Regulation of the maternal-to-embryonic transition in "C. elegans"

Reproduction is a fundamental process of life which allows the survival of each
species through the birth of young species members. Several modes of
reproduction evolved and are classified into sexual and asexual reproduction.
Sexual reproduction involves the fusion of two gametes: the female oocyte and
the male sperm. Parthenogenesis is a form of asexual reproduction in which
oocytes alone develop into viable progeny. Parthenogenesis most likely
independently evolved multiple times from sexual reproduction and has been
described in various invertebrate and vertebrate species.
In certain phyla, like in nematodes, closely related species differ in their modes of
reproduction. Studying reproduction and early embryogenesis in these species
allows the identification of molecular mechanisms which permit or restrict
parthenogenesis. From other model organisms, several regulatory candidates
are known. For example in flies, sperm independent oocyte activation and
spindle formation facilitate parthenogenetic development, whereas in mammals,
genetic imprinting, sperm dependent oocyte activation and oocytic centriole
elimination restrict parthenogenesis.
We study the sexually reproducing nematode C. elegans in which
parthenogenetic development has not been described. We analyzed the
influence of maternally inherited epigenetic modifiers and found that modifiers
involved in maintaining the soma - germ line distinction have no additional role in
regulating the onset of embryonic transcription. We further analyzed embryonic
features in two different maternal mutant backgrounds. We found that formation
of a maternal germ line tumor, the teratoma, has striking similarities with early
embryogenesis. In contrast, ovulated but unfertilized oocytes of a feminized
mutant show early embryonic-like features but are unable to differentiate. The
ability to differentiate is most likely not due to sperm inherited components but
partially depends on the sperm triggered formation of a functional eggshell.
We conclude that in C. elegans, the onset of the maternal-to-embryonic transition
is uncoupled from sperm dependent oocyte activation which might facilitate
parthenogenesis in nematodes. Further, next to sperm dependent oocyte
activation and centriole inheritance, sperm licensed eggshell formation might
restrict parthenogenetic development in C. elegans.