Having sex with only one partner makes males nicer and females less defensive, at least in fruit flies. It seems the flies don't benefit from having a choice of mate. In fact, they do worse.
Male Drosophila melanogaster are promiscuous and compete for available females. To try to ensure that their sperm fertilises the female's eggs, males have evolved toxic seminal fluid that disables the sperm of other males that have mated with the same female, and carries hormones that make the female less sexually receptive, reducing the chance that she will mate again. These toxins and hormones are harmful to females, however, so they have evolved defences against them.
In short, sex in fruit flies is a running battle in which males have evolved to fertilise as many eggs as possible, while females have evolved to protect their own health. "They can antagonistically coevolve just like predator and prey," says William Rice of the University of California at Santa Cruz.
Rice and his student Brett Holland hoped to find out far fruit flies have gone in this arms race. Holland divided a laboratory population of flies into four subpopulations in which females lived in individual vials. In two of the subpopulations, each female shared a vial with three males, which competed in the usual way to fertilise her.
In the other two subpopulations, Holland forced the flies to be monogamous by putting only one male with each female. Under such conditions, males have nothing to gain by emphasising quantity over quality. "In this case, what's good for a female is good for her mate and vice versa. That should convert the arms race into a mutualistic interaction," says Rice.
After 34 generations, Holland measured the toxicity of the flies' seminal fluid by mating the males with females that were specially bred to be particularly sensitive to seminal fluid toxins. The females were 20 per cent less likely to die after a single mating with monogamous males, Holland told last week's meeting. Monogamous males also spent 40 per cent less time harassing their females. Such harassment often reduces female survival, other researchers have found.
Monogamous females, in turn, proved more susceptible than their polygamous cousins to the seminal fluid toxins of normal males, Holland found. In other words, evolution stripped away much of their resistance.
To see the overall result of relaxing the arms race, Holland raised flies from each of the four lines under identical conditions. In every case, flies from monogamous lines produced more offspring than those from polygamous lines. That was a surprise, since biologists have usually assumed that competition leads to the selection of the fittest mates, leading to a net benefit from mate choice-not a net cost.
The work is elegant, says Michael Rose, an evolutionary biologist at the University of California at Irvine. He says it raises new questions about exactly how the adaptations arise: "The next step is breaking apart the seminal fluid toxicity and looking for the specific genes involved."
Author: Bob Holmes
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