Figure 3. Halving the recipient ooplasm prevents premature separation of paternal chromosomes in biparental meiosis.

1. Halving the recipient ooplasm mass reduced chromosome segregation errors. Errors were determined by tracking all chromosomes at anaphase (n = 39 and 17 oocytes; See also Fig 2B). The parental origin of errors is shown. Ooplasmic halving significantly reduced the rate of errors (**P < 0.01, Chi‐square test).
2. Predivision was predominant in biparental meiosis. Chromosome segregation error patterns were categorized based on anaphase trajectories: nondisjunction (0:4 segregation), balanced predivision (2:2 sister chromatid segregation), unbalanced predivision (1:3 segregation including sister chromatid segregation), and complex patterns including predivision (multiple errors including sister chromatid segregation). Chromosome breakages (chromosomes lacking centromeres) were also observed. Data from 39 and 17 oocytes, respectively.
3. Univalent‐like chromosomes. Images were 3D‐reconstructed as in Fig 2B and viewed from the top of the metaphase plate. Red surfaces with white arrowheads indicate univalent‐like chromosomes. Scale bar = 3 μm.
4. Halving the ooplasm volume suppressed the premature separation of paternal chromosomes. Oocytes were categorized based on whether the chromosomes exhibited premature separation into univalent‐like structures prior to segregation errors. Data from 39 and 17 oocytes, respectively.
5. Summary of biparental meiosis. In normal‐sized oocytes, biparental meiosis frequently exhibits premature separation of paternal chromosomes into univalent‐like structures. These chromosomes undergo predivision (premature segregation of sister chromatids), and thus result in separated chromatids in MII oocytes. Chromosome nondisjunction and breakage were relatively minor. Halving the ooplasmic volume reduced premature separation of paternal chromosomes.