The Germ Plasm ofDrosophila: An Experimental System for the Analysis of Determination

Abstract

The posterior polar plasm of Drosophila melanogaster may provide a model for understanding processes involved in cellular determination during early embryonic development. The presence of germ cell determinants in the posterior tip of the egg has been demonstrated by transplanting this material to new locations and showing that cells, induced at the site of transplantation, can function as germ cells. Furthermore, by utilizing similar procedures, the posterior polar plasm of late stage oocytes has been shown to be functional in inducing germ cells, thus demonstrating that the active components are formed during oogenesis. A number of maternal effect mutations affecting germ cell formation have been analyzed ultrastructurally and the mutant phenotype described. Ultrastructural studies have focused on polar granules, the unique organelle of the polar plasm, and these studies indicate that they are nonmembrane bound structures containing RNA and protein. Although ultrastructural observations have suggested that the protein components of polar granules may be continuous in the cytoplasm of germ cells during the whole life cycle, nucleo-cytoplasmic hybrid pole cells indicate that the structure of the polar granule is dependent upon the nucleus in each new generation of oocytes. Finally, attempts are being made to obtain a purified polar granule fraction in order to test their role in germ cell determination. Initial results indicate that clusters of ribosomes are attached to polar granules. Pole cells have recently been isolated as an enriched source of polar granules. These isolated cells will also provide an opportunity to analyze the unique traits of these cells at the time that they become segregated from the remaining cells of the embryo.