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identity, and reprogramming factors, which are provided by the oocyte to reprogram
gene expression in the zygote into a totipotent state. Although the majority of pater-
nal gene products defining male germs cell are eliminated during spermatogenesis,
including most of the cytoplasm and nucleosomal chromatin, some paternal factors
are still transmitted to the zygote. To acknowledge that OET also includes elimina-
tion of paternal factors, we will review two contributions from the father to the
zygote, which are eliminated during early development: paternal DNA methylation
and paternal mitochondria.
The process of clearance of parental products will be divided into phases
whose initiation or termination is defined by one of the major developmental tran-
sitions during OET (Fig. 10.4). We designate clearance of maternal products initi-
ated upon resumption of meiosis as Phase I since resumption of meiosis is
conventionally seen as OET initiation. Phase II is framed by fertilization and ini-
tiation of the first major wave of zygotic transcription. It includes clearance of
maternal products (e.g., mRNAs, proteins) as well as some paternal factors (e.g.,
paternal DNA methylation in mammals). Phase III is initiated by ZGA. In addi-
tion, we introduce two phases for events that occur outside the three phases above.
Phase 0 includes clearance of maternal products occurring already before resump-
tion of meiosis. Finally, we introduce a phase designated spermatogenic to high-
light the fact that the process of spermatogenesis is characterized by extensive
elimination of paternal factors (e.g., paternal mRNAs and nucleosomal
chromatin).
Fig. 10.4 Schematic depiction of different phases of parental product clearance during vertebrate
OET. Phase I is positioned at resumption of meiosis, which is traditionally taken as the first transi-
tion occurring during OET
P. Svoboda et al.