[Source of asymmetry in ontogeny: early polarization of the germline cyst and oocyte in Drosophila].

One or more body axes are already formed in the egg in many vertebrates and invertebrates. In Drosophila, the anteroposterior and dorsoventral axes are determined during oogenesis owing to the asymmetric localization of the bicoid, oskar, and gurken mRNAs in the oocyte (prospective egg). The localization of these transcripts depends on the polarized organization of the oocyte cytoskeleton and, consequently, the oocyte polarity. Initial asymmetry, leading to the oocyte polarity, is established in early ontogeny, during oocyte determination. The review considers the steps of early polarization and oocyte differentiation in Drosophila, the genetic control of these processes, and the findings that suggest an early oocyte polarity for vertebrates.