Depletion of maternal SMC2 causes abnormal nuclear morphology in zygotes
Considering the nucleolar maturation defects observed in SMC2-deficient oocytes and pronuclear stage arrest of SMC2-/+embryos, we next examined whether reorganization of the maternal and paternal pronuclei was affected after fertilization.
Abnormal pronuclear morphology was observed in SMC2 −/ + 1-cell embryos (Fig. 4A). Chromosomes were not properly separated between maternal pronuclei and PB2. The chromosomes were still connected by thin threads of decondensed chromatin. We also observed that numerous abnormal micronuclei existed in fertilized eggs. We also measured the sizes of male and female pronuclei. Maternal and paternal pronuclei were distinguished by their position in the fertilized eggs. The female pronucleus was closer to the second polar body (PB2) than the male pronucleus. First, we examined the pronuclear morphology of the SMC2+/+ and SMC2-/+ embryos, and quantified their relative diameter ratio. 1-cell SMC2−/+ embryos showed smaller male pronuclei and larger female pronuclei compared to the control 1-cell embryos (Fig. 4B). We calculated the size ratio of female and male pronuclei diameters (Fig. 4C). Strikingly, female pronuclei, which usually are smaller than male pronuclei, were larger than male pronuclei in SMC2-/+ zygotes, contrary to the SMC2-/+ embryos. In SMC2 +/ + 1-cell embryos, the nucleus ratio was close to 0.7, but in SMC2−/ + embryos, in addition to the abnormal female nucleus morphology, the ratio deviated up to 1.4. Thus, deletion of maternal SMC2 leads to abnormal female pronuclear organization.