The endless struggle for survival in nature inevitably boils down to finding food and eluding predators. To avoid the latter, many plants produce chemical weapons to discourage predators. A sound strategy overall, but the rules of co-evolutionary war suggest that an herbivore will evolve resistance to the toxic defenses of plants. The fruit fly Drosophila sechellia, for example, has a penchant for the fruit of a Polynesian shrub called Tahitian Noni (Morinda citrifolia) that smells so foul it's nicknamed 'vomit fruit.' Other Drosophila species treat the rank odor, which arises from the toxins hexanoic acid and octanoic acid, as a warning sign to stay away. And with good reason-if they alight on Tahitian Noni's fruit, they die. But D. sechellia blithely homes in on the malodorous fruit to lay its eggs, ensuring a bounteous meal for its larval offspring. D. sechellia's resistance to a plant that kills likely competitors gives the fly nearly exclusive access to its host-a distinct ecological advantage. But it also raises an important question for evolutionary biologists: are the factors that promote specialized ecological interactions between herbivore and plant host sufficient to drive herbivore speciation?
In a new study, published by PLoS Bioogy, Takashi Matsuo, Yoshiaki Fuyama, and colleagues explored the genetic factors underlying the behavioral differences between D. sechellia and other Drosophila species. Taking advantage of the robust genetic tools offered by D. melanogaster, the researchers traced the flies' divergent host-plant preferences to two olfactory genes, odorant-binding protein 57e (Obp57e) and Obp57d. Their findings suggest that as the expression patterns of these genes changed in D. sechellia, the fly lost the impulse to avoid Tahitian Noni, allowing an adaptive shift to this previously proscribed plant.
The researchers generated lines of "knock-out" flies that lacked either Obp57e genes, adjacent Obp57d genes, or both (cal
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