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

Abstract

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.