Questions for Thought

1. What is the logic behind this screen? Why are they looking for dominant mutations? Aren't most mutations recessive? What assumptions are they making? Were those assumptions correct here?

What are the advantages of such a screen compared to other types of mutagenesis screens? Think of at least three advantages. What are the disadvantages? Again, think of at least three disadvantages.

Could this general type of approach be used to analyze other genetic pathways in flies or in other organisms? Explain your reasoning.

2. What was the logic behind making temperature sensitive alleles of sevenless? If the authors did not have information about temperature sensitive alleles of src, what approach could they have used to make sevts alleles? How could they test mutant forms of the protein for temperature sensitivity?

3. Why did the authors use the reduced pseudopupil method to score the sevenless phenotype? Why not use the behavioral assay involving the choice test? How many flies can be scored with these two approaches? What sort of false positives or negatives would result from these two different approaches?

4. Simon et al. identified 20 mutant alleles that fell into seven complementation groups. Did they identify all the genes in the sev pathway? Should they have, given the fact that they screened 30,000 flies? How could they determine if they had identified all the genes? If they didn't identify all the genes, why did they recover so many alleles of E(sev)2A? The authors state at the outset (top of page 703) that they expect to obtain "downstream" genes. Why not boss?

5. Simon now has his own lab at Stanford. Two students begin to characterize the other E(sev) alleles.

A) Elaine chooses to work on the III-linked line E(sev)3D. She maps E(sev)3D very carefully and finds that Cass has already generated dozens of mutations in the region. She carries out complementation tests and finds that four embryonic lethal alleles fail to complement the pupal lethality of E(sev) 3D ^e0Q and also fail to complement each other. When placed in trans to e0Q, the Cass alleles are semilethal, most animals dying as pupae, and the adult escapers have rough eyes. By these criteria, all five of these alleles define the same gene. Thus, Elaine was very surprised to find that NONE of Cass's alleles enhanced the sevts phenotype. Propose a hypothesis to explain these data. (Hint: why are some transhets viable?) How could you test your idea?

B) Craig analyzes the X-linked alleles of E(sev)1A. He finds a deficiency for the region that fails to complement the lethality associated with E(sev)1A. The deficiency also acts as an E(sev). Surprisingly, however, the deletion is not as strong an enhancer as the two E(sev)1A alleles, e3C and e0P. What could explain this difference? How would you test your hypothesis?

6. The authors make mosaic animals in two different ways. How do these approaches differ? Why use both methods? What do they reveal? How do these results fit in with the analyses of Elp suppression?

When making clones of E(sev)3D, no phenotype was observed in the eye. How could this allele have been recovered if the gene doesn't seem to function in eye development?

7. Why are the authors so cautious about interpreting the pathway relationships of sev, Sos and Ras1? Could these genes function in parallel, rather than as a linear pathway? Are there genetic tests that would show that sev, Sos and Ras1 act in the same pathway?