Failure of pronuclear migration and repeated divisions of polar body nuclei associated with MTOC defects in Polo eggs of Drosophila

The meiotic spindle of Drosophila oocytes is acentriolar but develops an unusual central microtubule organising centre (MTOC) at the end of meiosis I. In polo oocytes, this common central pole for the two tandem spindles of meiosis II was poorly organised and in contrast to wild-type failed to maintain its associated Pav-KLP motor protein. Furthermore, the polar body nuclei failed to arrest at metaphase, and the four products of female meiosis all underwent repeated haploid division cycles on anastral spindles. This was linked to a failure to form the astral array of microtubules with which the polar body chromosomes are normally associated. The MTOC associated with the male pronucleus was also defective in polo eggs, and the sperm aster did not grow. Migration of the female pronucleus did not take place and so a gonomeric spindle could not form. We discuss these findings in relation to the known roles of polo like kinases in regulating the behaviour of MTOCs.