Telomere-associated endonuclease-deficient Penelope-like retroelements in diverse eukaryotes

The evolutionary origin of telomerases, enzymes that maintain the ends of linear chromosomes in most eukaryotes, is a subject of debate. Penelope-like elements (PLEs) are a recently described class of eukaryotic retroelements characterized by a GIY-YIG endonuclease domain and by a reverse transcriptase domain with similarity to telomerases and group II introns. Here we report that a subset of PLEs found in bdelloid rotifers, basidiomycete fungi, stramenopiles, and plants, representing four different eukaryotic kingdoms, lack the endonuclease domain and are located at telomeres. The 5 ' truncated ends of these elements are telomere-oriented and typically capped by species-specifictelomeric repeats. Most of them also carry several shorter stretches of telomeric repeats at or near their 3 ' ends, which could facilitate utilization of the telomeric G-rich 3 ' overhangs to prime reverse transcription. Many of these telomere-associated PLEs occupy a basal phylogenetic position close to the point of divergence from the telomerase-PILE common ancestor and may descend from the missing link between early eukaryotic retroelements and present-day telomerases.