Morphology is a poor predictor of interspecific admixture ‒ the case of two naturally hybridizing newts Lissotriton montandoni and Lissotriton vulgaris (Caudata: Salamandridae)

Compared to the parental species, interspecific hybrids often show, intermediate phenotypes which can affect their fitness directly and/or their attractiveness to potential mates. Lissotriton montandoni and Lissotriton vulgaris hybridize across their parapatric range along the Carpathian mountains. Hybrids are rare in natural hybrid zones, which may be related to strong assortative mating in parental species. In the present study, we investigated morphological male species-specific traits and their differentiation between both parental species and first generation of interspecific hybrids (F1), considering morphological variation between two L. vulgaris subspecies co-occurring with L. montandoni in nature: L. v. vulgaris and L. v. ampelensis. Male morphology of each group was assessed and compared based on species-specific and sexual secondary traits, selected based on the literature review. We confirmed that F1 hybrids of L. v. vulgaris and L. montandoni have an intermediate morphology between parental species. However, when morphological traits were compared with the level of genetic admixture of individuals in natural hybrid zones, we discovered that even early generations hybrids (i.e. F1 and/or F2 generation) can be similar to the parental species (especially L. v. ampelensis). As even early generations hybrids can be morphologically undistinguishable from L. v. ampelensis, sexual attractiveness of hybrids may be potentially less affected in direction of L. v. ampelensis backcrossing. Summarizing, morphology, especially morphology of male secondary sexual traits is not a reliable tool for the estimation of genetic admixture of an individual in natural populations within the hybrid zone.