Chromosome segregation in female meiosis in many metazoans is mediated by acentrosomal spindles, the existence of which implies that microtubule spindles self-assemble without the participation of the centrosomes. Although it is thought that acentrosomal meiosis is not conserved in fungi, we recently reported the formation of self-assembled microtubule arrays, which were able to segregate chromosomes, in fission yeast mutants where the contribution of the spindle pole body (SPB, the centrosome equivalent in yeast) was specifically blocked during meiosis. Here, we demonstrate that this unexpected microtubule formation represents a bonafide type of acentrosomal spindle. Moreover, a comparative analysis of these self-assembled spindles and the canonical SPB-dependent spindle reveals similarities and differences: for example, both spindles have a similar polarity, but the location of the γ-tubulin complex differs. We also show that the robustness of self-assembled spindles can be reinforced by eliminating kinesin-8 family members, whereas kinesin-8 mutants have an adverse impact on SPB-dependent spindles. Hence, we consider that reinforced self-assembled spindles in yeast will help to clarify the molecular mechanisms behind acentrosomal meiosis, a crucial step towards better understanding gametogenesis.

Chromosome segregation in female meiosis is inherently error-prone, among other reasons, because the acentrosomal spindle assembles and segregates chromosomes without the major microtubule-organizing centres in eukaryotes, the centrosomes, which causes high rate of aneuploidy. The molecular basis underlying formation of acentrosomal spindles is not as well-understood as that of centrosomal spindles and, consequently, strategies to improve spindle robustness are difficult to address. Recently, we noticed during fission yeast meiosis the formation of unexpected microtubules arrays, independent of the spindle pole bodies (yeast centrosome equivalent), with ability to segregate chromosomes. Here, we establish such microtubules formation as bonafide self-assembled spindles that depend on the canonical microtubule crosslinker Ase1/PRC1, share similar structural polarity and harbour the microtubule polymerase Alp14/XMAP215, while being independent of conventional γ-tubulin-mediated nucleation mechanisms. Remarkably, acentrosomal spindle robustness was reinforced by deletion of the Klp6/Kinesin-8, which, consequently, led to a reduced meiotic aneuploidy rate. Our results enlighten the molecular basis of acentrosomal meiosis, a crucial event in understanding gametogenesis.