The maternal-effect gene futile cycle is essential for pronuclear congression and mitotic spindle assembly in the zebrafish zygote

Embryos have been successfully used for the general study of the cell cycle. Although there are significant differences between the early embryonic and the somatic cell cycle in vertebrates, the existence of specialised factors that play a role during the early cell cycles has remained elusive. We analysed a lethal recessive maternal-effect mutant, futile cycle (fue), isolated in a maternal-effect screen for nuclear division defects in the zebrafish (Danio rerio). The pronuclei fail to congress in zygotes derived from homozygous fue mothers. In addition, a defect in the formation of chromosomal microtubules prevents mitotic spindle assembly and thus chromosome segregation in fue zygotes. However, centrosomal functions do not appear to be affected in fue embryos, suggesting this mutant blocks a subset of microtubule functions. Cleavage occurs normally for several divisions resulting in many anucleate cells, thus showing that nuclear- and cell division can be uncoupled genetically. Therefore, we propose that in mitotic spindle assembly chromosome-dependent microtubule nucleation is essential for the coupling of nuclear and cell division.