Chapter 19, Problem 20P

A temperature-sensitive allele of the gene encoding the Notch protein (N^ts) helped researchers understand the many roles of this protein in fly eye development. Notch is a transmembrane receptor that, when bound to a ligand, relays a signal to the nucleus. In one experiment, wild-type and N^ts homozygous developing eyes were allowed to grow in larvae for several hours at permissive temperature, and then the temperature was shifted to the restrictive temperature. After 4 hours, the eyes were dissected from the larvae and the photoreceptors were labeled with an antibody to a protein expressed in all photoreceptors (blue cells in the figure that follows are labeled with antibody). The black dots represent ommatidia at more advanced stages of development that are not shown in the figure.

Eye development occurs in a structure called the eye imaginal disc present in the larva.

Ommatidia develop behind an indentation called the morphogenetic furrow (mf in the diagram). The furrow forms at the posterior of the disc and moves anteriorly; every 2 hours, a new row of ommatidia initiates development posterior to the furrow, while the rows behind that row mature successively to the next stages of assembly. (Only one ommatidium is shown in the diagram, rather than an entire row.) Therefore, in a single eye disc, ommatidia at all stages of development are present. As you saw in Fig. 19.3, the first cells to join the ommatidium are the photoreceptors, R1–R8, and they do so in a particular order.

 

Describe the different roles of the Notch protein at different stages of ommatidial assembly.