The spatial arrangement of chromosomes determines fusion of nucleoli in diploid budding yeast.

The nucleolus is a nonmembrane-bound compartment that forms around tandem arrays of ribosomal RNA genes and provides the cell with ribosomes. Multiple nucleoli within the same nucleus coalesce, and fusion is thought to result mainly from intrinsic properties of nucleoli. However, ribosomal DNA (rDNA) arrays are mostly in chromosomal context, and chromosomes are not randomly organized. How the spatial arrangement of chromosomes affects nucleolar fusion is largely unknown. Using fluorescence microscopy, we investigated nucleolar fusion in diploid budding yeast. Nucleoli forming around homologous rDNA arrays efficiently fused during interphase but often individualized during late anaphase. Although nucleoli were far from the spindle pole body (SPB) in interphase, they came close during mitosis, suggesting that SPB-dependent positioning may affect nucleolar fusion. Indeed, disruption of microtubule-dependent centromere anchorage to the SPB by nocodazole promoted individualization of nucleoli. In contrast, impairment of rDNA tethering to the nuclear envelope had little or no effect. Hence, chromosome positioning by non-rDNA sequences facilitates nucleolar fusion.