Karyotype stasis but species-specific repetitive DNA patterns in Anguis lizards (Squamata: Anguidae), in the evolutionary framework of Anguiformes

Karyotype divergence may strongly affect the degree of hybridization between species. Western Palearctic slow worms (Anguis) are legless lizards forming different types of secondary contact zones. To identify the level of chromosomal variation in slow worms, we examined karyotype in multiple populations of all species except one and Pseudopus apodus as an outgroup. We applied conventional and molecular cytogenetic methods and whole-chromosome painting using macrochromosome probes from Varanus komodoensis to interpret results within the evolutionary framework of the common clade Anguiformes. All Anguis species and P. apodus have conserved karyotype structures composed of 44 chromosomes. Despite the conserved chromosome morphology, the phylogenetically oldest Anguis cephallonica living in partial sympatry with Anguis graeca, and parapatric Anguis colchica vs. Anguis fragilis exhibit distinct patterns of constitutive heterochromatin distribution and telomeric repeat accumulation. In contrast, the sister species A. colchica and A. graeca living in allopatry display highly similar karyotype features. Our findings thus indicate karyotype stasis in Anguis and Pseudopus for > 20 Myr, with fixed species-specific differences present in sympatric and parapatric species. These differences in repetitive DNA patterns may play a role as intrinsic factors co-maintaining species divergence. They may also be used as cytotaxonomic markers to identify slow worm species in practice.