Characterisation of an unusual telomere motif (TTTTTTAGGG)n in the plant Cestrum elegans (Solanaceae), a species with a large genome.

The characterization of unusual telomere sequence sheds light on patterns of telomere evolution, maintenance and function. Plant species from the closely related genera Cestrum, Vestia and Sessea (family Solanaceae) lack known plant telomeric sequences. Here we characterize the telomere of Cestrum elegans, work that was a challenge because of its large genome size and few chromosomes (1C 9.76pg; n=8). We developed an approach that combines BAL31 digestion, which digests DNA from the ends and chromosome breaks, with next-generation sequencing (NGS), to generate data analysed in RepeatExplorer, designed for de novo repeats identification and quantification. We identify an unique repeat motif (TTTTTTAGGG)(n) in C.elegans, occurring in ca.30400 copies per haploid genome, averaging ca.1900 copies per telomere, and synthesized by telomerase. We demonstrate that the motif is synthesized by telomerase. The occurrence of an unusual eukaryote (TTTTTTAGGG)(n) telomeric motif in C.elegans represents a switch in motif from the typical' angiosperm telomere (TTTAGGG)(n). That switch may have happened with the divergence of Cestrum, Sessea and Vestia. The shift in motif when it arose would have had profound effects on telomere activity. Thus our finding provides a unique handle to study how telomerase and telomeres responded to genetic change, studies that will shed more light on telomere function. Significance Statement We previously found that plants from related genera Cestrum, Vestia and Sessea (Solanaceae) lack known telomeric sequences. Here we characterise the Cestrum elegans telomere and the mechanism of its synthesis. We developed an approach combining NGS of untreated and terminally-truncated genomic DNA. We identified, using the RepeatExplorer bioinformatic pipeline, an unusual telomere repeat (TTTTTTAGGG)(n) synthesized by telomerase. That finding will enable studies on evolutionary divergence of telomeres and telomerase, shedding light on telomere interactions and functions.