Aloe spp. – plants with vertebrate-like telomeric sequences

Chromosome termini of most eukaryotes end in tracks of short tandemly repeated GC-rich sequences, the composition of which varies among different groups of organisms. Plant species predominantly contain (TTTAGGG)_n repeats at their telomeres. However, a few plant species, including members of Alliaceae and Aloe spp. (Asphodelaceae) were found to lack such Arabidopsis-type (T₃AG₃)_n telomeric repeats. Recently, it has been proposed that the lack of T₃AG₃ telomeric repeat sequences extends to all species forming the Asparagales clade. Here, we analysed the composition of Aloe telomeres by single-primer PCR and fluorescence in-situ hybridization (FISH) with directly labelled Arabidopsis-type (TTTAGGG)_28–43 DNA probe, and with vertebrate-type (TTAGGG)_33–50 DNA and a (C₃TA₂)₃ peptide nucleic acid (PNA) probe. It was found that Nicotiana tabacum contained Arabidopsis-type telomeric repeats, while Aloe telomeres lacked the corresponding FISH signals. Surprisingly, FISH with the highly specific vertebrate-type (C₃TA₂)₃ PNA probe resulted in strong T₂AG₃-specific FISH signals at the ends of chromosomes of both Aloe and Nicotiana tabacum, suggesting the presence of T₂AG₃ telomeric repeats in these species. FISH with a long (TTAGGG)_33–50 DNA probe also highlighted Aloe chromosome ends, while this probe failed to reveal FISH signals on tobacco chromosomes. These results indicate the presence of vertebrate-like telomeric sequences at the telomeres of Aloe spp. chromosomes. However, single-primer PCR with (T₂AG₃)₅ primers failed to amplify such sequences in Aloe, which could indicate a low copy number of T₂AG₃ repeats at the chromosome ends and/or their co-orientation and interspersion with other repeat types. Our results suggest that telomeres of plant species, which were thought to lack GC-rich repeats, may in fact contain variant repeat types. This revised version was published online in July 2006 with corrections to the Cover Date.