Germ plasm formation and germ cell determination in Drosophila.

In organisms as diverse as frogs, worms and flies germline precursor cells are set aside from the somatic cells early in development. It has been proposed that specific molecules, referred to as germ cell determinants, are deposited in the egg and direct the germ cell fate, but the molecular nature and function of these determinants is not fully understood. Genetic and molecular analysis in Drosophila melanogaster indicates that germ cell determination involves not only the synthesis of specific germ cell factors but also the proper localization and assembly of a morphologically distinct germ plasm. A pathway for germ plasm assembly has been established in which the oskar gene has a central role. The amount of oskar product in the embryo controls the number of germ cells formed and mislocalization of oskar RNA and protein in the egg cell leads to the formation of ectopic germ cells in the embryo. In addition to its role in anchoring germ cell-specific signals, the germ plasm also serves as the source of abdomen-specific signal. Such a colocalization of morphogenetic signals involved in germ cell formation and in the specification of the body axis is not unique to Drosophila but is also found in Caenorhabditis elegans and Xenopus.