Distribution of (TTAGGG)n telomeric sequences in karyotypes of the Xenopus species complex

The chromosomal distribution of the conserved vertebrate telomeric (TTAGGG)(n) sequence was studied by fluorescence in situ hybridization (FISH) in four Xenopus species and the triploid Silurana tropicalis. As expected, hybridization signals were observed at the distal ends of every chromosome in all species. In addition, the hybridization pattern demonstrates varied organization of (TTAGGG)(n) sequences in the different karyotypes. In X. borealis and X. muelleri hybridization signals intensely labeled one end of a homologous chromosome pair that coincides with the sites containing ribosomal RNA gene clusters. The karyotype of X. clivii remarkably differs from other Xenopus karyotypes in displaying numerous interstitial telomeric sites (ITS). C-banding analysis shows that the non-telomeric sites appear to correspond to the interstitially located constitutive heterochromatin. This suggests that interstitial telomeric sites in X. clivii do not necessarily represent the relic of ancestral telomeres resulting from the fusion of chromosomes, but their occurrence is due to the fact that (TTAGGG)(n) repeat arrays may be a constituent of highly repetitive DNA.