Chromatin-induced Spindle Assembly Plays an Important Role in Metaphase Congression of Silkworm Holocentric Chromosomes

The kinetochore plays important roles in cell cycle progression. Interactions between 4 chromosomes and spindle microtubules allow chromosomes to congress to the middle of the 5 cell and to segregate the sister chromatids into daughter cells in mitosis. The chromosome 6 passenger complex (CPC), composed of the Aurora B kinase and its regulatory subunits 7 INCENP, Survivin, and Borealin, plays multiple roles in these chromosomal events. In the 8 genome of the silkworm, Bombyx mori, which has holocentric chromosomes, the CPC 9 components and their molecular interactions were highly conserved. In contrast to 10 monocentric species, however, the silkworm CPC co-localized with the chromatin-driven 11 spindles on the upper side of prometaphase chromosomes without forming bipolar mitotic 12 spindles. Depletion of the CPC by RNAi arrested the cell cycle progression at prometaphase 13 and disrupted the microtubule network of the chromatin-driven spindles. Interestingly, 14 depletion of mitotic centromere-associated kinesin (MCAK) recovered formation of the 15 microtubule network but did not overcome the cell cycle arrest at prometaphase. These 16 results suggest that the CPC modulates the chromatin-induced spindle assembly and 17 metaphase congression of silkworm holocentric chromosomes