The most surprising feature of large C-positive regions containing rDNA in stick insects is the sheer size and their localization on a single chromosome pair. According to the present study, there are taxa including species with a few nucleolus organizing regions (NORs) per karyotype (most of the species studied here), while other taxa have species characterized by numerous NORs of distinct size (M. extradentata; Figure 1c). However, even in the first type of taxa, NOR locations are on different chromosomes also in related species, suggesting the transposition of rDNA followed by its amplification at the new spot and elimination of the old NORs. Also, in the three species studied here, and also in Leptynia , the large C-positive blocks are also enriched in telomeric repeats. Interestingly, in S. sipylus a large C-positive block with rDNA and telomeric repeats is present only on one chromosome; in addition, there is a smaller chromosome, with a smaller correspondingly stained region (Figure 4a–c). Either these two chromosomes are different autosomes, or this pattern is a hint of an unusual sex-chromosome system, one being the X and the other the Y chromosome.