Paper2InClassQ&A

(2) Briefly describe the strains that were used to generate the flies that expressed hunchback (hb) in the anterior at a time when it is normally downregulated (during cellularization). Dont forget to mention how they targeted the hb expression. Gal 4 UAS ( another activating sequence called GNC) was expressed under control of nanos and bicoid. This is weird, because nanos is localized to the posterior of the embryo. However, they appear to have used the bicoid localization signal to dictate that Gal4 expression would be at the very anterior of the embryo. The other strain included the UAS sequence hooked to the hb cDNA. Thus, GAL4 protein, made only at the anterior, would bind to the UAS seq upstream of hb (present in every cell), and activate expression of hb in the anterior. This activation of hb overcame normal repression of hb levels by torso. (2) In Table 1, what were the authors looking at Why did the authors look at flies from bicoid-/ mothers What does their data tell you about bicoid (is it a recessive or dominant maternal effect gene) Bicoid must be at least partially dominant, or offspring of a bicoid -/ mother would be totally wild type. We know from this table that reducing the amount of bicoid while overexpressing Hb has a negative effect on embryos (a lot of embryos die). This suggests , although the data is more clear in Figure 3, that bicoid might actually have an inhibitory effect on hb, even though it initially positively affects its translation. (2) In Figure 3, the authors are showing expression of the gene cnc (by in situ hybridization) in different genotypes of flies. The figure doesnt show this, but the text states that cnc expression is normal in Hb embryos at the early stages being shown in panels A-C and G-I. What, then, is the significance of the data shown in panels G and H The main point of this figure is to demonstrate that in embryos from bicoid het mothers, who are also overexpressing Hb, there is a downregulation in the cnc expression. Embryos from bicoid het mothers alone do not show this downregulation. This implies that bicoid might normally inhibit Hb from inhibiting cnc expression. When bicoid levels are lower than normal, this inhibition is removed, and the cnc expression is decreased. (2) In Figure 7, expression of cnc, wg and hh is shown to be normal in hunchback (lf) mutants. What do panels A,B and C indicate about the relationship between tor and hb Think about the phenotype being displayed by the double mutant vs. the phenotypes of the single mutants (ie, phenotype of hb (lf) and phenotype of torso (lf). Its an epistasis test. If torso acted directly upon hb (we already know it somehow inhibits hb expression, and thus should act upstream), then in the double mutant with hb, the embryos should look like hb mutants. But they dont, they look like the torso mutants. This implies torso acts indirectly on the downregulation of hb. Note that from Figure 5, we already know that a tor- and Hb phenotype are similar wg and hh expression are downregulated in both situations, because higher levels of hb inhibit their expression. (2) Finally, looking at the pathway proposed in Figure 8C, and using information from the paper, suggest how hb, torso and bcd must be working to designate the very anterior end of the Drosophila embryo. At the very anterior end, hb and bicoid appear to have opposing functions. Bcd induces anterior expression of genes like cnc, wng, hh, while Hb, if present, antagonizes the expression of these genes. Thus, although bcd is originally positively regulating hb, the anterior terminal requires absence of hb function. Furthermore, torso has a role in inhibiting hb levels, although the interaction is not direct (ie, in an epistasis test, hb and torso double mutants did not look like hb mutants). MCDB 4650 DEVELOPMENTAL BIOLOGY Spring 2006 Name ___________________________________________