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Ribosomal DNA clusters and telomeric repeats are important parts of eukaryotic genome. However, little is known about their organization and localization in karyotypes of organisms with holocentric chromosomes. Here we present first cytogenetic study of these molecular structures in seven blue butterflies of the genus    Polyommatus    Latreille, 1804 with low and high chromosome numbers (from n=10 to n=ca.108) using fluorescence  in situ  hybridization (FISH) with 18S rDNA and (TTAGG)  n   telomeric probes. FISH with the 18S rDNA probe showed the presence of two different variants of the location of major rDNA clusters in    Polyommatus    species: with one or two rDNA-carrying chromosomes in haploid karyotype. We discuss evolutionary trends and possible mechanisms of changes in the number of ribosomal clusters. We also demonstrate that    Polyommatus    species have the classical insect (TTAGG)  n   telomere organization. This chromosome end protection mechanism probably originated  de novo  in small chromosomes that evolved via fragmentations.

Ribosomal DNA clusters and telomeric (TTAGG)n repeats in blue butterflies ( Lepidoptera , Lycaenidae ) with low and high chromosome numbers