Abstract
The requirements for oocyte meiotic cytokinesis during polar body extrusion are not well understood. In particular, the relationship between the oocyte meiotic spindle and polar body contractile ring dynamics remains largely unknown. We have used live cell imaging and spindle assembly defective mutants lacking the function of CLASP/CLS-2, kinesin-12/KLP-18, or katanin/MEI-1 to investigate the relationship between meiotic spindle structure and polar body extrusion in C. elegans oocytes. We show that spindle bipolarity and chromosome segregation are not required for polar body contractile ring formation and chromosome extrusion in klp-18 mutants. In contrast, oocytes with similarly severe spindle assembly defects due to loss of CLS-2 or MEI-1 have penetrant and distinct polar body extrusion defects: CLS-2 is required early for contractile ring assembly or stability, while MEI-1 is required later for contractile ring constriction. We also show that CLS-2 both negatively regulates membrane ingression throughout the oocyte cortex during meiosis I, and influences the dynamics of the central spindle-associated proteins Aurora B/AIR-2 and MgcRacGAP/CYK-4. We suggest that proper regulation by CLS-2 of both oocyte cortical stiffness and central spindle protein dynamics may influence contractile ring assembly during polar body extrusion in C. elegans oocytes.
Highlights
Oocyte meiosis comprises a single round of genome replication followed by two highly asymmetric cell divisions that produce one haploid gamete and two small polar bodies that contain discarded chromosomes [1,2,3,4]
We suggest that CLS-2 influences oocyte cortical stiffness to promote polar body extrusion, and we further suggest that disruption of central spindle-associated protein dynamics may contribute to proper oocyte meiotic contractile ring assembly and ingression
The -340s time point corresponds to roughly the beginning of anaphase in control oocytes. (F) Control and mutant oocytes expressing GFP::ASPM-1 and mCherry::histone 2B (H2B) during meiosis I. (G) Percent of control and mutant oocytes that extrude a polar body during meiosis I, as determined by mCherry or GFP-fused histone signal remaining stably ejected from the oocyte cytoplasm for the duration of imaging
Summary
Oocyte meiosis comprises a single round of genome replication followed by two highly asymmetric cell divisions that produce one haploid gamete and two small polar bodies that contain discarded chromosomes [1,2,3,4]. The contractile ring ingresses past both the membrane-proximal pole and one set of the segregating chromosomes to constrict and separate the nascent polar body from the oocyte [6]. These dynamics contrast substantially with mitotic cytokinesis (Fig 1A), during which signals from astral microtubules and the central spindle position the contractile ring midway between the two spindle poles [8,9,10]. While the signals required for contractile ring assembly and constriction during mitotic cytokinesis are relatively well understood, how oocyte meiotic spindles influence contractile ring dynamics during polar body extrusion is not known
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