Germ Cell Development and Sex Differentiation.

It is our great pleasure to have been able to publish cutting-edge monographs written by leaders of germ cell studies. Currently, a tremendous number of studies on germ cells can be read in the scientific journals of many fields. At this timing, it would be good to recall that the germ cell studies on early days conceived an issue of sexual fate [McClung, 1902; Stevens, 1905]. Behavior of a curious chromosome during meiosis led to the recognition of sex chromosome and of development of sexually two different gametes. The modern studies of germ cells began with addressing the timing of sexual fate decision of germ cells to develop into eggs or sperm [Burgoyne et al., 1988; McLaren, 1988]. These were done by a group of Dr. McLaren in 1988, more than 30 years ago before the first identification of the sex determination gene, Sry [Sinclair et al., 1990]. In the studies, XY germ cells isolated from mouse embryos were transplanted in XX body and vice versa. Since then, the germ cell studies have been prosperous at the levels of molecular and cellular mechanism. Recently in relation with stem cell biology, it is getting relevant to understand how germline stem cells leave the state of stemness and commit to either oogenesis and spermatogenesis. This stems a modern understanding of the sexual fate decision of germ cells [Spiller et al., 2017]. This issue will provide a current view of mechanisms of germ cell sexual fate decision. Germ cells in mammals have a sexually asymmetric feature, development of germline stem cells in male gonad but no germline stem cells defined in female gonad [Zhang et al., 2015]. This is different from other vertebrate species and shows that germ cells in mammals are not everything. In this context, germ cell studies in Drosophila and Nematoda (and other vertebrates) have been complementary to mammals and giving a deep insight on an essential nature of germ cells. In addition to the two chapters of Drosophila and Nematoda, the other chapters constitute different aspects of mechanisms underlying sexual fate of germ cells and describe the recent progress in mammals. These include signal transduction, cellular interaction, epigenetic regulation, recombination, and metabolism. In these chapters, the audience would recognize the detailed mechanisms analyzed by development of sophisticated techniques with big data. The editors believe that this special issue represents current trends of germ cell studies and will chart a scientific path in this field. Minoru Tanaka Nagoya University, Nagoya, Japan Katsuhiko Hayashi Osaka University, Suita, Japan