This thesis was done in collaboration with Sorbonne University as part of a double degree Cotutelle. / During development, cell differentiation frequently occurs upon signaling from concentration or activity gradients of molecules called morphogens. These molecules control in a dose-dependent manner the expression of sets of target genes that determine cell identity. A simple paradigm to study morphogens is the Bicoid gradient, which determines antero-posterior patterning in fruit fly embryos. The Bicoid transcription factor allows the rapid step-like expression of its major target gene hunchback, expressed only in the anterior half of the embryo. The general goal of my thesis was to understand how the information contained in the Bicoid morphogen gradient is rapidly interpreted to provide the precise expression pattern of its target. Using the MS2 system to fluorescently tag specific RNA in living embryos, we were able to show that the ongoing transcription process at the hunchback promoter is bursty and likely functions according to a two-state model. At each nuclear interphase, transcription is first observed in the anterior and it rapidly spreads towards the posterior, as expected for a Bicoid dose-dependent activation process. Surprisingly, it takes only 3 minutes from the first hints of transcription at the anterior to reach steady state with the setting of a sharp expression border in the middle of the embryo. Using modeling taking into account this very fast dynamics, we show that the presence of only 6 Bicoid binding sites (known number of sites in the hunchback promoter) in the promoter, is not sufficient to explain the establishment of a sharp expression border in such a short time. Thus, either more Bicoid binding sites or inputs from other transcription factors could help reconcile the model to the data. To better understand the role of transcription factors other than Bicoid in this process, I used a two-pronged strategy involving synthetic MS2 reporters combined with the analysis of the hunchback MS2 reporter in various mutant backgrounds. I show that the pioneer factor Zelda and the Hunchback protein itself are also critical for hunchback expression, maternal Hunchback acting at nuclear cycle 11-12, while zygotic Hunchback is acting later at nuclear cycle 13-14. The synthetic reporter approach indicate that in contrast to Hunchback and Caudal, Bicoid is able to activate transcription on its own when bound to the promoter. However, the presence of 6 Bicoid binding sites only leads to stochastic activation of the target loci. Interestingly, the binding of Hunchback to the Bicoid-dependent promoter reduces this stochasticity while Caudal might act as a posterior repressor gradient. Confronting these experimental data to theoretical models is ongoing and should allow to better understand the role of transcription factors, other than Bicoid, in hunchback expression at the mechanistic level. / Thesis / Doctor of Philosophy (PhD) / Have you ever wondered how a single cell can become a full grown organism? Well it starts when an egg and sperm fuse together. As time passes this single cell divides over and over again until an organism is formed. During this developmental process, somehow the cells know exactly where they are and what they need to become so that they form the organism. However, we don’t fully understand this process and this is what we hope to answer with our research: How do the cells know where they are and what they need to become during development? We study this process in the fruit fly. Although fruit flies might not look a lot like us, during early embryonic development we are quite similar, so we can try to answer these questions in fruit flies and what we find might be relevant to other organisms like us. During development, the first element that an embryo needs to know is the orientation of its body, where the head and tail, the left and right and the back and front of the body will be. We concentrate on studying how the head to tail axis, which we call the anterior-posterior axis, is formed. To know where the head is going to be, the embryo releases proteins called morphogens that broadcast instructions to other genes so that cells know where they are and what they should become. We study a morphogen called Bicoid. Its concentration is high in the anterior, the region that will become the head of the embryo, and lower as you move towards the posterior where the tail will form. Bicoid activates a gene called hunchback, which ends up dividing the embryo in two large parts, the top and the bottom. However, Bicoid’s message fades away during each cell division and needs to be read again at the beginning of each new nuclear cycle. So how is the message read and how long does this process take? This last question is particularly critical during the period of very fast cell division. My thesis tries to answer this question. We found out that it takes 3 minutes for a nuclei to read the Bicoid concentration, activate hunchback and express it correctly. However, in contrast to what was believed before, or namely, that only Bicoid was involved in this process, we found out that other players are involved in helping relay this message. This way hunchback can accurately divide the body in two parts exactly in the middle and without mistake in such a short period of time.