteristics, but do not provide further details. The presence of kosswigi west of the Bosphorus would not make sense in light of the biogeographical scenario outlined above, unless it could be proven that the Bosphorus on initial formation had a more westward position or formed only after kosswigi reached European Turkey. The rerouting of the marine connection between the Marmara and Black Seas is not yet fully understood and a matter of debate in the paleogeological literature (e.g. Nazik et al. 2011, 2012, Yaltırak et al. 2012). We suggest that historical biogeographical patterns such as shown by Lissotriton (and Triturus) newts might assist paleogeological reconstruction.

Genetic data from the potentially admixed kosswigi and schmidtleri populations are as yet lacking, but would provide more insight in the matter. However, considering the expansion of the Istanbul agglomeration it should be taken into account that these populations might well have gone extinct. We conclude that potential kosswigi-schmidtleri admixture represents, at most, the remnants of a former contact zone. The main ranges of the two taxa are currently isolated in the region by the Bosphorus and hence the influence of potentially admixed populations on the genetic integrity of the two taxa can be expected to be negligible. In this light we make some remarks on the not (yet) generally accepted treatment of the Turkish Lissotriton taxa as distinct species (Dubois and Raffaëlli 2009, Frost 2014).

Following the taxonomy of Babik et al. (2005), the Lissotriton vulgaris group consists of seven taxa, namely ampelensis, graecus, kosswigi, lantzi, meridionalis, schmidtleri and the nominal vulgaris. Four of these taxa, graecus, kosswigi, lantzi and meridionalis, are sometimes regarded as specifically distinct (Dubois and Raffaëlli 2009, Frost 2014). The split of graecus and meridionalis has been criticised (Speybroeck et al. 2010) as a misinterpretation of the phylogenetic position of the congener L. montandoni which, due to mitochondrial DNA introgression, is nested within the L. vulgaris group from the perspective of mitochondrial DNA (Babik et al. 2005, Zieliński et al. 2013). However, the taxa kosswigi and lantzi are genuinely genetically diverged for mitochondrial DNA (Babik et al. 2005).

Within the Lissotriton vulgaris group mitochondrial DNA suggests a basal split between lantzi and the rest (Babik et al. 2005). Although the distinction of lantzi from the perspective of the nuclear genome has as yet not been determined, the divergence in the mitochondrial genome and the at least currently disjunct distribution support a scenario of long-term disrupted gene flow with other Lissotriton newts.

The next split in the Lissotriton vulgaris group is between kosswigi and the remaining taxa (Babik et al. 2005). The distinction of kosswigi from its geographical neighbour schmidtleri has been supported in a study exploring gene flow based on eight nuclear DNA markers (Nadachowska and Babik 2009). Given that kosswigi is genetically distinct and currently allopatric from other Lissotriton taxa, its treatment at the species level seems justified. From the conservation perspective it is important whether this geographically restricted, Turkish endemic is treated as a ‘unique species’ or ‘merely a subspecies’.

Although schmidtleri represents a distinct mitochondrial DNA clade as well, it is genetically nested within the European Lissotriton taxa (Babik et al. 2005, Pabijan et al. in prep.). The phylogeography of Lissotriton on the Balkan Peninsula is highly complex, with morphologically distinct subspecies being highly polyphyletic from the mitochondrial DNA perspective (Babik et al. 2005, Pabijan et al. in prep.). Furthermore, no doubt in part because of its turbulent taxonomical history, the morphological distinctiveness of schmidtleri is not well understood (Schmidtler and Franzen 2004). Hence, we refrain from making further comments on the taxonomic status of schmidtleri and rather await further research on nuclear gene flow between schmidtleri and the other Lissotriton taxa on the Balkan Peninsula.