Supplementary Figure 7: Chr III is not inherently protected from centromere assembly defects.

A potential alternative explanation for the attachment of Chr III to the spindle in a ‘circular + internal telo’ background could stem from the unique presence on Chr III of the rDNA repeats, whose heterochromatic nature could conceivably protect the centromere of Chr III from effects of bouquet deficiency. To assess this possibility, we monitored the attachment of Chr III to the spindle in linear chromosome-containing bqt1Δ meiocytes via endogenously GFP-tagged and functional Reb1, a rDNA binding protein. Of 29 cells with functional spindles, 10 had chromosomes remaining in the centre of the cell at anaphase I or II. In 4 of these 10, the unsegregated chromosome harboured Reb1 signal, indicating that the centromere of Chr III does not enjoy singular protection from non-attachment; rather, the internal telomere stretch affords proper kinetochore assembly on circular Chr III. Representative films are shown. Numbers below frames represent time relative to metaphase I. Scale bar represents 5 μm. Chr III resides in the middle of the cell at anaphase I (arrow), indicating failed segregation.