Vipera berus is the species with the largest range of snakes on Earth and one of the largest among reptiles in general. It is also the only snake species found in the Arctic Circle. Vipera berus is the most involved species of the genus Vipera in the process of interspecific hybridization in nature. The taxonomy of the genus Vipera is based on molecular markers and morphology and requires clarification using SC-karyotyping. This work is a detailed comparative study of the somatic and meiotic karyotypes of V. berus, with special attention to DNA and protein markers associated with synaptonemal complexes. The karyotype of V. berus is a remarkable example of a bimodal karyotype containing both 16 large macrochromosomes and 20 microchromosomes. We traced the stages of the asynchronous assembly of both types of bivalents. The number of crossing-over sites per pachytene nucleus, the localization of the nucleolar organizer, and the unique heterochromatin block on the autosomal bivalent 6—an important marker—were determined. Our results show that the average number of crossing-over sites per pachytene nucleus is 49.5, and the number of MLH1 sites per bivalent 1 reached 11, which is comparable to several species of agamas.

Nucleolar organizer regions (NORs) play a key role in ribosome biogenesis and serve as important cytogenetic markers. In reptiles, in particular, vipers (genus Vipera), NOR localization on chromosomes has remained poorly understood. The aim of the work was immunocytological identification of nucleoli in meiocyte nuclei of three viper species (V. berus, V. nikolskii, V. renardi). Primary spermatocytes were studied using antibodies to SYCP3 and fibrillarin proteins. Nucleoli were visualized as large rounded areas in the proximal part of microbivalents, which was accompanied by the absence of a DAPI signal in the area of their localization. For V. berus, the nucleolus is predominantly associated with the bivalent 17, and for V. nikolskii and V. renardi, it is associated with the bivalent 15. Analysis of synaptonemal complexes of microchromosomes provides more precise mapping of nucleoli compared to classical studies of mitotic metaphase plates. The results highlight the potential of this method for comparative and evolutionary studies of viper karyotypes, as well as the possibility of using localization of nucleoli as species-specific cytogenetic markers.