[The structure of long telomeres in chromosomes of the Iberian shrew]

It is shown that the size, localization, and structure of telomeres in the Iberian shrew (Sorex granarius) are not characteristic of mammals. In this species, long telomeres of an average size of 213 kb are localized on the short arms of all 32 acrocentrics; ribosomal blocks and active nucleolus-organizing regions (NORs) were also discovered there. At the remaining chromosome ends the average size of telomeres is 3.8 kb. However, in a closely related species, Sorex araneus, all telomeres have size similar to that of human telomeres, i.e., 6.8-15.2 kb. Despite the fact that some long telomeres contain ribosomal repeats in addition to telomeric ones, the long telomeres have preserved asymmetry of G- and C-rich strands as in functional telomeres. It is probable that long telomeres were formed in meiosis at the stage of chromosome bouquet as a result of global reorganization of the chromosome ends. The provoking factors for such reorganization might be the fission of several metacentrics and the necessity of telomerization of the resulting acrocentrics.