Evolution of Sexual Dimorphism

Sexual dimorphism makes life interesting for many species. In the case of the fruit fly Drosophila melanogaster, the rear end of a male is so much darker than that of a female that a seasoned fly pusher can distinguish he from she even without the aid of a microscope. A telling investigation by Artyom Kopp and Sean Carroll at the Howard Hughes Medical Institute and Laboratory of Molecular Biology at the University of Wisconsin-Madison and Ian Duncan at the Department of Biology at Washington University in St. Louis suggests how sexual selection could have maintained the distinctive abdominal differences in two steps.1 Sexual selection is a key part of Charles Darwin's theory of the origin of species. Inherited traits that foster mating obviously boost certain gene variant frequencies in the next generation, and therefore drive microevolutionary change. Darwin knew nothing of genes, but he described the ins and outs of sexual selection in a veritable zoo of alligators, salmon, stag-beetles, lions, and various birds. He wrote, "This form of selection depends on a struggle between the individuals of one sex, generally the males, for the possession of the other sex. A hornless stag or spurless cock would have a poor chance of leaving numerous offspring."2 Reprinted with permission from Nature 408:553-9(2000), Macmillan Magazines. Ltd. The homeotic function of bab The work on fly rears focused on a gene called bric-a-brac (bab), whose protein product, a transcription factor, is itself regulated by specific homeotic and sex determination genes. The researchers demonstrated that male flies avoid females whose rears are pigmented due to bab mutations. However, visually impaired males were just as attracted to the mutant females as to the normal ones. In contrast, females were equally likely to mate with normal males as with pale-ended mutant males. "Bab is normally not activated in the male's behind, but we can activate it there by artificial means," explains postdoctoral researcher at the University of Wisconsin and lead author Kopp. Therefore, noticing abdomen color among these flies seems to be a male preference. The researchers also compared abdominal color and topography among various types of fruit flies. And they found that specific patterns of pigmentation and epidermal outgrowths (trichomes, which are projections of the cuticle, and bristles) distinguish the sexes in D. melanogaster and several species subgroups, but that these three characteristics vary independently in related species subgroups whose sexual cues are not based in the abdomen. Overall, the observations point to the dark abdomen of the modern male D. melanogaster as a relatively new acquisition that has been the subject of intense sexual selection. Biologists determine whether a trait is recent or ancient by considering its presence or absence among many species arranged in a treelike diagram, or phylogeny, according to molecular sequence similarities. Mutation rates in known genes are used to superimpose a very approximate timeline. Explains professor of genetics Carroll, "Molecular phylogeny indicates that dimorphic pigmentation in Drosophila melanogaster originated about 17 million years ago. That's recent, compared to other types of changes." Reprinted with permission from Nature 408:553-9(2000), Macmillan Magazines. Ltd.Abdominal segments of D. melanogaster female (a) and male (b), and D. Willistoni female (c) and male (d). Selection and Counterselection Dark rears aren't all-important in the sexual stakes, however. Mutant males with pale rears, but normal bristles and trichomes, mate as often as other males. The researchers applied their observations to a scenario of "runaway" sexual selection first proposed by R.A. Fisher in 1930 to reconstruct what might have happened.3 Perhaps females at some time in the past encountered and preferred males who, by the luck of genetic roulette, had slightly darker rears. Because the pigmentation is under genetic control, the mating preference increased the representation of the responsible gene in populations, until it came to be the norm. Then, like Friends actress Jennifer Aniston's trademark hairstyle that lost some of its oomph once myriads of women copied it, a male fly's dark-endedness was no longer special once everyone had it. So females began to pay more attention to other features, such as the tapered shape or bristle patterns of the male rear, or perhaps other attractions not obvious to human eyes. "I don't think they give a damn about trichomes, but there are a lot of other cues involved in mating, such as pheromones, behavior, and wing vibration. I suspect these things took over from pigmentation," says Kopp. Meanwhile, or perhaps somewhat later, males learned to avoid attempting to mate with dark-ended individuals. It's hard to resist comparing or even extrapolating sexual selection among the birds and the bees to Homo sapiens, or at least describing it in human terms. In this sense, Carroll sees the first phase of sexual selection among the flies as an "arms race, the bells and whistles that the male evolves and the female selects. For a trait to become sexually dimorphic, according to Darwin, sexual selection has to drive morphological change in both sexes. But sexual selection dissipates if one sex habituates. For a trait to stay dimorphic, it has to be selected by males against being in females. This maintains the lack of pigmentation in females." Carroll is excited about the results because they merge schools of thought that rarely grace the pages of the same journal, let alone the same paper. "The work considers development and gene regulation, phylogeny, and a behavioral explanation for why the trait is there," he says. And it places researchers in the role of "human on the wall" in observing the mating habits of fruit flies. Ricki Lewis (rickilewis@nasw.org) is a contributing editor for The Scientist. References 1. A. Kopp et al., "Genetic control and evolution of sexually dimorphic characters in Drosophila," Nature, 408;553-9, Nov. 30, 2000. 2. C. Darwin, The origin of species by means of natural selection or the preservation of favoured races in the struggle for life, Mentor Book: New York, 1859. 3. R.A. Fisher, The genetical theory of natural selection, Dover: New York, 1958.

Evolution of Sexual Dimorphism

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