Exercise 12.2

Flies That Evolve with a Selfish Genetic Element Are More Likely to Multiply Mate

(This exercise is based on Price, T. A. R., D. J. Hodgson, Z. Lewis, G. D. Hurst, and N. Wedell. 2008. Selfish genetic elements promote polyandry in a fly. Science 322: 1241–1243.)

(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.)


In most species of Drosophila, a female mating with a single male usually supplies sufficient sperm to fertilize her lifetime supply of eggs. Moreover, mating is often costly to female flies. So why do females of some species mate more than once? Perhaps they do this to assess the quality of males and/or the sperm males produce via sperm competition.

Tom Price, Nina Wedell, and their colleagues at the University of Exeter provide evidence for the sperm competition hypothesis. They show multiple mating increases in frequency when a selfish genetic element that reduces sperm quality is prevalent.

In Drosophila, females are XX and males are XY. They normally occur in equal numbers. In D. pseudoobscura, males that harbor the X-linked selfish genetic element sex ratio (SR) have produced nearly all female progeny, as SR sabotages Y-bearing sperm. The SR element gains a tremendous transmission advantage but causes males that bear it to have reduced sperm numbers.

Females that mate with SR males will have, on average, fewer grandchildren due to the reduced sperm count of these males. Thus, an allele that would allow females to preferentially mate with non-SR males and avoid SR ones would be selectively favored. Yet, these females show no mating preference against SR males. However, SR males are inferior at sperm competition; if a female mates with both an SR and a non-SR male, most of her progeny will be sired by the non-SR male. If SR is prevalent in the population, multiple mating would substantially increase the likelihood that a female will mate with a non-SR male and thus have mainly SR males.



Question 1. If the frequency of SR in a population is 0.3, what is the probability that a female will mate with a non-SR male if she mates once? If she mates twice? (Assume mating is at random.)


Question 2. What would these same probabilities be for females that mate once versus twice, if the frequency of SR were 0.1? Compare this with what you obtained in Question 1.

Use the information in the paragraph below to answer question 3.


Price and colleagues took the progeny of wild-caught D. pseudoobscura from an area where SR is present, divided the progeny into lines, and subjected the lines to one of three different treatments. In treatment A, SR was present at an initial frequency of 30 percent, and the sex ratio of the population was maintained at two females for every male. In treatment B, there was no SR, and the sex ratio was maintained at one female for every male. In treatment C, there was no SR, and the sex ratio was maintained at two females for every male. These treatments were maintained for ten generations.


Question 3. Why did the researchers use treatment C in addition to treatments A and B?

Use the information in Figure 1 to answer Questions 4 and 5.


Figure 1 The researchers assayed females from each line for their likelihood to remate immediately after mating. The data here show the proportion of females observed remating at generation 10, for each selection regime (N = 12 lines), showing median, interquartile range, and range. ***P1 0.0005; n.s., not significant.


Question 4. What can you infer about the effect of the presence of SR on the frequency of remating?


Question 5. What can you infer about the effect of the population sex ratio on the frequency of remating?

Use the information in Figure 2 to answer Questions 6 and 7.


Figure 2 The mean number of days to remating, for each selection regime (N = 11 lines), when females were mated to standardized stock males, showing median, interquartile range, and range. ***P, 0.0001; n.s., not significant.


Question 6. By how much did the females from lines with SR differ with respect to their average time to remating as compared with those from lines without SR?


Question 7. Did the population sex ratio significantly affect the mean time to remating?