Which one of the following morphogens is involved in anterior-posterior patterning in Drosophila?
Anterior-posterior axis in Drosophila is initially established by maternally localized mRNAs along oocyte cortex. Bicoid transcript anchored anteriorly by cytoskeletal proteins, Nanos posteriorly, creates opposing gradients determining polarity. Bicoid protein diffuses posteriorly forming concentration gradient that activates gap genes hunchback and orthodenticle in threshold-dependent manner specifying head and thorax, while repressing caudal translation posteriorly refining pattern. Notch signaling mediates later segmentation boundary sharpening and neuroectoderm decisions but does not provide primary AP positional information. Thus morphogen driving AP patterning is Bicoid, functioning as anterior determinant and concentration-dependent transcription activator of zygotic genome.
Ref: St Johnston and Nusslein-Volhard, Development 1992, Bicoid Establishes Drosophila AP Polarity.