Abstract
Parental pronuclei (PN) are asymmetrical in several points but the underlying mechanism for this is still unclear. Recently, a theory has been become broadly accepted that sperm are more than mere vehicles to carry the paternal haploid genome into oocytes. Here, in order to reveal the formation mechanisms for parental asymmetrically relaxed chromatin structure in zygotes, we investigated histone mobility in parthenogenetic-, androgenic-, ROSI-, ELSI-, tICSI-, and ICSI-zygotes with several numbers of PNs with the use of zygotic fluorescence recovery after photobleaching, a method previous established by our group. The results showed that sperm played a role to cause chromatin compaction in both parental PNs. Interestingly, during spermiogenesis, male germ cells acquired this ability and its resistance. On the other hand, oocytes harbored chromatin relaxation ability. Furthermore, the chromatin relaxation factor was competed for between PNs. Thus, these results indicated that the parental asymmetrically relaxed chromatin structure was established as a result of a competition between the PNs for the chromatin relaxation factor that opposed the chromatin compaction effect by sperm. Together, it was suggested that parental germ cells cooperated for their just arisen newborn zygotes by playing a distinct role in the regulation of chromatin structure.