Practice question

Existence of morphogen gradients was substantiated by visualizing Bicoid protein distribution in Drosophila early syncytial blastoderm using antibody staining. Immunostaining revealed anterior-high posterior-low exponential gradient correlating with fate map and hunchback expression boundary. Manipulating bicoid dosage by genetics or mRNA injection shifted gap gene expression boundaries anteriorly or posteriorly in predictable threshold manner, and ectopic gradient induced mirror-image anterior structures posteriorly. This quantitative correlation between protein concentration, enhancer occupancy and transcriptional outcome provided direct evidence that graded distribution of single factor could impart positional values and specify distinct segment identities along embryonic axis without additional signals.