Gene expression and distribution of Swi6 in partial aneuploids of the fission yeast Schizosaccharomyces pombe

Imbalances of gene expression in aneuploids, which contain an abnormal number of chromosomes, cause a variety of growth and developmental defects. Aneuploid cells of the fission yeast Schizosaccharomyces pombe are inviable, or very unstable, during mitotic growth. However, S. pombe haploid cells bearing mini- chromosomes derived from the chromosome 3 can grow stably as a partial aneuploid. To address biological consequences of aneuploidy, we examined the gene expression profiles of partial aneuploid strains using DNA microarray analysis. The expression of genes in disomic or trisomic cells was found to increase approximately in proportion to their copy number. We also found that some genes in the monosomic regions of partial aneuploid strains increased their expression level despite there being no change in copy number. This change in gene expression can be attributed to increased expression of the genes in the disomic or trisomic regions. However, even in an aneuploid strain that bears a minichromosome containing no protein coding genes, genes located within about 50 kb of the telomere showed similar increases in expression, indicating that these changes are not a secondary effect of the increased gene dosage. Examining the distribution of the heterochromoatin protein Swi6 using DNA microarray analysis, we found that binding of Swi6 within ~50 kb from the telomere occurred less in partial aneuploid strains compared to euploid strains. These results suggest that additional chromosomes in aneuploids could lead to imbalances in gene expression through changes in distribution of heterochromatin as well as in gene dosage.