In all animals information is passed from parent to offspring via the germline, which segregates from the soma early in development and undergoes a complex developmental program to give rise to the adult gametes. Many aspects of germline development have been conserved throughout the animal kingdom. Here we review the unique properties of germ cells, the initial determination of germ cell fates, the maintenance of germ cell identity, the migration of germ cells to the somatic gonadal primordia and the proliferation of germ cells during development invertebrates and invertebrates. Similarities in germline development in such diverse organisms as Drosophila melanogaster, Caenorhabditis elegans, Xenopus laevis and Mus musculus will be highlighted.

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The Bicaudal‐C (Bic‐C) gene of Drosophila melanogaster is required for correct targeting of the migrating anterior follicle cells and for specifying anterior position. Females lacking any wild type copies of Bic‐C produce only eggshells open at the anterior end, because of the failure of the columnar follicle cells to migrate in the correct position at the nurse cell‐‐oocyte boundary. Embryos which develop from eggs produced in females with only one wild type copy of Bic‐C show defects in anterior patterning and an abnormal persistence of oskar RNA in anterior regions. We cloned Bic‐C and found that, in ovaries, Bic‐C RNA is expressed only in germline cells. Bic‐C RNA is localized to the oocyte in early oogenesis, and later concentrates at its anterior cortex. The Bic‐C protein includes five KH domains similar to those found in the human fragile‐X protein FMR1. Alteration of a highly conserved KH domain codon by mutation abrogates in vivo Bic‐C function. These results suggest roles for the Bic‐C protein in localizing RNAs and in intercellular signaling.

Bicaudal-C (Bic-C) is required during Drosophila melanogaster oogenesis for several processes, including anterior-posterior patterning. The gene encodes a protein with five copies of the KH domain, a motif found in a number of RNA-binding proteins. Using antibodies raised against the BIC-C protein, we show that multiple isoforms of the protein exist in ovaries and that the protein, like the RNA, accumulates in the developing oocyte early in oogenesis. BIC-C protein expressed in mammalian cells can bind RNA in vitro, and a point mutation in one of the KH domains that causes a strong Bic-C phenotype weakens this binding. In addition, oskar translation commences prior to posterior localization of oskar RNA in Bic-C ؊ oocytes, indicating that Bic-C may regulate oskar translation during oogenesis.Anterior-posterior polarity in Drosophila melanogaster is established during oogenesis through the asymmetric localization of many RNAs and proteins in the egg (17, 45). Localized molecules include the oskar (osk) and nanos (nos) RNAs, which are localized at the posterior of the developing oocyte during midoogenesis and are required to specify posterior pattern information (11,22,48,49). Eggs with osk or nos RNA mislocalized at the anterior produce bicaudal embryos whose posterior structures are duplicated at their anterior ends (12,14). In addition to asymmetric RNA distribution, the localization of many maternally expressed proteins occurs through translational regulation of their RNAs (30). For example, translation of osk is repressed until posterior localization of its RNA at stage 9 of oogenesis. This translational repression is mediated in part by Bruno, a protein which interacts with specific sequences (termed BREs, for Bruno response elements) in the osk 3Ј untranslated region (UTR). In oocytes produced by females with a transgene lacking BREs (osk-BRE Ϫ

Homeotic genes encode DNA-binding transcription factors that specify the identity of a segment or segments in particular body regions of Drosophila.