Primordial germ cell migration and the assembly of the Drosophila embryonic gonad

During embryogenesis Drosophila pole cells, like germ cells in many other vertebrates and invertebrates, actively migrate before assembling into the gonad. Five separate steps can be distinguished: an initial passive displacement of the germ cells by gastrulation movements, an amoeboid migratory phase during which the pole cells pass through the endoderm, migration on endoderm toward mesoderm, separation into two bilateral groups associated with the gonadal mesoderm precursors, followed by condensation into the gonad itself. We have analyzed gonad assembly in embryonic pattern mutants to determine whether distinct cues are required in this process. We show that the initiation of migration does not require the presence of the mesoderm, the eventual target of the germ cells. Rather, migration is triggered as a consequence of the differentiation of the endoderm. Examination of embryos mutant for maternal genes of the terminal group suggests that a primary role of the endoderm in this process is to allow the germ cell access to the interior of the embryo. In addition, we show that normal gonad assembly requires a region of the embryo that includes the posterior compartment of the fifth and the sixth abdominal segments.