JCB_201604079 1597..1608

Eukaryotic cells possess robust mechanisms that safeguard against genomic damage. In the event of damage, cells activate checkpoint pathways that inhibit cell cycle progression, providing time for repair or elimination of the genetically compromised cells through apoptosis (Ermolaeva and Schumacher, 2014). Although we have learned a great deal regarding the function of checkpoint and DNA repair pathways during interphase (G1, S, and G2), little is known concerning the cellular response to genetic damage as cells progress through metaphase. Studies show that entry into metaphase with damaged DNA elicits either the spindle assembly or DNA damage checkpoint depending on the cell type (Mikhailov et al., 2002; Royou et al., 2005). In spite of these safeguards, cells occasionally exit metaphase with unrepaired double-strand breaks (DSBs). The presence of DSBs at metaphase is particularly troublesome, because they can result in the formation of chromosome fragments lacking a centromere. Known as acentrics, these fragments are incapable of forming the microtubule–kinetochore attachments that drive poleward chromosome segregation. Consequently, acentrics are expected to lag on the cell equator and exhibit severe segregation defects. However, several recent studies demonstrate poleward migration of acentric chromosomes. In budding yeast and Drosophila melanogaster, acentrics are transmitted through many generations (Sandell and Zakian, 1993; Malkova et al., 1996; Ahmad and Golic, 1998; Galgoczy and Toczyski, 2001; Titen and Golic, 2008). Segregation of acentrics has been observed in Drosophila polyploid cells, Caenorhabditis elegans meiosis, Scadoxus multiflorus, and mammalian cells (Bajer, 1958; Liang et al., 1993; Khodjakov et al., 1996; Kanda et al., 1998; Kanda and Wahl, 2000; Muscat et al., 2015; Bretscher and Fox, 2016). Poleward directed movements of acentrics during mitosis have been previously reported in several different species (Bajer, 1958; Khodjakov et al., 1996; Kanda et al., 1998; Platero et al., 1999; Kanda and Wahl, 2000; Ishii et al., 2008; Titen and Golic, 2008). Proposed mechanisms of acentric segregation include neo-centromere formation and direct association of the acentrics with a kinetochore-bearing chromosome (Platero et al., 1999; Kanda et al., 2001; Ishii et al., 2008; Ohno et al., 2016). Despite these studies of poleward-directed movements of acentrics, the forces that power acentric segregation during mitosis remain poorly understood. Recent studies have also demonstrated the presence of a DNA tether that connects acentrics to the main chromosome mass, suggesting that the tether is required for acentric segregation. The DNA tether consists primarily of DNA along with histones and is coated with Polo, Bub3, and BubR1 kinases and the chromosomal passenger proteins Aurora B and INC ENP and Cdc20 (Royou et al., 2010; Derive et al., 2015). During metaphase, the acentric sisters are held together and are positioned toward the outer edge of the metaphase plate. At the onset of anaphase, through an unknown mechanism, acentric sisters remain held together on the spindle equator while the rest of the chromosome mass travels toward the pole. Eventually, the Although poleward segregation of acentric chromosomes is well documented, the underlying mechanisms remain poorly understood. Here, we demonstrate that microtubules play a key role in poleward movement of acentric chromosome fragments generated in Drosophila melanogaster neuroblasts. Acentrics segregate with either telomeres leading or lagging in equal frequency and are preferentially associated with peripheral bundled microtubules. In addition, laser ablation studies demonstrate that segregating acentrics are mechanically associated with microtubules. Finally, we show that successful acentric segregation requires the chromokinesin Klp3a. Reduced Klp3a function results in disorganized interpolar microtubules and shortened spindles. Normally, acentric poleward segregation occurs at the periphery of the spindle in association with interpolar microtubules. In klp3a mutants, acentrics fail to localize and segregate along the peripheral interpolar microtubules and are abnormally positioned in the spindle interior. These studies demonstrate an unsuspected role for interpolar microtubules in driving acentric segregation. The chromokinesin Klp3a and microtubules facilitate acentric chromosome segregation