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

The posterior polar plasm of Drosophila melanogaster may providea model for understanding processes involved in cellular determinationduring early embryonic development. The presence of germ celldeterminants in the posterior tip of the egg has been demonstratedby transplanting this material to new locations and showingthat cells, induced at the site of transplantation, can functionas germ cells. Furthermore, by utilizing similar procedures,the posterior polar plasm of late stage oocytes has been shownto be functional in inducing germ cells, thus demonstratingthat the active components are formed during oogenesis. A numberof maternal effect mutations affecting germ cell formation havebeen analyzed ultrastructurally and the mutant phenotype described.Ultrastructural studies have focused on polar granules, theunique organelle of the polar plasm, and these studies indicatethat they are nonmembrane bound structures containing RNA andprotein. Although ultrastructural observations have suggestedthat the protein components of polar granules may be continuousin the cytoplasm of germ cells during the whole life cycle,nucleo-cytoplasmic hybrid pole cells indicate that the structureof the polar granule is dependent upon the nucleus in each newgeneration of oocytes. Finally, attempts are being made to obtaina purified polar granule fraction in order to test their rolein germ cell determination. Initial results indicate that clustersof ribosomes are attached to polar granules. Pole cells haverecently been isolated as an enriched source of polar granules.These isolated cells will also provide an opportunity to analyzethe unique traits of these cells at the time that they becomesegregated from the remaining cells of the embryo.