GERMLINE BLASTOMERES LACK NEWLY TRANSCRIBED mRNAs A difference in transcriptional activity between somatic and germline blastomeres was first observed in in situ hybridization experiments aimed at characterizing patterns of gene expression in early embryos

The mechanisms that set the germline apart from the soma have fascinated biologists since the work of Weismann in the late 1800s (Weismann, 1893). Several developmental characteristics distinguish germ cells from somatic cells: during early development, germ cells show relative mitotic inertness compared to somatic cells; later, they are the only cells to undergo meiosis and gametogenesis. Because the germline is the only lineage to contribute its genetic material to the next generation, it is often referred to as an immortal and totipotent lineage, capable of “outliving” its somatic host to regenerate an entirely new organism (e.g. Wylie, 1999). These unique characteristics have led some biologists to wonder whether germ cell specification may involve molecular processes fundamentally different from those used by somatic cells. For example, examination of germ cell development across species has shown that primordial germ cells are often formed in locations and/or at times that appear to exclude them from the inductive events that specify the fates of somatic cells (Dixon, 1994). These observations have suggested that “protective mechanisms” that shield germ cells from somatic signals may be crucial for the proper establishment of the germline. In this review, we describe recent studies in Caenorhabditis elegans which suggest that such “protective mechanisms” indeed exist, and that these mechanisms function, at least in part, by repressing transcription in developing germ cells.