Untangling the mysteries of maternal inheritance with polycomb.

The inheritance of maternal factors in oocytes plays a crucial role in development, particularly in non-mammalian species, where asymmetric segregation of developmental factors among early blastomeres regulates early patterning of the embryo. Whereas this role is less important in mammals, a number of studies have shown that maternal inheritance is equally critical for regulating development through epigenetic instructions and initiating transition from the zygote to embryonic transcription programme (Li et al, 2010). A recent study by Posfai et al (2012) now shows the important role played by maternally inherited Polycomb complexes during development of the mouse oocyte, and thereafter during early development. Germ cell development is a highly dynamic process, which has to be precisely orchestrated to ensure proper development of the embryo. A totipotent embryo has the ability to give rise to all the three germ layers including extra embryonic tissues. Development follows after the formation of a totipotent zygote, which involves fusion between two differentiated cell types, the sperm and the oocyte. However, nuclear transfer experiments (whereby a somatic nucleus is injected into an enucleated oocyte) can also result in development to term, albeit infrequently. Thus, the cellular milieu of the oocyte has a unique composition, which enables the oocyte to reprogram the nucleus of even a terminally differentiated somatic cell (Meissner and Jaenisch, 2006). To understand the mechanisms that govern totipotency, it is essential to know what maternal factors give an oocyte this unique potential. Upon fertilization, the maternal and paternal nuclei undergo dramatic chromatin changes, wherein the sperm nucleus is stripped off its protamines, which are rapidly replaced by