Epigenetics of Reproduction in Animals - Building the Most Complex Structure on Earth - page 109

Epigenetic Programming of Gene Expression in the Egg
Procreation in animals is a monopoly of the gametes—egg and sperm—in dioecious
animals and the egg in parthenogenetic animals. No other type of cell can develop
into an adult organism. What makes gametes, or the egg cell in parthenogenetic ani-
mals, uniquely capable of producing adult organisms?
For a long time, it was taken for granted that the genome provides gametes the
singular ability to reproduce. However, it has never been argued—let alone proved—
that this is indeed the case.
There are at least three essential facts that unambiguously argue against a determin-
ing role of the genome in gametes' ability to start and proceed with individual develop-
ment. First, that the other somatic cells in metazoans are incapable of developing into
adult organisms, although they have the same species-specific genome and genes that
gametes/zygotes have. Second, early individual development following the first cleav-
age division of the zygote depends not on zygotic genes but on the epigenetic infor-
mation deposited in gametes mainly in the form of parental (maternal and paternal)
cytoplasmic factors, primarily mRNA and also on other chemicals such as hormones
and neurotransmitters. Third, cloning experiments require the oocyte cytoplasm exclu-
sively, and no cytoplasm of any other cell can substitute for the oocyte cytoplasm.
Two important landmarks in the development of the knowledge of the role of cyto-
plasmic factors in development is the transformation of putative somatic cells into
germ cells by transplanting germ cytoplasm by Illmensee and Mahowald (1974) and
demonstration by Freeman and Lundelius (1982) that a maternal cytoplasmic factor
in the egg cell is responsible for left/right coiling in a snail. We know that it is the epi-
genetic information that the egg cell is provided with maternally that is responsible
for the unique procreational ability of eggs. This information is provided to the egg
in the form of maternal factors that are deposited in an orderly fashion within the egg
and determine the early embryonic development up to the phylotypic stage.
Accumulating evidence shows that cytoplasmic factors of the sperm cell are
involved in the initial regulation of early development. The sperm cell plays a deter-
mining role in asymmetric cleavage divisions that lead to the formation of different
types of cells. Here, we will deal only with the deposition of the maternal factors in
the egg cell because it is the primary source of cytoplasmic factors (epigenetic infor-
mation) in the zygote.
Deposition of Maternal Determinants in the Egg
Despite the crucial importance of the egg in development, studies of the ordered
placement of maternal factors in the egg cytoplasm has never been central in bio-
logical investigation. It is generally taken for granted that concentration gradients of
morphogenetic substances may be responsible in determining their location in the
egg cell. This statement has not been tested, and it seems implausible that concen-
tration gradients of numerous substances in the same nanospace might maintain the