Pheromone evolution and sexual behavior in Drosophila are shaped by male sensory exploitation of other males

Animals exhibit a spectacular array of traits to attract mates. Understanding the evolutionary origins of sexual features and preferences is a fundamental problem in evolutionary biology, and the mechanisms remain highly controversial. In some species, females choose mates based on direct benefits conferred by the male to the female and her offspring. Thus, female preferences are thought to originate and coevolve with male traits. In contrast, sensory exploitation occurs when expression of a male trait takes advantage of preexisting sensory biases in females. Here, we document in Drosophila a previously unidentified example of sensory exploitation of males by other males through the use of the sex pheromone CH503. We use mass spectrometry, high-performance liquid chromatography, and behavioral analysis to demonstrate that an antiaphrodisiac produced by males of the melanogaster subgroup also is effective in distant Drosophila relatives that do not express the pheromone. We further show that species that produce the pheromone have become less sensitive to the compound, illustrating that sensory adaptation occurs after sensory exploitation. Our findings provide a mechanism for the origin of a sex pheromone and show that sensory exploitation changes male sexual behavior over evolutionary time.Sexual selection is widely regarded as an important mechanism for the origin of new traits and species. Darwin first proposed that the elaboration of male secondary sexual traits is driven by female preferences (1, 2). This concept has been refined by models suggesting that females select male traits that indicate genetic quality or confer direct reproductive benefits (3–7). In contrast, sensory exploitation occurs when expression of a male trait takes advantage of preexisting sensory biases in females (8). In this case, female preference does not coevolve with the male trait but rather precedes it. In one of the first examples documenting sensory exploitation, female Physalaemus coloradorum frogs were shown to prefer male calls that contain a low-frequency “chuck” component despite the absence of this feature in calls from conspecifics. The sensory bias for chucks was shown to have its mechanistic basis in the tuning properties of the inner ear, a physiological feature that predated the appearance of chucks (9). Similarly, female platyfish exhibit a preference for males with swordtails despite the absence of swordtails in male platyfish. Females consistently chose to spend more time with conspecific males exhibiting an artificially attached plastic sword (10). In both these examples, female preference predates expression of the trait. Sensory exploitation has since been documented for numerous other visual cues, across a diversity of taxa (11–14). In each case, females prefer traits that are not found naturally in their own species but appear in males of other species. Moreover, both the sensory bias and behavioral response to the trait already were present before expression of the trait.Pheromones are taste and olfactory cues that, in many species, play an important role in mate selection (15). As with courtship cues detected by other sensory modalities, pheromones are shaped by sexual selection and, thus, may exhibit enormous structural diversity and exquisite stereochemical specificity. In insects, exogenously secreted lipids advertise mating status, availability, and reproductive fitness (16). In some cases, male pheromones serve as a nuptial gift, thus providing direct reproductive benefits to females and offspring in the form of either nutritive or defensive compounds (17). Little is known, however, about the mechanisms underlying the diversification and the origin of chemical specificity. Here, we provide an example of a pheromone that has evolved from sensory exploitation. In Drosophila melanogaster, CH503 [formally, (3R,11Z,19Z)-3-acetoxy-11,19-octacosadien-1-ol; Fig. 1A] functions as an antiaphrodisiac (18). The pheromone is secreted in the anogenital region, is transferred to females during mating, and suppresses courtship from males. Our findings indicate CH503 evolved from males exploiting the preexisting sensory biases of other males to gain mating advantage by limiting access to females. Moreover, the use of CH503 has altered male sexual behavior over evolutionary time such that males have adapted by becoming less sensitive to the pheromone.Open in a separate windowFig. 1.Characterization of CH503 expression in Drosophila. (A) Chemical structure of CH503 and representative UV-LDI mass spectra measured from the male anogenital region of different Drosophila species. Each spectrum is recorded from a single fly. Signals corresponding to the mass-to-charge ratio (m/z) for cVA (m/z 349.24) and CH503 (m/z 503.38) were detected in D. melanogaster, D. simulans, D. yakuba, D. sechellia, and D. ananassae. No signal for CH503 was detected from D. willistoni, D. mojavensis, or D. virilis. Potassium-bearing molecular compounds [M+K]⁺ constitute the major ion species in all cases. (B) The HPLC chromatogram shows distinct retention times (RT) for each of the eight synthetic CH503 stereoisomers following derivatization. HPLC analysis of derivatized CH503 isolated from D. simulans, D. yakuba, and D. sechellia reveals that (3R,11Z,11Z)-CH503 is the only expressed stereoisomer. The retention times for the major peaks are noted in each chromatogram. The compound isolated from D. ananassae has the same m/z and elemental composition as CH503 but a different structure.