In wild-type flies, every ommatidium has a single R7 cell. The ligand Boss, prod
ID: 265565 • Letter: I
Question
In wild-type flies, every ommatidium has a single R7 cell. The ligand Boss, produced in the R8 cell, activates the receptor tyrosine kinase Sev in the prospective R7 cell, causing it to adopt the R7 fate. If either Boss or Sev are absent, no R7 is produced. Activation of Sev by Boss transduces signals via a canonical Ras- Raf-Mek-Erk cascade (shown in the figure) Boss (ligand) SE GR82 ? ( RAS TP P If activated Ras is expressed using a fragment of the Sev promoter, all five cells in which it is expressed become R7 cells MEK You can perform tissue-specific RNA against various components in flies (using the Sev promoter to express Gal4, which drives P UAS-shRNA in potential R7 cells) ERK nscnpbu fa A. If a fly has the RasG12V transgene and UAS-shRNA is directed against RAF, would you expect the RNAi-treated flies to look (i) more wild-type (i.e. have fewer R7 cells, suppressed), (ii) the same, or (iii) more mutant (i.e. have more R7 cells, enhanced) than a control strain with the RasG12V transgene but no RNAi? Why? (4 points) B. If a fly the RasG12V transgene and UAS-shRNA is directed against SEV, would you expect the RNAi-treated flies to look (i) more wild-type (i.e. have fewer R7 cells, suppressed), (ii) the same, or (iii) more mutant (i.e. have more R7 cells, enhanced) than a control strain with the RasG12V transgene but no RNAi? Why? (4 points)Explanation / Answer
Answer A.
Option (i)
The iRNAI treated cells are not expected to look like R7 cells. This is due to the fact that although all the signal cascaded proteins are produced by the mutatnt cell, the UAS-shRNA is directed againts RAF which is the downstream regulator of Ras signalling pathway,
Answer B
Option (ii)
The fly cells having activated Ras transgene expressed look like wild type cells even though UAS-shRNA is directed against Sev gene. This is because Sev directed shRNA will inhibit the fromation of Sev receptor and since cells are expressing activtaed Ras, this will not affect downstream signalling cascade.