Oct. 6, 1998 ATHENS, Ga. -- In the game of dating, the word "chemistry" has nothing to do with the periodic table of the elements. Nothing is worse than finding out there is no "chemistry" with your date. Something just doesn't feel right, and later, a man or a woman will shrug and say, "There was just no chemistry between us."
That statement may be more accurate than anyone could have ever imagined. A new study by geneticists at the University of Georgia shows that when female fruit flies are given a choice between mates, their offspring live longer as adults than females who have only a single mate from which to choose. The research is bringing new 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," said Daniel Promislow. "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."
The research, which was partially supported by a grant from the National Institute on Aging, was published today in the Proceedings of the National Academy of Sciences.
Drosophila melanogaster, a variety of fruit fly, is ideal for studying the process of sexual selection. Much of what scientists know of sexual selection models has come from studying these tiny creatures. Scientist can study their sexual behavior much easier than they can follow humans to singles bars and athletic clubs. Promislow's idea with the current research was to test the so-called "good genes" model, which proposes that female preference for certain male traits evolves because that male trait is an indicator of genetic quality.
The study presented some thorny problems. First, many, if not most, animal signals may fall outside the range of unassisted human perception. Second, very little is known about adult fitness components and how females select mates to pass them on to offspring. And finally, many previous studies have focused largely on observable appearance and how it relates to female choosiness or male attractiveness.
"We attempted to avoid each of these potential pitfalls by using an artificial selection approach in which we varied levels of selection over multiple generations," said Promislow.
To do this, 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 big lug chosen for them. 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 is a structure resembling a stunted hair comb with long teeth. It 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 eye 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, however, 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," said Promislow. This fact 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 life-span. The choice may also account even 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 most important finding is the confirmation that adult survival rates increased in populations with an increased opportunity for sexual selection. The result is consistent with the idea that females choose males on the basis of relatively high genetic quality. However, it seemed clear to the researchers that "enforced monogamy" was bad news for the "S" lines of flies. These findings may have some effect on how we view some social and cultural practices as well.
The research opens some new doors for investigation. Promislow said that not all fitness traits necessarily will benefit from sexual selection and that gene-environment interactions could create unexpected consequences generations down the line. And genetic benefits to one sex may 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," said 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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