(This exercise is based on Green, D. M., A. R. Kraaijeveld, and H. C. J. Godfray. 2000. Evolutionary interactions between Drosophila melanogaster and its parasitoid Asobara tabida. Heredity 85: 450–458.)
(Note: The reference above links directly to the article on the journal’s website. In order to access the full text of the article, you may need to be on your institution’s network [or logged in remotely], so that you can use your institution’s access privileges.)
As we saw in the Chapter 13 of the textbook, host–parasite interactions often involve arms races; both the host and its parasites (or parasitoids) are likely to be under evolutionary pressure. Natural selection will likely act on a host such that individuals that possess attributes making them more resistant or more tolerant of parasites will be more likely to survive and reproduce. Likewise, parasites that are better at evading the host’s defenses will be more likely to survive and reproduce, and thus would be favored by natural selection.
In nature, a number of different wasp parasitoids attack larvae of Drosophila. Through use of its ovipositor, a female wasp will lay eggs in the fly larva. The eggs of the parasitoids hatch, and these wasp larvae feed internally on the fly larva, eventually causing the fly to die. A Drosophila larva is not without defenses; it may be able to mount a cellular immune response against parasitoid eggs. If the fly does mount a successful immune response against a parasitoid egg, a capsule will form around an egg, thus killing the parasitoid. One way that a parasitoid can evade being encapsulated is to damage the immune system of the host before it has had the chance to encapsulate; in this way, parasitoids that are exposed to hosts that often encapsulate would be expected to evolve higher virulence.
Charles Godfray and his associates at Silkwood Park at Imperial College in London have been examining the interactions between Drosophila melanogaster and its parasitoid wasps for many years. In this study, they focused on the outcomes if D. melanogaster were allowed to coevolve with one particular parasitoid, Asobara tabida, in a semi-natural setting.
Prior to the study, the base population of Asobara tabida had been cultured with a different fly species (D. subobscura), one that does not encapsulate eggs. Thus, it had not been subjected to selection for increased virulence.
Fly populations were maintained under three different regimes: (A) without parasitoids, (B) with outbred parasitoids (i.e., parasitoids whose parents were not closely related), and (C) with somewhat inbred parasitoids. The difference between regime B and regime C is that the outbred parasitoids should have more genetic variation than the inbred parasitoids.
After ten fly generations (equivalent to about five parasitoid generations), the flies from each treatment were tested for their ability to encapsulate parasitoid eggs. Note that all flies were tested with the same reference parasitoid strain (an inbred laboratory strain of A. tabida that had not been exposed to flies that encapsulate eggs).
Figure 1
Question 1
Figure 1 shows the percentages of flies (and standard errors) that are able to successfully encapsulate eggs from the standard parasitoid. A, B, and C refer to the different treatments (see Introduction). The bar designated “B & C” represents the pooled data from treatments B and C. The placement of the same lowercase letter above more than one bar indicates a group of treatments whose means did not differ statistically from one another. What conclusions can be drawn from these results?
Figure 2
Question 2
Figure 2 shows the relative virulences (and standard errors) of the base population of A. tabida and those wasps that evolved with D. melanogaster. Treatment B are the wasps that are outbred, and treatment C are the wasps that are partially inbred. The placement of the same lowercase letter above more than one bar indicates a group of treatments whose means did not differ statistically from one another. What conclusions can be drawn from these results?
Question 3
What possible explanations exist for the lack of increase in virulence in wasps that were exposed to flies that encapsulate eggs? What role can genetic variation play?
Figure 3
Question 4
Figure 3 shows the feeding rates (and standard errors) of flies subjected to the three treatments (as well as the pooled data from treatments B and C). The placement of the same lowercase letter above more than one bar indicates a group of treatments whose means did not differ statistically from one another. What conclusions can be drawn? Do the results indicate that a trade-off may affect the evolution of resistance to parasitism in this fly species?
