Chromosomal puzzle in snakes: adjacent interstitial telomeric sites on chromosome 5 in three species of genus Vipera.

Large interstitial telomeric regions are considered remnants and markers of chromosomal rearrangements or a result of several suggested molecular mechanisms of telomere repeats accumulation. More rare are cases when large interstitial repeats are found not close to, but at a distance from the centromere. However, synapsis, recombination, and effects on chromatin near these regions during meiotic prophase I have not been sufficiently studied. Using the model of three snake species of the genus Vipera: V. berus, V. nikolskii, V. renardi, we studied interstitial telomere sites (ITSs) in the pachytene nuclei of primary spermatocytes. We discovered an unusual composite chromosome in the species under study with two ITSs located far from the centromere. In V. berus, two very large adjacent ITS blocks were found on bivalent 5. In the other two species, V. nikolskii and V. renardi, two ITSs are also present on bivalent 5, but they are significantly smaller and barely distinguishable by FISH on pachytene bivalents. The possibility of forming crossing-over sites is shown between the two ITSs. Apparently, the three studied viper species received this complex structure of chromosome 5 from their common ancestor. However, the transformation of these telomeric repeat regions during evolution in the species under study occurred differently. Possible mechanisms of modifications of the telomeric regions are discussed.