Think about it: The former is and will remain an egg if left alone, while the latter has within it the means to unleash the dazzling cascade of events needed to create a viable organism. What's already in the egg that enables it to make the transition? How is it the same and how is it different across species? And what changes occur when the egg is fertilized? Scientists led by Drs Barbara Knowles and Alexei Evsikov at The Jackson Laboratory have certainly been thinking about this topic for years. A new paper in the journal Genes & Development presents significant new information about what is happening within a fully grown (pre-ovulatory) egg and some of the changes that occur during transition to the two-cell stage embryo.
The researchers performed a large-scale survey of thousands of fragments of mouse egg cDNA. The cDNA fragments are reverse coded from messenger RNA and therefore indicate which genes are being expressed. They identified 5,400 active genes in the fully grown egg, about a tenth of which are unique to egg cells. Interestingly, the process of getting ready for fertilization requires more genetic activity than early embryo development. There's a far greater diversity of RNAs in the egg cell than in the two-cell embryo, perhaps because the embryo is independent and the pre-ovulatory egg must still coordinate signals with surrounding tissues.
Another discovery was that 80% of the expressed genes identified in mouse eggs are also expressed in the eggs of two other organisms previously studied, a frog and a sea squirt. Obviously early development is a very stable process and many of the crucial steps associated with it have remained the same along the vertebrate evolutionary pathway. But at the same time there's evidence of instability in other areas.
According to Dr. Evsikov, "There's tantalizing evidence for rapid change in a few critical mammalian genes specifically expressed in eggs. These fast-paced changes may pinpoint the molecular mechanisms for reproductive isolation, a primary cause for the origin of new species, and an astonishing diversity of mammals inhabiting Earth."