The Transitioning Germ Line

Abstract

![Figure][1] CREDIT: GETTY IMAGES/VISUALS UNLIMITED Germline constancy is a must for species continuity, but the life of the germ cell itself is full of change. The trek to and from the germ line includes numerous transitions and decisions deftly choreographed for the gametes' unique ability in nature to develop into a complex animal and pass on genetic information. This special section highlights the transitioning germ line and otherwise expands on intriguing aspects of these cells of heredity. In the early embryo, cells decide between becoming soma or germ line. Mechanisms protect germ cells from a somatic fate, just as somatic cells require insulation from the germline differentiation pathway. Strome and Lehmann (p. [392][2]) outline activities in the fly and worm, such as transcriptional quiescence and inheritance of germ cell determinants, and Hayashi et al. (p. [394][3]) tell of the murine system, invoking an inductive signal. Although model systems use different strategies, common themes exist. A new class of molecules reportedly participates in germ cell development. Lin (p. [397][4]) describes the generation and role of germline-specific small RNAs termed piRNAs. Functions as diverse as posttranscriptional regulation and epigenetic programming are suggested. Another epigenetic event—de novo DNA methylation—silences imprinted genes and repeat elements during vertebrate gametogenesis. Schaefer et al. (p. [398][5]) detail this heritable silencing mechanism and its sexual dimorphism. As the germ cell transitions to the mature form, more decisions are made: to enter mitosis or meiosis and to differentiate as sperm or egg, with possible linkage of these decisions as proposed by Kimble and Page (p. 400). Stem cell self-renewal versus differentiation in the Drosophila male and female germ lines is discussed by Fuller and Spradling (p. [402][6]). Switching to vertebrate germline stem cells, Brinster (p. [404][7]) reports methods for the recovery, culture, and transplantation of spermatogonial stem cells; and Daley (p. 409) summarizes advances in embryonic stem cell (ESC) differentiation to a germline fate: procedures with research and clinical applications but not without limitations. In an Editorial, McLaren (p. [339][8]) takes the discussion further, highlighting ethical and social issues surrounding ESC-derived germ cells. Changes continue as the oocyte transitions to the fertilized egg. Stitzel and Seydoux (p. 407) detail the oocyte-to-zygote transition, with associated alterations in protein synthesis, protein and RNA degradation, and organelle remodeling; and Schier (p. 406) discusses zygotic genome silencing and maternal messenger RNA degradation around the maternal-zygotic transition. In News, Travis (p. 390) discusses “urbisexuality” with a biologist pondering the evolution of germ cell specification, and Leslie (p. 388) details how oocyte freezing has become more effective and acceptable despite questions about the long-term health of offspring. Finally, in [ Science 's STKE][9], McClure discusses how poppies reject self-pollen, and Boldajipour and Raz highlight similarities in Drosophila and mouse mechanisms involved in the migration of primordial germ cells and elimination of ectopic cells. The articles here give but a sampling of exciting ongoing research on the seeds of life—themselves products of change. Advances bear fruit in understanding development and its underlying genetics, as well as infertility. [1]: pending:yes [2]: /lookup/doi/10.1126/science.1140846 [3]: /lookup/doi/10.1126/science.1137545 [4]: /lookup/doi/10.1126/science.1137543 [5]: /lookup/doi/10.1126/science.1137544 [6]: /lookup/doi/10.1126/science.1140861 [7]: /lookup/doi/10.1126/science.1137741 [8]: /lookup/doi/10.1126/science.1142267 [9]: http://www.sciencemag.org/sciext/germcells/#section_in-stke