Autonomous determination of anterior structures in the early Drosophila embryo by the bicoid morphogen.

A small number of maternal effect genes determine anterior-posterior pattern in the Drosophila embryo. Embryos from females mutant for the maternal gene bicoid lack head and thorax. bcd mRNA becomes localized to the anterior tip of the egg during oogenesis and is the source for the morphogen gradient of bcd protein. Here we show that in vitro transcribed bicoid mRNA that has its own leader sequences substituted by the Xenopus beta-globin 5' untranslated sequences is translated more efficiently than bicoid mRNA with the natural 5' mRNA leader when tested in vitro and in Drosophila Schneider cells. When injected into bicoid mutant embryos, only the bcd mRNA with the beta-globin leader sequence, substituted for the natural leader, is able to induce anterior development. We used P-transformation to show that sequences in the 5' leader are neither necessary for localization of the transcript nor for the translational block of the bcd mRNA during oogenesis. For our injection experiments, we used only one of the identified splicing forms of bcd mRNA. The bcd protein species derived from this mRNA is able to induce anterior development at any position along the anterior-posterior axis. Thus bicoid protein can induce development of head and thorax independent of any other specifically localized morphogenetic factor. Our findings further support the notion that the concentration gradient of bcd protein, and not the existence of different forms of bcd protein, is responsible for specifying subregions of the embryo.