In many species primordial germ cells are set aside early in development from the somatic cells that give rise to the body.
Germ cells navigate through and along different tissue to reach the somatic part of the gonad.
Once the gonad is formed, the ovary and testis differentiate different cell types that facilitate formation of egg and sperm. Many fundamental questions regarding germ cell development remain to be answered. To form an embryonic gonad, germ cells move from the site of germ cell formation at the posterior pole to the somatic part of the gonad, which is composed of mesodermal cells. A genetic dissection of germ cell migration has revealed that both attractive as well as repulsive factors guide germ cells from the midgut pocket towards the mesoderm. We found that Drosophila PAP2 is encoded by two redundant genes, wunen and wunen-2 .