Contact zone of slow worms Linnaeus, 1758 and (Nordmann, 1840) in Poland.

Background: Legless lizards, the slow worms of the genus are forming secondary contact zones within their Europe-wide distribution.

Methods: We examined 35 populations of and to identify the level of morphological and genetic divergence in Poland. We applied a conventional study approach using metric, meristic, and categorial (coloration) features for a phenotype analysis, and two standard molecular markers, a mitochondrial (NADH-ubiquinone oxidoreductase chain 2; ) and a nuclear (V(D)J recombination-activating protein 1; ) one.

Results: We found clear differences between and in molecular markers and phenotype-in meristic features, ear opening, number of scales rows around the body, and higher than so far known diversity in and haplotypes. The presence of five hybrids was detected in three populations in the Polish part of the European contact zone. In all hybrids, homozygous alleles of were detected, which suggests a back-crossing within the genus.

Conclusions: The ability to produce fertile offspring by x hybrids shows inefficient mechanisms of reproductive isolation of the two legless lizards. The hybrids were indistinguishable from parental species in head proportions (principal components and discriminant analyses) but more resembling in meristic traits.