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Phil Williams
 In the dating game, the word “chemistry” has nothing to do with the periodic table. After a boring evening, the participants may shrug and say, “There was just no chemistry between us.”
They may be more right than they know. A new study shows that when female fruit flies are given a choice between mates, their offspring live longer as adults than do those of females who have only a single male from which to choose. The research offers insights into both female choice and male competition.
“We know there are fitness consequences to sexual selection, but past studies have largely focused on juvenile survival,” says Daniel Promislow of the genetics department. “Our results focus on adults, and we showed that the process of sexual selection can lead to a genetically related increase in the components of adult fitness.”
Drosophila melanogaster, a variety of fruit fly, is ideal for studying the process of sexual selection. Instead of going to singles bars or athletic clubs, the pinhead-size flies will readily mate anywhere, even in test tubes. Much of what scientists know of sexual-selection models has come from studying these tiny creatures.
Promislow wanted to test the so-called “good genes” model, which proposes that female preference for certain male traits evolved because that trait is an indicator of genetic quality.
Promislow and his co-authors, Emily A. Smith and Louise Pearse, created two sets of artificial selection lines of D. melanogaster. In the first--or “S”--line, they reduced the opportunity for sexual selection by mating one virgin male and female in each vial. In the second--or “M”--line, they placed one female with five males, so that female choice and male competition would come into play.
Not every vial was the site of a hot romance. Even in some of the “S” vials, females chose not to mate rather than consort with the only male available. To adjust for such problems, the researchers carried out three complete replications per treatment over 10 generations and compared offspring for such qualities as age-specific adult survival, larval competitive ability, wing size and sex-comb tooth number. (The sex comb, a structure resembling a stunted hair comb with long teeth, is found only on the legs of males and is used to sense females.) Finally, the study compared so-called fluctuating asymmetry between the lines for wing size and sex-comb tooth number. Fluctuating asymmetry refers to subtle changes in bilateral symmetry--for instance, if one of your eyes is higher than the other.
For some of these traits, there was little difference in offspring between the “M” and “S” lines. Wing size, sex-comb tooth number and larval competitive ability had only weak statistical differences. The significant difference came in adult survivability.
“We found that the females and males in the ‘M’ lines lived significantly longer than the ‘S’ line males and females,” says Promislow. This result supports the proposition that females with a choice of mates are able to select males whose genetic makeup gives their offspring a chance for a longer lifespan. The choice may even account for an increase in body size.
The success of this research may lie in the fact that selection was carried out over multiple generations, which increased the likelihood of identifying small genetic effects. Some problems remain, however. For instance, the scientists cannot distinguish between the effects of female choice and male competition in the “M” lines.
Still, the findings confirm that adult survival rates increased with an increased opportunity for sexual selection, a result consistent with the idea that females choose males on the basis of relatively high genetic quality. It was clear that enforced monogamy was bad news for the “S” lines of flies.
The research opens some new doors for investigation. Promislow says that not all fitness traits will necessarily benefit from sexual selection and that gene-environment interactions could result in unexpected consequences generations later. Genetic benefits to one sex might even prove detrimental to the other.
In the meantime, the current study confirms the validity of at least one model of sexual selection.
“We wanted to provide an explicit test of the “good genes’ model,” says Promislow. “And we can say that what we found was consistent with that model and with others. But I don’t think we will ever find one unifying model that explains the patterns of sexual selection in all organisms.”
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