Cooper previously showed that a telomere-binding protein called Taz1 is required for forming bouquets in fission yeast. The taz1 mutants are moderately defective in homologous recombination, which is thought to be the bouquet's main purpose. But their dominant fault lies in spore formation after meiosis—mutants often have too few spores containing uneven amounts of DNA.
To question why bouquet mutants disrupt meiosis, the team has now followed the dynamics of bouquet formation in live cells. In meiotic prophase of wild-type cells, the bouquet associated with the spindle pole body (SPB)—the yeast microtubule-organizing center. The SPB then dissociated from the telomeres, divided, and set up the bipolar spindle for the meiosis I division. It then divided again and set up the meiosis II spindle.
Deleting taz1 disrupted this prophase telomere–SPB association. As the bouquet mutants progressed through meiosis I, the SPB became disorganized, failed to divide properly, and sometimes even appeared outside the nucleus altogether. The problems resulted in weak, monopolar, or tripolar spindles during one or both meiotic divisions.
Closer inspection of the wild-type telomere–SPB dissociation event revealed “telomere fireworks,” in which the telomere ends simultaneously dissociated from the SPB. Since these fireworks directly preceded the first SPB division and failed in bouquet mutants, Cooper speculates that the event somehow “marks” the SPB for proper division and spindle organization for the rest of meiosis.
A version of Taz1 that cannot bind telomeres did not rescue the SPB or spindle defects, hinting that a connection to chromosome ends is needed, perhaps because a mechanical force must be generated or because bouquet proteins only function in the context of a telomere complex.
Cooper will next investigate the effects of bouquet association and dissociation on the SPB. Because they are highly conserved, bouquets may be critical for mammalian gametogenesis, too.