Rapid evolution and functional divergence of the monkeyflower Mimulus lewisii telomerase

Telomeres are nucleoprotein structures that protect chromosome ends and are maintained by the Telomerase Reverse Transcriptase (TERT) protein that uses a noncoding Telomerase RNA (TR) as a template. In monkeyflowers, Mimulus lewisii had an ancient TR gene duplication, synthesizing an evolutionarily atypical sequence heterogeneous telomere. How TERT interacts with both TR paralogs during telomere maintenance is unknown and answers can shed novel insights underlying telomere function. Using new genome assemblies we discovered TERT is rapidly evolving in lineages sharing the TR duplication. We investigated the functional consequences arising from the rapid evolution, first by using yeast-three-hybrid and testing the physical binding between conspecific and heterospecific TERT-TR combinations. Results showed M. lewisii TERT binds both ancestral (TR1) and derived (TR2) TR paralogs, but not in species without the TR duplication. We located the region of TR binding to amino acids near the KRxR motif of TERT. We then combined next-generation sequencing with Telomeric Repeat Amplification Protocol and discovered M. lewisii had high telomerase activity. Comparative transcriptomics indicated no strong evidence of expression divergence in telomere maintenance genes for M. lewisii, suggesting rapid evolution shaped the telomerase protein sequence. In vivo activity of M. lewisii telomerase was investigated by analyzing F1 telomeres generated by crossing M. lewisii and M. verbenaceus, which doesn't have a functioning TR2. Results showed M. verbenaceus chromosome ends in the F1 had converted into M. lewisii telomeres, suggesting dominance of the M. lewisii telomerase. We demonstrate TERT-TR coevolution can have significant consequences on the evolution of plant telomeres.