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Drosophila and other insects, males transmit seminal fluid as well as sperm during copulation.
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.
QUESTION: 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?
