The early Drosophila embryo develops through a se-tion (7,8), thus complementing the gradient of Bicoid (Fig. ). ries of rapid syncytial nuclear divisions. The nuclei migrate to the egg membrane at the periphery to form
The third maternal transcription factor is the zinc finger protein Hunchback, which becomes asymmetri-individual cells. This formation is achieved by the membrane extending between the nuclei to generate a sin-cally distributed in response to nanos activity [reviewed in (3)]. The nanos mRNA is localized to the posterior gle-layered cellular epithelium, the blastoderm (1). These processes occur in parallel with pattern-forming pole region, and the protein emanating from this source results in a posterior-to-anterior concentration gradi-events that establish a molecular prepattern of the Drosophila body plan as defined by the expression of seg-ent, which is required for abdomen formation. Nanos (with uniformly distributed Pumilio and other not yet mentation genes in a series of stripes along the anterior-posterior axis of the blastoderm embryo (2).
identified proteins) binds to the nanos response element located within the 3 untranslated region of Pattern formation along the anterior-posterior axis is initiated by three asymmetrically distributed maternal evenly distributed maternal hunchback mRNA and causes its translational repression in the posterior re-transcription factors, and their activities are mediated through zygotic transcription factors that subdivide the gion of the embryo (3,9,10). Hunchback acts as a strong transcriptional repressor of posteriorly expressed gap embryo into increasingly smaller units [for reviews, see (2 -4)]. At the first zygotic level of this cascade of tran-genes such as knirps and giant [reviewed in 4)]. The generation of a Hunchback-free region in the posterior scriptional events, the locally activated gap genes add a new series of overlapping short-range transcription half of the embryo (by combined nanos and pumilio activities) is therefore necessary for the expression of factor gradients to the preexisting maternal long-range gradients. The gap gene-encoded transcription factors the posterior gap genes by Caudal and Bicoid [reviewed in (4)].
(2) act horizontally by locally repressing the activation of neighboring gap genes and vertically through inter-CONTROL OF FIRST ZYGOTIC GENE actions with distinct cis-acting modules mediating ACTIVITIES: MATERNAL INPUT AND stripe expression of the pair-rule genes (2). We report CROSS TALK that most of the established genetic interactions within the segmentation gene cascade depend on direct DNA -Activation of the gap genes depends on Bicoid, Caudal, Hunchback, [reviewed in (3,4)] and an as yet un-protein interactions. The mechanisms allowing to generate positional information for localized gene expres-known transcription factor, which is activated in the terminal regions of the embryo in response to the torso-sion along the anterior-posterior axis are discussed.
dependent raf/ras signal transduction pathway [re-MATERNAL GRADIENTS THROUGH viewed in (11)]. Expression of the gap genes are found DIFFUSION AND TRANSLATIONAL in specific regions of the preblastoderm embryo (Fig.
REPRESSION
1b), which fail to develop in the respective mutants [reviewed in (2)]. They include the terminal gap genes The homeodomain transcription factor Bicoid forms tailless and huckebein, the head segmentation genes an anterior-to-posterior concentration gradient that orthodenticle, empty spiracles, and buttonhead, and the emanates from maternally prelocalized mRNA in the central gap genes hunchback, Kru ¨ppel, knirps, and gianterior pole region of the egg (3). After translation, ant, which encode transcription factors containing ho-Bicoid diffuses to form a concentration gradient exmeodomains, zinc fingers, or a basic leucin zipper as tending from the source, the site of mRNA localization, their DNA-binding motif [reviewed in (2)]. Gene intertoward the posterior. Bicoid is required for the activaaction studies (2,4) have revealed an elaborate genetic tion of zygotic segmentation genes, which establish network (Fig. ) showing that (a) terminal gap genes head and thoracic segments in the embryo [reviewed are activated by the maternal terminal system, the in (2 -4)]. Recently, a second function of Bicoid aptorso-dependent ras/raf signal transduction pathway, peared with the discovery that Bicoid binds to evenly distributed maternal caudal mRNA via its homeodomain and represses its translation (5,6).