The embryonic vertebrate forebrain: the prosomeric model.

C. elegans early embryos extends beyond the parallels between hunchback and glp-1 translational regulation. Both embryos contain cytoplasmic "granules" in the posterior region of the zygote that are segregated ultimately to germ cell precursors during em-bryogenesis-the polar granules of Dro-sophila (14) and the P granules of C. elegans (15) (see figure). In Drosophila, maternal nanos RNA is associated with polar granules ; perhaps in C. elegans, a homolog of nanos is associated with P granules. What about vertebrates? Does transla-tional repression in the posterior cyto-plasm establish embryonic polarity in these "higher" animals? A hint that this mechanism may indeed function in vertebrates comes from the identification of a maternal transcript that encodes a nanos-like protein called Xcat-2 in Xenopus embryos (16). Although the function of Xcat-2 is unknown , its location at the vegetal pole suggests a role in early pattern formation. Furthermore , a "germ plasm" exists in the veg-etal cytoplasm of amphibian embryos that may be analogous to P granules and polar granules of worms and flies (17). Over the past decade, a handful of molecular mechanisms have been implicated in the pattern-ing of Drosophila, C. elegans, and Xenopus embryos (1, 18, 19). On the basis of the diversity of these mechanisms, the prevailing view has been that each embryo has differentially employed a handful of common molecular mechanisms to create its own coordinate system. For example, localized transcriptional activators are utilized for patterning of both C. elegans and Dro-sophila early embryos (20-23), but the mechanisms for localization, types of DNA binding protein, and specified fates are not obviously similar. By contrast, the molecular parallels between hunchback and glp-1 regulation suggest the existence of an ancient mechanism for creating asymmetric patterns of gene expression in early embryos (see figure). This mechanism is predicted to depend on a transacting regulator similar to nanos and to act through cis-acting sequences similar to NREs in the 3'UTRs of maternal transcripts. If this molecular machinery regulates polarity in embryos as diverse as worms, flies, and frogs, it becomes plausible that it influences axis formation in all animal embryos, including mammals. "Molecu-lar tinkering" (24) may then come into play to reinforce this primitive patterning control and to derive other axes from it. Research in Drosophila has pioneered our understanding of the molecular mechanisms that can establish the body axes in an early embryo. Now, phylogenetic comparisons will tell us which mechanisms are primitive …