Meiosis produces haploid gametes by accurately reducing chromosome ploidy through one round of DNA replication and two subsequent rounds of chromosome segregation and cell division. The cell divisions of female meiosis are highly asymmetric and give rise to a large egg and two very small polar bodies that do not contribute to development. These asymmetric divisions are driven by meiotic spindles that are small relative to the size of the egg and have one pole juxtaposed against the cell cortex to promote polar body extrusion. An additional unique feature of female meiosis is that fertilization occurs before extrusion of the second polar body in nearly all animal species. Thus sperm-derived chromosomes are present in the egg during female meiosis. Here, we explore the idea that the asymmetry of female meiosis spatially separates the sperm from the meiotic spindle to prevent detrimental interactions between the spindle and the paternal chromosomes.

Fertilization occurs during female meiosis in most animals, which raises the question of what prevents the sperm DNA from interacting with the meiotic spindle. In this study, we find that Caenorhabditis elegans sperm DNA stays in a fixed position at the opposite end of the embryo from the meiotic spindle while yolk granules are transported throughout the embryo by kinesin-1. In the absence of F-actin, the sperm DNA, centrioles, and organelles were transported as a unit with the yolk granules, resulting in sperm DNA within 2 µm of the meiotic spindle. F-actin imaging revealed a cytoplasmic meshwork that might restrict transport in a size-dependent manner. However, increasing yolk granule size did not slow their velocity, and the F-actin moved with the yolk granules. Instead, sperm contents connect to the cortical F-actin to prevent interaction with the meiotic spindle.

In a wide range of eukaryotes, chromosome segregation occurs through anaphase A, in which chromosomes move toward stationary spindle poles, anaphase B, in which chromosomes move at the same velocity as outwardly moving spindle poles, or both. In contrast, Caenorhabditis elegans female meiotic spindles initially shorten in the pole-to-pole axis such that spindle poles contact the outer kinetochore before the start of anaphase chromosome separation. Once the spindle pole-to-kinetochore contact has been made, the homologues of a 4-μm-long bivalent begin to separate. The spindle shortens an additional 0.5 μm until the chromosomes are embedded in the spindle poles. Chromosomes then separate at the same velocity as the spindle poles in an anaphase B-like movement. We conclude that the majority of meiotic chromosome movement is caused by shortening of the spindle to bring poles in contact with the chromosomes, followed by separation of chromosome-bound poles by outward sliding.

Anaphase chromosome movement is thought to be mediated by pulling forces generated by end-on attachment of microtubules to the outer face of kinetochores. However, it has been suggested that during C. elegans female meiosis, anaphase is mediated by a kinetochore-independent pushing mechanism with microtubules only attached to the inner face of segregating chromosomes. We found that the kinetochore proteins KNL-1 and KNL-3 are required for preanaphase chromosome stretching, suggesting a role in pulling forces. In the absence of KNL-1,3, pairs of homologous chromosomes did not separate and did not move toward a spindle pole. Instead, each homolog pair moved together with the same spindle pole during anaphase B spindle elongation. Two masses of chromatin thus ended up at opposite spindle poles, giving the appearance of successful anaphase.

Oocyte meiotic spindles orient with one pole juxtaposed to the cortex to facilitate extrusion of chromosomes into polar bodies. In Caenorhabditis elegans, these acentriolar spindles initially orient parallel to the cortex and then rotate to the perpendicular orientation. To understand the mechanism of spindle rotation, we characterized events that correlated temporally with rotation, including shortening of the spindle in the pole-to pole axis, which resulted in a nearly spherical spindle at rotation. By analyzing large spindles of polyploid C. elegans and a related nematode species, we found that spindle rotation initiated at a defined spherical shape rather than at a defined spindle length. In addition, dynein accumulated on the cortex just before rotation, and microtubules grew from the spindle with plus ends outward during rotation. Dynactin depletion prevented accumulation of dynein on the cortex and prevented spindle rotation independently of effects on spindle shape. These results support a cortical pulling model in which spindle shape might facilitate rotation because a sphere can rotate without deforming the adjacent elastic cytoplasm. We also present evidence that activation of spindle rotation is promoted by dephosphorylation of the basic domain of p150 dynactin.