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Telomeres cap the physical ends of eukaryotic chromosomes to ensure complete DNA replication and genome stability. Heritable natural variation in telomere length exists in yeast, mice, plants and humans at birth; however, major effect loci underlying such polymorphism remain elusive. Here, we employ quantitative trait locus (QTL) mapping and transgenic manipulations to identify genes controlling telomere length set point in a multi-parentArabidopsis thalianamapping population. We detect several QTL explaining 63.7% of the total telomere length variation in the Arabidopsis MAGIC population. Loss-of-function mutants of theNOP2Acandidate gene located inside the largest effect QTL and of two other ribosomal genesRPL5AandRPL5Bestablish a shorter telomere length set point than wild type. These findings indicate that evolutionarily conserved components of ribosome biogenesis and cell proliferation pathways promote telomere elongation.

Reef‐building corals can increase their resistance to heat‐induced bleaching through adaptation and acclimatization and/or by associating with a more thermo‐tolerant strain of algal symbiont (Symbiodiniumsp.). Here, we show that these two adaptive pathways interact. We collectedAcropora milleporacorals from two contrasting thermal environments on the Great Barrier Reef: cooler, mid‐latitude Orpheus Island, where all corals hosted a heat‐sensitive clade CSymbiodinium, and warmer, low‐latitude Wilkie Island, where corals hosted either a clade C or a more thermo‐tolerant clade D. Corals were kept in a benign common garden to reveal differences in baseline gene expression, reflecting prior adaptation/long‐term acclimatization. Model‐based analysis identified gene expression differences between Wilkie and Orpheus corals that were negatively correlated with previously described transcriptome‐wide signatures of heat stress, signifying generally elevated thermotolerance of Wilkie corals. Yet, model‐free analyses of gene expression revealed that Wilkie corals hosting clade C were distinct from Wilkie corals hosting clade D, whereas Orpheus corals were more variable. Wilkie corals hosting clade C symbionts exhibited unique functional signatures, including downregulation of histone proteins and ion channels and upregulation of chaperones andRNAprocessing genes, putatively representing constitutive “frontloading” of stress response genes. Furthermore, clade CSymbiodiniumexhibited constitutive expression differences between Wilkie and Orpheus, indicative of contrasting life history strategies. Our results demonstrate that hosting alternativeSymbiodiniumtypes is associated with different pathways of local adaptation for the coral host. These interactions could play a significant role in setting the direction of genetic adaptation to global warming in the two symbiotic partners.