Question

Genetic mosaics are created within the proneural clusters in the background of a heterozygous Notch-/+ organism. One of the cells within such a cluster gets a Notch-/Notch- genotype. Predict whether this cell will become a neuroblast or an epidermoblast. Briefly explain your answer.

Answer 1

Lateral inhibition. A) All cells of a proneural cluster express the proneural genes of the achaete-scute (a-sc) complex and thus all have neurogenic potential. The neurogenic genes Notch and delta are also evenly but weakly expressed. Activated Notch (bound to Delta) is cleaved and the intracellular domain (NICD) is liberated at low levels. B) By chance one cell within the cluster has higher levels of Delta expression (D in the cluster, right cell). This cell activates Notch in surrounding cells more strongly (N in the cluster, left cell). Higher levels of NICD are translocated to the nucleus (together with Suppressor of Hairless (Su(H)) and activate the enhancer of split (E(Spl)) genes. These in turn shut off expression of the a-sc complex and eventually also delta, resulting in a loss of neuronal competence in the surrounding cells. This feedback-loop is called lateral inhibition and guarantees that C) only one cell of a proneural cluster delaminates as neuroblast (NB), while the other cells remain in the ectoderm and become epidermoblasts (Epi).