Chapter 13, Problem 29P

Drosophila P elements were discovered because of a phenomenon called hybrid dysgenesis—sterility of particular hybrid progeny. When scientists in the 1970s crossed their D. melanogaster laboratory strains to flies of the same species obtained from natural environments outside the lab, they observed a remarkable result: The progeny of the crosses were sterile, but only when outside males were crossed with lab strain females. Progeny resulting from crosses of outside females with lab males were perfectly normal.

DNA analysis revealed that while the genomes of the outside flies contain P elements, the lab fly genomes have none. Apparently, P elements spread throughout the wild population of D. melanogaster after the capture of the originators of present-day laboratory strains over 100 years ago.

a.	The hybrid progeny are sterile because their germ-line cells have a high rate of mutation and chromosomal rearrangement (dysgenesis) caused by high rates of P element mobilization. Explain how P element movement can cause dysgenesis.
b.	Scientists first hypothesized that the deposition of P element-encoded repressor protein (see Fig. 13.27) in egg cytoplasm is behind the observation that dysgenic progeny result only from crosses of laboratory females with outside males, and not vice versa. Explain this hypothesis. Why do the P elements mobilize when the cross occurs in one direction but not the other? (You will see in Chapter 17 that this hypothesis is correct, but it accounts for only part of the story.)
c.	When males from certain outside strains are mated to lab females, the hybrid progeny are only partially sterile rather than completely sterile. Given this information, describe crosses that would allow you to isolate loss-of-function mutations in the X-linked Drosophila gene yellow that are caused by P element insertion. (These recessive mutant alleles will produce yellow rather than the wild-type tan body color.) At the molecular level, what do you think explains the difference between outside strains whose hybrid progeny are all sterile and outside strains whose progeny are only semisterile?
d.	In wild-type fruit flies, researchers can observe rare patches on the bodies that have yellow rather than tan color. Interestingly, the frequency of these yellow patches did not increase in the progeny of a cross between outside males and lab females. What property of hybrid dysgenesis does this result suggest?