(This exercise is based on Holland, B., and W.R. Rice. 1999. Experimental removal of sexual selection reverses intersexual antagonistic coevolution and removes a reproductive load. Proc. Natl. Acad. Sci. 96: 5083–5088.)
(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 Drosophila and other insects, males transmit seminal fluid as well as sperm during copulation. This seminal fluid contains proteins and other chemicals that cause females to be more likely to lay eggs and less likely to mate again. This altered female behavior is adaptive for the male because it increases the number of his progeny that the female will produce. In contrast to these positive effects, these chemicals have negative consequences, as they reduce the longevity of the females.
Because they have positive fitness effects in one sex and negative effects in the other, seminal fluid chemicals are an example of a sexually antagonistic trait. Sexual antagonism occurs because the evolutionary interests of males do not completely coincide with the evolutionary interests of females. In this case, the interests of the sexes diverge because female Drosophila can and often do mate with multiple males. The first male that a female mates with gets an evolutionary benefit if he somehow manipulates the female to be less receptive to mating and more receptive to egg laying.
This is not necessarily in the interest of the female. Even if the chemicals used to manipulate the female were somewhat toxic, a male would benefit if he gained more when she switched from mating to egg laying than he lost from her shortened lifespan. Moreover, it is in the evolutionary interest of females to evolve resistance to both the manipulation and the longevity-reducing effects of the seminal fluid. Thus, an arms race with males evolving more powerful chemicals and females evolving greater resistance could arise.
Courtship itself is potentially a sexually antagonistic trait. It, too, could evolve in an evolutionary arms race, where females evolve resistance to male courtship and males evolve more intense courtship to counteract the female resistance.
What if a population of flies became strictly monogamous, with each female (and each male) mating only once? Under strict monogamy, the evolutionary interests of males and females would coincide. In a classic experiment, Brett Holland and William Rice subjected experimental lines of Drosophila melanogaster to enforced monogamy, wherein the line was started each generation with pairs of males and females that mated only with each other. In contrast, in the control lines, lines were started each generation with groups of three males for every female. This was continued for 34 generations.
Question 1. Under these conditions of monogamy, what would you expect to happen to the toxicity of seminal fluids over the course of the selection experiment? Why?
Question 2. Assuming that female resistance to seminal fluid involves some costs, what would you expect to happen to female resistance to seminal fluids over the course of the selection experiment?
Use the information in Figure 1 to answer questions 3 through 5.
Question 3. Relative to the test females mated to control males, do the test females mated to the monogamy males have greater or shorter survivorship? Is this consistent with the expectations?
Question 4. Relative to the test females mated to control males, do the test females mated to the monogamy males have higher fecundity?
Question 5. Based on this graph, do females housed with monogamy males have greater or lower reproductive success than females housed with control males?
Use the information in Figure 2 to answer questions 6 and 7.
Question 6. Do monogamy males court more or less than control males? How great is the difference in each of the replicates?
Question 7. Provide a plausible reason for why the change in courtship patterns may be an adaptation as the lines evolved under monogamy.
Use the information in Figure 3 to answer questions 8 and 9.
Question 8. How does survivorship of the monogamy females compare to that of the control females?
Question 9. What can you infer about female resistance to toxic effects of male courting and/or seminal fluid chemicals during the evolution of monogamy?