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MODEL Drosophila melanogaster
In Drosophila melanogaster is observed a structure called pole plasm in pole cells which are precursors of germ cells( Ikenishi, 1998). The pole plasm has the germ-cell inducing potencial (Illmensee and Mahowald, 1974; Okada et al. 1974). Eight genes belonging to the "posterior group" maternal effect genes (capuccino, spire, staufen, oskar, vasa, valois, mago nashi, and tudor) are required for the normal formation of the pole cells ( Ephrussi and Lehmann, 1992). In particular, the proteins products of oskar, vasa, and tudor have been shown to be components of pole plasm immunoelectron microscopic analysis (Hay et al., 1988; Bardsley et al., 1993; Lehmann and Ephrussi 1994).
Among the eight genes mentioned above, vasa gene products are specifically found in germ cells of a wide range of animals from insects to mammals (Ikeninishi, 1998). The vasa gene of Drosophila melanogaster has been shown to be a member of the DEAD protein family of ATP-dependent RNA helicases (Liang et al., 1994). Futhermore, the vasa gene is expressed in germ cell lineages throughout life in Drosophila melanogaster and Xenopus (Ikenishi, 1998). Although the function of the vasa gene product is still unclear, the observation mentionated above suggest that vasa plays important roles in germ cell determination and development.
