Origin, Diversity, and Evolution of Telomere Sequences in Plants

Abstract

Telomeres are basic structures of eukaryote genomes. They distinguish natural chromosome ends from double-stranded breaks in DNA and protect chromosome ends from degradation or end-to-end fusion with other chromosomes. Telomere sequences are usually tandemly arranged minisatellites, typically following the formula (T_xA_yG_z)_n. Although they are well conserved across large groups of organisms, recent findings in plants imply that their diversity has been underestimated. Changes in telomeres are of enormous evolutionary importance as they can affect whole-genome stability. Even a small change in the telomere motif of each repeat unit represents an important interference in the system of sequence-specific telomere binding proteins. Here, we provide an overview of telomere sequences, considering the latest phylogenomic evolutionary framework of plants in the broad sense (Archaeplastida), in which new telomeric sequences have recently been found in diverse and economically important families such as Solanaceae and Amaryllidaceae. In the family Lentibulariaceae and in many groups of green algae, deviations from the typical plant telomeric sequence have also been detected recently. Ancestry and possible homoplasy in telomeric motifs, as well as extant gaps in knowledge are discussed. With the increasing availability of genomic approaches, it is likely that more telomeric diversity will be uncovered in the future. We also discuss basic methods used for telomere identification and we explain the implications of the recent discovery of plant telomerase RNA on further research about the role of telomerase in eukaryogenesis or on the molecular causes and consequences of telomere variability.

Keywords: Allium; Cestrum; Genlisea; circular chromosomes; green algae; linear chromosomes; telomerase; telomeres.

Figure 1

Schematic representation of the telomerase activity cycle with the Arabidopsis-type telomere template. TERT, Telomere Reverse Transcriptase; TR, telomerase RNA subunit. Figure based on Sekhri (2014).

Figure 2

Telomere motifs in Archaeplastida (plants in the broad sense), based on the APG IV (The Angiosperm Phylogeny Group 2016) and on the One Thousand Plant Transcriptomes Initiative (2019). Branch lengths do not express real time scales. For simplicity and to save space, certain polyphyletic “groups” (grades) marked with an asterisk in the tree have been represented by a single branch; for the same reason, several minor orders (listed in the blue square at the left upper side of the figure) are not depicted on the tree. The first tip label usually refers to plant orders and in a few cases, to divisions, grades and even families; the second label displays representative families and in a few cases, representative orders or genera.