Cytoplasmic competition between separate parental pronuclei in zygotes

Animals All animal experiments conformed to the Guide for the Care and Use of Laboratory Animals and were approved by the Institutional Committee of Laboratory Animal Experimentation of the RIKEN Centre for Biosystems Dynamics Research. C57BL/6 (B6), JF1/Ms (JF1), BDF1 (C57BL/6 × DBA/2) and R26R-H2B-EGFP +/− (C57BL/6 × DBA/2 background) 43 mice, aged 8–10 weeks, were used to produce oocytes and sperm. Surrogate pseudopregnant females used as embryo transfer recipients (see ‘Embryo transfer’) were ICR strain mice mated with vasectomized males of the same strain. BDF1 mice were purchased from Japan SLC Inc. Oocyte collection Mature oocytes were collected from the oviducts of 8- to 10-week-old female mice that had been induced to superovulate with 5 IU of equine chorionic gonadotropin (ASKA Pharmaceutical) followed by 5 IU of human chorionic gonadotropin (ASKA Pharmaceutical) 48 h later. Cumulus-oocyte complexes were collected from the oviducts approximately 16 h after human chorionic gonadotropin injection. Cumulus-oocyte complexes were placed in M2 medium and treated with 0.1% (w/v) bovine testicular hyaluronidase. After several minutes, the cumulus-free oocytes were washed twice and then transferred to Chatot, Ziomek and Bavister medium (CZB). Mature MII oocytes were subjected to ICSI. Micromanipulation For cytoplasmic removal (Extended Data Fig. 3a,b ), micromanipulation was performed as described previously 44 with some modifications. …