The Evolution of Telomeres in Algae

Telomeres, protective nucleoprotein structures of linear chromosome termini, are a ubiquitous and conserved structure of eukaryotic genomes. But telomeric DNA formed by a long array of minisatellite motif may vary between eukaryotic groups. Algae are of interest for telomere biology, because they represent a substantial portion of the eukaryotic phylogenetic diversity. We employed an experimental approach to probe telomeric minisatellites. Our results support the view that there are few variants of the telomeric motifs that are conserved in particular groups of algae. Telomeric motif TTTAGGG, conserved in most green algal lineages, is ancestral for Chloroplastida. However, an interesting diversity of telomeres was described in the order Chlamydomonadales, where at least two independent evolutionary transitions from the ancestral type to TTTTAGGG occurred; moreover a change to TTAGGG was found in a subclade of Caudivolvoxa. Algae of the streptophyte class Klebsormidiophyceae possess TTTTAGGG telomeric repeat or a novel TTTTAGG motif. The TTTAGGG telomeric sequence is also present in telomeres of the photosynthetic alveolate Chromera velia and of Xanthophyceae, in contrast to the presence of the TTAGGG motif in other ochrophytes studied. Glaucophytes and haptophytes exhibit TTAGGG telomeric type. To expand knowledge about the distribution of different telomere types in framework of the eukaryotic phylogeny, we performed in silico analyses of genomic data from major eukaryotic lineages. These analyses confirm the TTAGGG telomeric repeat as the most common and possibly ancestral in eukaryotes, but alternative motifs replaced it along the phylogeny of diverse eukaryotic lineages.