Chapter 19, Problem 23P

The yan gene encodes a transcription factor that represses R7 development in the five cells that express Sevenless and are therefore competent to become R7. In yan null mutants, all five cells in each ommatidium can become R7 (just as in sev-Ras^G12V> transgenic flies; see Fig. 19.7b). The eyes of sev^- yan^- double null mutants display the yan^-> mutant phenotype.

a.	Describe the epistasis relationship between sev- and yan- mutants.
b.	Based on what you know about the roles of Sevenless and Yan proteins in the signaling pathway (Fig. 19.6), is this the epistatic interaction you would have expected? Explain.
c.	Epistasis analysis can be performed only if two mutants in genes in the same pathway have different mutant phenotypes. How is it possible for two different null mutations in the R7 signaling pathway, sev and yan, to elicit different mutant phenotypes?
d.	The constitutively active RasG12V mutation is epistatic to the sev- null mutation, and so is a yan- null mutation; yet both Ras and Yan function downstream of Sevenless in the signaling pathway. Considering your answer to part (c), explain why this makes sense.
e.	Suppose that a researcher identifies flies with a dominant constitutively active mutant allele of yan (yanD) that expresses yan even in cells where the Sevenless pathway is activated. Predict the mutant phenotype of the flies. Would you expect yanD to be epistatic to sev- ? to RasG12V? Explain.