The function and regulation of vasa-like genes in germ-cell development

In sexually reproducing organisms, primordial germ cells (PGCs) give rise to gametes that are responsible for the development of a new organism in the next generation. These cells must remain totipotent - able to differentiate into each and every cell type of all the different organs. In many organisms, maintenance of totipotency is achieved by the specification of germ cells early in embryogenesis: a small group of cells is set aside to follow a unique pathway of differentiation into gametes (reviewed in [1,2,3]).Information on the specification of PGCs has been gained from detailed microscopical analysis, embryological experiments (for example transplantation of cells or cytoplasm) and gene identification through genetic screens for maternal-effect mutations in Drosophila and Caenorhabditis elegans (reviewed in [1,2,3,4]). The main conclusion from these two invertebrate model organisms is that asymmetrical localization of cytoplasmic determinants - the germ plasm - is responsible for the early specification of the germline lineage. The importance of localized cytoplasmic determinants for germ-cell development has been most clearly shown in Drosophila. Here, for example, cytoplasmic germ plasm determinants concentrated at the posterior pole of the embryo in 'pole plasm' can direct cells towards a germ-cell fate when transplanted to an ectopic location. In mutants in which the formation of this morphologically characteristic cytoplasm is disrupted, germ-cell formation is impaired (reviewed in [5]). The pole plasm is characterized by the presence of the polar granules, electron-dense structures not delimited by a membrane that contain many RNAs and proteins and that are associated with mitochondria. The distribution of the pole plasm correlates with the site of PGC formation. On the basis of their unique morphology, germ plasm components have also been identified in other organisms such as C. elegans (where they are termed P granules), Xenopus laevis (germinal granul