Lagging chromosomes (arrow) result from the loss of KLP-19.

Holocentric chromosomes, like those of C. elegans, pose a dangerous attachment problem during mitosis. Their kinetochores are distributed along the length of the chromosomes and are thus easy targets for microtubules from both poles. If a single kinetochore is caught by two poles, the chromatid is held up at anaphase, causing segregation defects (as these merotelic attachments create tension, the mitotic checkpoint is not activated).

On page 991, Powers et al. uncover a chromosome-associated kinesin in worms that prevents persistent merotelic attachments. Although homologues of this plus-end microtubule motor, called KLP-19, are found in many systems, they may be especially critical in organisms with holocentric chromosomes.

Dividing worm cells that lacked KLP-19 had high rates of segregation errors due to merotelic attachments. Chromosomes congregated at the metaphase plate normally, but some then moved quickly back toward the poles before anaphase, suggesting that KLP-19 resists poleward pulling forces on chromosomes that initially attach to just one pole.

By pushing chromosomes away from the poles, KLP-19 may put immediate and persistent tension on the first spindle kinetochore–microtubule attachment, thus swinging the initial kinetochore toward its pole and away from contact with microtubules from the second pole.

Similar activities in monocentric vertebrate cells are not essential for segregation fidelity. By compacting kinetochores into a discrete region that is recessed within surrounding chromatin, vertebrate chromosomes may rely more on geometry than tension to prevent merotelism. ▪