Chiasmata Promote Monopolar Attachment of Sister Chromatids and Their Co-Segregation toward the Proper Pole during Meiosis I

The chiasma is a structure that forms between a pair of homologous chromosomes by crossover recombination and physically links the homologous chromosomes during meiosis. Chiasmata are essential for the attachment of the homologous chromosomes to opposite spindle poles (bipolar attachment) and their subsequent segregation to the opposite poles during meiosis I. However, the overall function of chiasmata during meiosis is not fully understood. Here, we show that chiasmata also play a crucial role in the attachment of sister chromatids to the same spindle pole and in their co-segregation during meiosis I in fission yeast. Analysis of cells lacking chiasmata and the cohesin protector Sgo1 showed that loss of chiasmata causes frequent bipolar attachment of sister chromatids during anaphase. Furthermore, high time-resolution analysis of centromere dynamics in various types of chiasmate and achiasmate cells, including those lacking the DNA replication checkpoint factor Mrc1 or the meiotic centromere protein Moa1, showed the following three outcomes: (i) during the pre-anaphase stage, the bipolar attachment of sister chromatids occurs irrespective of chiasma formation; (ii) the chiasma contributes to the elimination of the pre-anaphase bipolar attachment; and (iii) when the bipolar attachment remains during anaphase, the chiasmata generate a bias toward the proper pole during poleward chromosome pulling that results in appropriate chromosome segregation. Based on these results, we propose that chiasmata play a pivotal role in the selection of proper attachments and provide a backup mechanism that promotes correct chromosome segregation when improper attachments remain during anaphase I

A non-ring-like form of the Dam1 complex modulates microtubule dynamics in fission yeast

The Dam1 complex is a kinetochore component that couples chromosomes to the dynamic ends of kinetochore microtubules (kMTs). Work in the budding yeast Saccharomyces cerevisiae has shown that the Dam1 complex forms a 16-unit ring encircling and tracking the tip of a MT in vitro, consistent with its cellular function as a coupler. Dam1 also forms smaller, nonring patches in vitro that track the dynamic ends of MTs. However, the identity of Dam1’s functional form in vivo remains unknown. Here we report a comprehensive in vivo characterization of Dam1 in the fission yeast Schizosaccharomyces pombe. In addition to their dense localizations on kinetochores and spindle MTs during mitosis, we identify that Dam1 is also localized onto cytoplasmic MTs as discrete spots in interphase, providing the unique opportunity to analyze Dam1 oligomers at the single-particle resolution in live cells. Such analysis shows that each oligomer contains one to five copies of Dam1, and is able to “switch-rail” while moving along MTs, precluding the possibility of a 16-unit encircling structure. Dam1 patches track the plus ends of the shortening, but not the elongating, MTs and retard MT depolymerization. Together with Mal3, the EB1-like MT-interacting protein, cytoplasmic Dam1 plays an important role in maintaining proper cell shape. In mitosis, kinetochore-associated Dam1 appears to facilitate kMT depolymerization. Together, our findings suggest that patches, instead of rings, are the physiologically functional forms of Dam1 in pombe. Our findings help establish the benchmark parameters of the Dam1 coupler and elucidate the mechanism of its functions