Stable twin rDNA loci form a single nucleolus in brewer's yeast

The nucleolus is the most prominent membraneless compartment within the nucleus1, 2 - dedicated to the metabolism of ribosomal RNA. Nucleoli are composed of hundreds of ribosomal DNA (rDNA) repeated genes that form large chromosomal clusters3-5, whose high recombination rates can cause nucleolar dysfunction and promote genome instability6-8 related to metabolic and genetic diseases9-13. Intriguingly, the evolving architecture of genomes appears to have favored two strategic rDNA locations in a broad range of species - where a single locus per chromosome is situated either near the centromere or the telomere14, 15. To delve into how organisms may benefit from these nuclear organizations, we used a fused-karyotype strain of Saccharomyces cerevisiae16 to megabase-engineer a chromosome with twin chromosome-collinear rDNA loci. We showed that the twin-rDNA yeast readily adapts exhibiting wild-type growth and maintaining rRNA homeostasis. Using imaging and chromosome conformation capture, we found that the twin loci merge into a single subnuclear compartment throughout the cell cycle. Unexpectedly, we found that rDNA locus size is dependent on its position relative to the centromere, whereby the locus that is centromere-distal undergoes size reduction at a higher frequency compared to the centromere-proximal counterpart. In sum, our work sheds light on the structural evolution of rDNA loci and provides new tools to study the rDNA dosage effect on cellular metabolism.