Immunocytological Identification of Nucleoli in Spermatocytes of Three Viper Species of the Genus Vipera

Abstract 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.