Maternal effect genes, which have been studied extensively in systems such as flies, fish and frogs, are much less commonly encountered in the mouse. Such genes in these non-mammalian model organisms are expressed in the oocytes and produce messenger RNAs that segregate to specific regions of the egg. These mRNAs typically encode proteins that function to pattern the embryo from the very earliest stages of development. However, mouse embryogenesis is characterized by the equivalency of cells that arise from the first few divisions from the egg, suggesting that in mice maternal effect genes play a limited role at best.

In a study designed to determine stella’s physiological roles, the Wellcome Trust team and Saitou knocked out the gene, which is specific to nascent germ cells, maturing oocytes and preimplantation embryos in mice, and found that even mice homozygous for the mutation were viable and had no apparent defects in the development of either sperm or eggs. However, they dsicovered that although the knockout males exhibited normal fertility, the females exhibited greatly reduced birth rates, which prompted the investigators to look more closely at development in the mice in the period immediately following fertilization attempts. They found that fertilized oocytes failed to implant in the uterine wall, a critical step in the early embryonic development of the mouse. The dramatic reduction in number of live offspring was consistent regardless of whether the father was a knockout or wild type mouse, a pattern characteristic of maternal effect genes.

The role of stella in later development remains a puzzle. The failure of stella mutations to produce any discernible effect in germ cells suggests that there may be other proteins capable of compensating for the loss of stella function in the developing germline. The Stella protein is known to share structural characteristics with proteins involved in maintaining pluripotency and the development of embryonic and germ cell tumors in humans, and the researchers now expect to focus on the functions of this group of molecules in the earliest stages of development and the establishment of the germline.