Real-time imaging of germ cell migration in Drosophila. H. Sano, R. Lehmann. Developmental Genetics, Skirball Inst, NYU Medical Ctr, New York, NY.

   Germ cell migration is a dynamic process in which cells must find their way by interacting with different tissues. To clarify how germ cells migrate to the target tissues in vivo, we carried out real-time imaging of migrating germ cells. In Drosophila, primordial germ cells (PGCs) arise at the posterior pole of the embryo. During gastrulation, PGCs are carried into the pouch of the posterior midgut (PMG), and they then pass through the lumen of the PMG to enter into the embryo. They migrate toward somatic gonadal precursors (SGPs) that are specified as three clusters at bilateral positions in the mesoderm. The majority of PGCs first associate with the nearest SGPs, but they go on to occupy the rest of the SGP clusters. Eventually, PGCs and SGPs coalesce into the tightly compacted embryonic gonads. We filmed PGCs labeled with EGFP in wild type embryos and analyzed the trajectory and detailed structure of migrating PGCs. We found that PGCs never crossed the midline on the way from the PMG to the SGPs, suggesting that a repellant moves PGCs laterally. This process was severely affected in wunen/wunen-2 double mutant, which has been reported to act as repellants for PGCs in the other migration step. The early bilateral migration of PGCs was relatively normal in abdominal A mutants in which SGPs are not specified. This suggests that there are two steps to reach the SGPs, repulsion from the midline and then later capture by SGPs. While migrating toward SGPs, PGCs showed specific migratory features, such as filopodia in the front and uropods in the back of the cell. We also found that PGCs did not move from one SGP cluster to another until germband retraction occurred. Since germband retraction makes the SGP clusters closer, it might help PGC migration between SGPs. Once PGCs enter into the mesoderm, they rarely changed position with respect to each other. Combining real-time imaging of wild type and mutant embryos should give us novel insights into PGC migration.