Chromosome Segregation in <i>C. Elegans</i> Female Meiosis is Accompanied by a Switch in Lateral to End-On Microtubule Orientation

Abstract

Acentrosomal female meiotic spindles of many organisms undergo a transition of the microtubule cytoskeleton from a bipolar spindle in metaphase to an inter-chromosomal array in anaphase. Two opposing modes of microtubule-to-chromosome orientation have been previously proposed during chromosome partitioning in C. elegans oocytes, i.e., lateral attachment of microtubule bundles to chromosomes versus inter-chromosomal organization of microtubules. Here we applied large-scale electron tomography of staged C. elegans oocytes in meiosis to show that the re-organization of microtubules is accompanied by a switch from a lateral to an end-on orientation of microtubules to chromosomes. We further show that this transition in microtubule orientation involves the microtubule depolymerase KLP-7 to remove laterally associated microtubules around chromosomes, thus contributing to a high fidelity in chromosome segregation. These results indicate that both modes of microtubule-to-chromosome orientation operate successively in oocyte meiosis to segregate chromosomes.