Monkeyflower (Mimulus) uncovers the evolutionary basis of the eukaryote telomere sequence variation

Abstract

Telomeres are nucleoprotein complexes with crucial role of protecting chromosome ends. Because of its vital functions, components of the telomere, including its sequence, should be under strong evolutionary constraint. Yet across the tree of life there are numerous examples of telomere sequence variation and the evolutionary mechanism driving this diversification is unclear. Here, we studied the telomeres in Mimulus by investigating the noncoding telomerase RNA (TR), which is a core component of the telomere maintenance complex and determines the telomere sequence in eukaryotes. We conducted de novo transcriptomics and genome analysis of 18 species, and discovered Mimulus has evolved at least three different telomere sequences: (AAACCCT)n, (AAACCCG)n, and (AAACCG)n. We discovered several species with TR duplications, implying functional consequences that could influence telomere evolution. For instance, M. lewisii harbored two sequence-divergent TR paralogs while its sister species the paralog had pseudogenized. Nanopore-sequencing and fluorescence in situ hybridization indicated M. lewisii had a sequence heterogeneous telomere, and Telomeric Repeat Amplification Protocol combined with Terminal Restriction Fragment analysis confirmed the telomerase can use both TR paralogs for telomere synthesis. Interestingly in closely related species M. cardinalis, TR was also duplicated and both paralogs were expressed but its telomere consisted of a single telomere repeat. Evolutionary analysis indicated the TR paralogs arose from an ancient duplication, which also underlies the evolutionary origin of multiple Mimulus species with divergent telomere sequences. We propose sequence variation in eukaryotic telomeres arises from an evolutionary process involving TR duplication, sequence divergence, and loss of TR paralog.