Chapter 19, Problem 16P

Suppose a particular gene is required for early development and also later for development of a particular tissue, such as the adult nervous system. By generating a homozygous mutant clone in that tissue of a heterozygote, researchers can circumvent the lethality that would result if the entire animal is homozygous for a loss-of-function mutation in that gene.

A technique called MARCM (Mosaic Analysis with a Repressible Cell Marker) was developed to enable Drosophila geneticists to generate homozygous mutant cell clones that are marked by the presence of a reporter protein such as GFP. Marker expression enables the investigator to observe clearly the mutant phenotype within a clone of mutant cells. This technique relies on a yeast protein called Gal80 that is a negative regulator of the Gal4 protein described previously in Solved Problem II. Gal80 binds to Gal4 and prevents it from activating transcription. The idea of MARCM is that Gal4/UAS -driven GFP expression is blocked by Gal80 throughout the fly, except within the homozygous mutant clone where the Gal80-expressing transgene is lost by mitotic recombination.

a.	Diagram the chromosomes and the mitotic crossover that generate a homozygous m mutant clone marked by GFP expression.
b.	How could you restrict the clones to the adult nervous system?