Exercise 13.2

Adaptation by Symbiosis: A Bacterium Mitigates the Effects of Nematode Parasitism in a Fly

(This exercise is based on Jaenike, J. et al. 2010. Adaptation via symbiosis: recent spread of a drosophila defensive symbiont. Science 329: 212–215.)

(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.)


Species interactions seldom involve just two species. They often involve several species, and the interacting species can be very distant phylogenetically. One case involves a fly that can be infected by a nematode and have a symbiotic bacterium.

The fly Drosophila neotestacea, which lives in the United States and Canada, feeds on mushrooms. In much of its range, this fly is parasitized by the nematode Howardula aoronymphium. Mated female nematodes attack larval flies. Inside the developing fly, the “motherworm” grows and her nematode offspring develop. As adult flies spend time on mushrooms feeding and (in the case of the females) laying eggs, they release nematode offspring onto mushrooms. There the nematodes mate, and the life cycle is renewed.

Nematode infection often partially or completely sterilizes female flies, thus selecting for mitigation of fertility reduction.


Use the information in Figure 1 to answer questions 1 and 2.


Figure 1 Average number of eggs per ovary in female flies collected in Rochester, New York (1989). Nematode-infected flies (top) versus nematode-free flies (bottom).


Question 1. On average, how many eggs do parasitized flies produce per ovary?


Question 2. On average, how many eggs do unparasitized flies produce per ovary?

Use the information in the paragraph below and in Figure 2 to answer questions 3 and 4.


Starting in the mid-1990s, Jaenike and colleagues noted an increase in the fecundity of nematode-infected female flies. They also found that many of the flies now contained a symbiontic bacterium, Spiroplasma, whereas flies from prior years seldom, if ever, contained the bacterium. They hypothesized that the bacterium reduces the negative effects of the nematode.


Figure 2 Average number of eggs per ovary in female flies collected in Rochester, New York (2008). Nematode-infected flies (top) versus nematode-free flies (bottom).


Question 3. About what percent of flies harbor the bacterium?


Question 4. What pattern do you observe with respect to female fecundity in (a) flies with the bacterium and (b) flies without the bacterium?

Use the information in Figure 3 to answer question 5.

Figure 3 Size of motherworms in nematode-infected flies.


Question 5. Based on these data, what can you infer about the effect of bacteria on the size of the nematode motherworm living in the flies?

Use the information in Figure 4 to answer questions 6 and 7. (Click on the image to enlarge figure.)


Figure 4 Percent of flies in northern United States and southern Canada with the bacterium Spiroplasma. The blackened area in the pie charts represents the percentage of flies that have the bacterium.

Question 6 . How does the percentage of flies with the bacterium change as samples go from east to west?


Question 7. What could explain this pattern?