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2005_pset3 - 7.06 Problem Set#3 Spring 2005 1 The...

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1 7.06 Problem Set #3, Spring 2005 1. The Drosophila compound eye is composed of about 800 units called ommatidia. Each ommatidium contains eight photoreceptor neurons (R1 through R8), which develop in a fixed order. Mutations have been isolated that perturb the differentiation of the R7 cells. Two such mutations are in genes called Sev ( Sevenless ) and Boss ( Bride Of Sevenless ). R7 progenitor cells in fly eyes that do not express either (or both) of these genes differentiate into non-neural cone cells (instead of R7 cells). Too many R7 cells develop in fly eyes that are overexpressing either BOSS or SEV (or both). a. Overexpression of SEV in fly eyes that do not express functional Boss leads to ommatidia with too many R7 cells. Overexpression of BOSS in fly eyes that do not express functional Sev generates ommatidia with no R7 cells. What do these results imply about the relationship between Sev and Boss ? b. You have isolated a gain of function mutation in Sos ; you call this allele Sos *. You find that flies that are expressing Sos* in their eyes have high numbers of R7 cells. What two strains would you make (given that you have gain of function and loss of function alleles in SEV and BOSS available to you) if you didn’t already know the order of Sos in the fly eye developmental pathway with Sev and Boss , and what phenotype would each strain have? c. Given what is known now about the RTK/Ras pathway and fly eye development, how do you think the SOS* protein operates molecularly that makes it different from the wild- type SOS protein? d. To learn about the expression pattern of your genes Sev, Boss and Sos , you perform in situ hybridizations (a molecular technique for detecting where in a cell or an organism a specific mRNA is expressed). In your in situ hybridization experiment, you incubate three different sets of fly eye cells with antisense probes that are complementary to the Sev , Boss , and Sos mRNAs. You can then use a colormetric assay to detect where in the fly eyes these mRNAs are being expressed. Given what is known about fly eye development, what do you predict that you will see?
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2 e. You next perform a “mosaic” experiment. In this experiment, you make mosaic flies, or flies that are a mix of two cell types that differ in their genotypes. In one line of mosaic flies, the R1, R2, R3, R4, R5, R6 and R7 progenitor cells have one functional copy of Boss, while the R8 progenitor cells have no functional copies of Boss. In another line of mosaic flies, the R1-R7 progenitor cells have no functional copies of Boss , but the R8 progenitor cells have one functional copy of Boss . For each of these flies, predict whether their ommatidia will form normally.
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