One of the major components of the world's biological diversity are the differences between males and females in traits related to mating, including weapons used when competing for mates and display traits used to seduce them. Such gender differences are thought to arise because selection acts differently on each sex. The conflicting interests of males and females in reproduction are thought to be a key source of sex-specific selection on such traits.
The evolution of sexual dimorphism is hampered, however, because the sexes share the majority of their genomes; an adaptive response to selection in one sex may therefore cause nonadaptive changes in the other as a correlated response.
In a recent study, Steve Chenoweth and colleagues Howard Rundle and Mark Blows from the University of Queensland, Australia, tackle the combined issues of genetic control and sex-specific selection in a comprehensive investigation of the evolution of male-female differences in a set of sexual display traits in the Australian fruit fly, Drosophila serrata.
Both male and female Drosophila serrata use a set of pheromonal displays during mating, but differ in the relative concentrations of the individual compounds. Using a series of natural populations collected across a wide latitudinal gradient along the east coast of Australia, the authors show that the amount of sexual dimorphism varies and that these differences among populations are genetic. Using classic quantitative genetic methods, they demonstrate that the relaxation of genetic constraints appears to have occurred largely via genes on the X chromosome.
"There are a number of mechanisms by which genetic constraints can be overcome," says Dr. Steve Chenoweth, "Our results suggest that sex-linkage may be key." By manipulating both natural and sexual selection and observing the evolutionary response in the laboratory, the authors also show that, as expected, sexual selection appears to generate sex-specific optima for these pheromones. Contrary to expectation, however, females and not males responded to sexual selection.
"This suggests that the classic scenario of males as the sole target of sexual selection may be overly simplistic," states Dr. Howard Rundle. "If you want to know why males and females differ, you need to consider all forms of selection on both sexes."
This research was published in the January issue of the American Naturalist.
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